Transcript for:
IGCSE Physics Paper 4 Review Essentials

Hi everyone. This is it. The final stretch before your IGCS physics paper 4. Just bear this in mind. You have studied. You have practiced. Right now you just need something that is powerful, something that is real quick, clear and complete. So in this video I am giving you the final boost to your IGCSA physics paper 4. I will recap every essential concept the key definition and must know formula so that you can walk into that examination hall with confidence and clarity. So are you ready? Let us power up. So this is it. This is the final boost to your IGC DSA physics paper 4 right I'm going through the topic one by one tell you what are the important things important concept important key words that you're supposed to write when you enter your examination hall and you see the question I pray heart that everything come clear to you and let us get started now first of all this is about motion. Do you ever realize that when you try a lot of passive questions there are surely right or most of the time more than 90% the question related to motion will be asked so right these are the things that are very important right first of all define speed what is speed I'm going to highlight the key words speed is being defined as distance per time and speed is a scalar quantity Remember because distance is a scalar quantity. Velocity is being defined as displacement per unit time. What is displacement again? Displacement is a direction. It's a distance with a direction. North, south, east, forward, backward. These are this is displacement. Right? Acceleration is being defined as rate of change of velocity. But if you don't want this definition, you can also define it almost similar meaning right as change in velocity per time taken. But you cannot put change in speed because speed is a scalar quantity. Acceleration is a vector quantity. So you have to put change in velocity. If you put change in speed, I'm very sorry. is very hard to give you mark right average speed is being defined as total distance traveled per total time. So if you have three speeds given for example 10 m/s for example 10 m/s uh 20 m/s or 5 m/s you cannot add up all these speeds and divide it by three and claim that's your average speed. Average speed is being calculated differently. So for 10 m/s if the time is being given you have to find let's say uh for 10 second. So you have to find the distance traveled by the object within that 10 second. Maybe 20 m/s the time is only 5 second. T uh 5 m/s the time is maybe 20 seconds. So you have to find the distance for every section and then you just add up all the distances divided by the total time. That's your average speed. Now, graphs, graphs are quite popular. In your syllabus, you only need to know about these two graphs. When a graph is being given, bear this in mind, right? From the graph, you can obtain two information. One is the area under the graph. The other one is gradient. Only about area and gradient. Graph is only about area and gradient. Right? Bear in mind if the distance time graph is being given area represents nothing but the gradient of the graph represents speed. If a speed time graph is being given the area under the graph represents total distance traveled. gradient of the graph represents acceleration. I would say that if you ask me to compare a distance time graph and speed time graph, I will say that speed time graph is more popular. Why? Because you can obtain more information from the graph. You can find the area of the graph. You can find the gradient of the graph. But distance time graph, you can only focus on gradient. Now, wow. Free fall. For free fall you need to consider two situation. One is without air resistance. That means you conduct a free fall right. You just drop an object right in space because there are no air resistance right. So object without air resistant all objects if you conduct the experiment on earth right will fall with gravitational acceleration and if you assume that there is no air resistant acting on the object then the gravitational acceleration on earth is 9.8 8 m/s squared. But if there is air resistance acting on the object, of course you need to understand this. Air resistance increases as the speed of the object increases. So when the speed of the object is increasing air resistance will also increase. And for every object that is falling the speed is going to be increasing not decreasing. Right? Now let me explain on how the terminal velocity is reached. But before that I need to explain the definition of terminal velocity. Terminal velocity is the maximum velocity when air resistance balances weight of the object. So when the terminal velocity is reached there is no more acceleration because air resistant has already balanced the weight of the object. Acceleration resultant force is equal to zero. Acceleration on the object is equal to zero. So the object will fall at terminal velocity right constant speed until it reaches the ground. So when you are being asked with how the terminal velocity is reached, you can explain in term of forces that are acting on the object or the acceleration there is the variation of the acceleration on the object. So I will explain both just in case right. So I am going to divide into three stages right I'm going to use different colors. So first stage always start with at the start of the four object is always at rest. Is that okay? So you must assume that the initial speed of the object is equal to zero or initial velocity of the object is equal to zero. Initial since they ask about terminal velocity. So I put initial velocity of object is zero. Remember what I wrote just now. Let us go back to the my previous slide. I wrote that air resistant increases as the speed of the object increases. But at the start of the fall since there is no velocity there's no speed air resistant is also equal to zero. air resistance on object is zero. So the only force if you talk about force if you want to write about forces the only force acts on object is its weight object. So due to the weight object falls with gravitational acceleration if you want to get more information that is even better of 9.8 8 m/s squared. That's it. That's the first stage. Okay. I'm going to use different color for the second stage. Right. Second stage. You know once the object is falling the speed is going to increase when the velocity of the speed velocity of object is increasing air resistance acting on object increases. You know air resistance is the resistive force that opposing the motion of the object. When the object is falling, object goes down. Right? Object goes down, weight is acting downwards. But the air resistant try to push the object outwards. Right? That makes the resultant force of the object decreases. Do you still remember how to find the resultant force? You take weight plus negative air resistance. Why I say plus negative air resistant? Because weight and air resistance are forces. Force is a vector quantity. So you have to consider direction. One weight is acting downwards. Air resistance is acting upwards. Right? Could you see that? So you have to plus a negative air resistance. That makes your resultant force acting on the object decreases. Right? Uh so you just write here resultant force acting on object decreases when the resultant force decreases. What about acceleration? Of course, the acceleration is also decreasing because F mama, right? Acceleration on object decreases. And then after that what happens the last stage. Okay. Okay. Last stage when the velocity of the object when the because air resistance is going to increase until it can balance the weight of the object. When the velocity of object is maximum, air resistance increases to a magnitude equal to weight of object. object. So what happened? Because one outward force is balanced by the downward force. Resultant force acting on object is zero decreases to zero. So what happens to the acceleration? acceleration on object decreases uh is zero. Sorry. So just write down zero will do because I have already explained decreasing. So is zero and at that moment object is falling with terminal velocity. Object falls with terminal velocity. So this is how you're supposed to explain. Let me highlight the key words. If the question asks about forces and acceleration, I highlight both. Okay. At the start tell the examiner before the object starts falling initial velocity is equal to zero. So therefore the air resistant must be equal to zero. The only force is weight object falls gravitational acceleration velocity increases air resistance will also increase resultant force decrease. Air resistance decreases. Air resistant increases to a okay equal to weight. Resultant force is equal to zero. Acceleration is equal to zero. Object falls with terminal velocity. These are all the important words, key words, key points that you were supposed to write. Now the next one, mass v weight. Uh state what is meant by mass? Mass is amount of substance contained in your body. Our body is made up of atoms. How many atoms in our body gives you your mass? Just take it like that. Right? The unit of mass is kilogram could be in gram as well. And apparatus instrument to measure the mass instrument to measure the mass is called electronic balance. And bear in mind mass is a scalar quantity. Scalar quantity means mass cannot take negative value. Right? Weight on the other hand is being defined as gravitational force acting on the body. So the unit of weight is Newton. instrument to measure the weight is called Newton balance or or Newton's meter or Newton spring balance. Weight is a vector quantity. Why vector quantity? Because the gravitational force acting on you, on me, on everybody here is directing towards the center of the earth. That is why weight is a vector quantity. Now density density is being defined as mass per unit volume. So I have seen some of the past question ask you about how to determine the density of an object. So first of all when you are asked to describe a simple experiment always write down the apparatus. So what are what is the apparatus that we use to measure mass? Okay, just write down it is electronic balance. What is the apparatus that we use to measure volume? So to measure volume, you have to look at the questions. Is the object right? Does the object has a regular shape? What is the shape of the object? If the object is a cuboid, then you just multiply. You just use your ruler, measure the length, the height, the width. But if the object is irregular shape, you don't know what's the shape is that then you have to use then you have to look at the size of the object. If the size of the object is small, then you could use measuring cylinder, right? I'm going to talk about that later. Okay, this is it. I'm going to talk about that later. So density, the definition is mass per unit volume. So formula, if you want to write down formula, it's not been given. Density. This is a symbol of density. It's called row. m over v. m represents mass. V represents volume. Row represents density. And the unit of density could be in kilogram per meter cube because volume is measured in me cube or gram per cm cube. Density bear in mind it is a scalar quantity. That's all. Now, how to calculate the density of an irregular shaped object? So, maybe a small rock, right? So first thing okay you are supposed to prepare a measuring cylinder and but first thing fill in water okay into measuring cylinder I measure the volume first or if you want to write the mass first because when you see how to calculate the density always write down the equation of density. Oh density depends on mass and volume. It is okay. You can write down the mass first. Okay. Mass is easier because we just place that object on top of the electronic balance. Place or use. Okay. Uh measure mass of object uh suggest the apparatus by using electronic balance. And after that um fill in water into measuring cylinder. Take its initial volume and after that carefully or you can say please object into measuring cylinder. take its final volume and tell us how do I calculate the volume of the object. So volume of object is equal to final volume minus initial volume. And last part density of object can be calculated by mass of object. Maybe if you want to put a unit just put gram divided by volume of object because mass measured by electronic balance is in gram. put cm Q. Is that okay? So, I'm going to highlight the keywords right now. Measure mass, electronic balance. We want to see that. Fill in water, measuring cylinder, initial volume. I don't want to see that. And then volume of the object, final volume minus initial volume. And density, mass per volume. These are all the things that we want to see. The next one, Hook's law. Robert Hook. Do you see remember not Captain Hook? Cook's law states that extension of an elastic material is always directly proportional to the force applied one beneath the limit of proportionality. Now what is limit of proportionality? Bear in mind limit of proportionality is not equal to elastic limit. What is limit of proportionality? I'm going to write this down. Limit of proportionality means that okay maximum point in which hook's law is obeyed. But what is elastic limit? Elastic limit that means maximum. Okay, a point. Okay, a point. Sorry, I just put a point. I just write this down here. Elastic limit. Elastic limit is a point in which exceeded. If you exceed that point material material under goes deformation. What is deformation? That means changing of shape, changing of size, length will not return to its original shape or size or length. Is that okay? But Hook's law is never about elastic limit. Hook's law is only until limit of proportionality. Right? So formula I'm going to use red color. Formula F= KX. K F F represents force. K represents a spring constant. X represents extension. Right? Graph. Okay. You should know how to plot the graph as well since it is directly proportional. It could be extension against force or force against extension. Force/ the unit extension could be in mm or cm. And the graph must always start from origin because when there's no force, there's no extension. So a straight line graph. Wow, we drew a graph. Remember what I told you when the graph is being drawn, there are two information that you could obtain from the graph. One is the gradient of the graph. Let me write down. What is the gradient of the graph? Gradient of the graph in this case represents spring constant. Okay, this is called spring constant. So if you see a graph is being plotted f against extension bear in mind that's gradient of the graph represents spring constant. Area under the graph area represents work done in stretching spring or elastic material. or or you can call it as elastic potential energy. So there are two equation of elastic potential energy. One is I put E represents energy half FX. You see this is straight line graph. If you find the area of the straight line graph that will be the area of triangle. F represents force. X represents extension. If you don't want this, you could replace it with half kx² spring constant multiplies by extension square it. So all these formula are very very important. Yeah. And the hook's law itself the statement of hooks law you need to know how to state. Right. And the graph itself just in case they ask you to sketch, they ask you to plot. Right? Remember this is the graph. The next one, centripedalo force. Bear in mind centrial force is not a new type of force. So you don't have to be terrified. It is just a normal force. Maybe from friction, maybe from normal reaction force etc. that causes object to travel in a circle. And the direction of the centripal force right always directed towards center of circle or center of the band. Uh sometime they use the word bend right and then centripal force. I want you to bear in mind one more thing is that centriater force adds perendicular to motion of object. So in other words like this ah okay let me draw okay a circle here I'll draw a circle for you. You just imagine that this is a very nice circle. This object is moving in clockwise direction around the circle. Right? So this is the direction of motion. Direction of motion is called the direction of the velocity. So if I want to find the direction of motion at this point, I put a cross here. I draw a tangent. Uh this is the direction of motion. Velocity. Centripal force will act perpendicular. Can you see? 90° towards the center of the circle. This is centripetal force. That is why since the centripal force is acting perpendicular to the motion of the object or to the velocity direction of velocity of the object. Centriped force does not change the speed of the object. It doesn't make the object move faster or slower. It is just responsible to change the direction of the object so that the object can complete a perfect circle. can travel around the circle. That's the centriu force. Now turning effect of a force is called moment. You know force can do many things. Force can make things move faster. Force can slows things down and force can also cause a turning effect. The turning effect that caused by the force is called moment. So the definition of moment is the product of force and perpendicular distance from pivot. What is pivot? Pivot is a turning point. So if you want formula definitely formula is equal to okay moment formula moment equals to force I always write it like this multiplies by perpendicular distance right okay the symbol of moment is t with a tail we call it as to but it doesn't matter it's just a symbol right the unit of moment could be newton meter newton cm newton mm it is all depending on the perpendicular distance, right? Bear in mind moment, right? Causes the object to turn. So moment itself is a vector quantity. So when the object turn, object can only turn in two direction, right? Anticlockwise direction or clockwise direction. Just look at the clock. Turn in this direction is called clockwise. turn in this direction is called anticlockwise. Now principle of moment you know when in paper four sometimes they ask you to state two conditions for an object to be in equilibrium. One of it is this principle of moment. Principle of moment talks about um what is the condition of the moment acting on the object in order for the object to be in equilibrium. So that is why I say for an object to be in equilibrium object equilibrium total anticlockwise moment must always be equal to total clockwise moment. So we use the principle of moment to form equation to solve problem related to moment. Right? So you have to identify. Do you know how to see which what is clockwise moment? What is anticlockwise moment? Let me show you. Right? This is an object for example and this is a pivot. Sometimes it is called falcrum. Don't worry. Right? So let's say I have a force acting upwards here and uh a force acting downwards here. Uh this is f_sub_1. This is f_sub_2 I call it. And this is f uh three I call it. Is that okay? Now I want you to see that. Okay. This is a distance. I call it as distance one. Okay. This is what we call perpendicular distance. You see when a force is in vertical perpendicular distance must occur in horizontal direction X direction because the force is acting in Y direction. So X direction this is called perpendicular distance one for force one. This is perpendicular distance from the PD itself. Pendicular distance for F_sub_2. This is perpendicular distance of F_sub_3. Now since we have all the perpendicular distances and then we have all the forces here I will highlight okay only three forces involved here how do you determine which is clockwise which is anticlockwise direction now I want to you to look at f_sub_1 f_sub_1 is going upwards right so because moment causes turning effect so I want you to think it like this f_sub_1 is going upwards while going upwards it will seek for where's the turning point so it has to turn towards the turning point. So f1 moving outwards while turning to the turning point. This direction what is this direction represents? This is called clockwise because you follow the direction of the clock. Right? Look at f_sub_2. F_sub_2 is going down. While going down where's the pivot is on the right hand side. So you have to turn to the right hand side. So when f_sub_2 is turning towards the right hand side this is called anticlockwise moment. What about F3? F3 going up. While going up, where's the pivot? Pivot is on the left. So, we will turn towards left. Turning towards left. This is also called anticlockwise moment, right? Ah. So, if the whole system is in equilibrium, you have to look at it. So, uh the question sometimes they don't mention equilibrium. The question would mention um remain uh stationary uh that means equilibrium or uh remain remains horizontal. Uh these are all the key words for equilibrium. So I will say that this plank remains horizontal. So you can use anticlockwise moment principle of moment to solve is equal to clockwise moment. Just put everything anticlockwise on left hand side which is f_sub_2 force * perpendicular distance x2 plus f_sub_3 force * perpendicular distance x3 is equal to f_sub_1 * x1. Tada. Then you just sub in the values that you have to find the unknown. Now conditions for object to be in equilibrium. If a question regarding moment is coming up, normally the first part they'll ask you to list down two conditions for an object to be in equilibrium. Just memorize this. Resultant force acting on the object is equal to zero. What do I meant by resultant force? Let's say this is the object. That means the upward force of the object is balanced by the downward force. Forward force on the object is balanced by the backward force. Up is balanced by down. Left is forward is balanced by backward. uh that is called resultant force. Resultant moment acting on the object must also be equal to zero. It means that the clockwise moment must always be equal to anticlockwise moment. That's it. Next one. Center of mass v center of gravity. Remember center of mass it is regarding mass. Center. Okay, I will highlight the key words here. Center means a point. You have to write that a point. Center of mass cm which the whole mass is considered to at center of gravity is a point in which the whole weight of the object is considered to act. In your syllabus IGCSE syllabus we always assume that the center of mass is equal to the center of gravity when we do perform our calculation. It doesn't matter. But it comes to the definition. You have to put center of mass as a point which the whole mass is considered to act. Whereas center of gravity is a point where the whole weight of the object is considered to act. The next one momentum. We always say that if you want to be successful, you need to have a lot of momentum. Let us have a lot of momentum. Now how about that? Right? Momentum is being defined as a moving mass. Right? As long as you are moving, you have mass. you have momentum but if you are not moving you're not progressing you just sit down there procrastinating then you have no momentum give a lot of momentum to physics how about that right the definition of momentum is mass multiplies by velocity if you want formula definitely formula is equal to mv and bear in mind due to momentum is a vector quantity yeah most of the student make a lot of callous mistake I will show you later. So, and the unit of momentum is equal to kilogram meter per second or newton meter. No, not Newton meter. Sorry, it's not Newton meter. It's a Newton second. Newton second. Why? I will explain later. Now, this has something to do with impulse. Impulse as change in momentum. Impulse has two equations, right? Impulse can be calculated from two equations. One is force multiplies by time. The other one is okay change in momentum. So if the impulse is calculated using force multiply by time, you can use the unit of kilogram meter per second, not a problem or Newton second. Impulse represent change in momentum. So it all it is all depending on the question. If the question gives you force and time and asks you to find the impulse, just take force multiplies by time. If the question gives you momentum, initial momentum, initial velocity and mass ask you to find the impulse, just put impulse as change in momentum. Right? Final momentum minus initial momentum. MV minus mu. That is also called impulse. M represents mass. V represents final velocity and U represents initial velocity. So if you must know how can impulse be equal to change in momentum. Let me prove it. Do you remember force F equal to mama? Yeah. And then the definition of acceleration change in velocity per time taken. Sub into the equation you have m v minus u over t. Right? Times in the m, you have mv minus mu over t. Could you see that? So you times out cross multiplication times out the the t. So you have f_t is equal to mv minus mu which is equal to impulse. That is how we get the next one. Resultant force. Okay. Resultant force is the sum of all the forces that are acting on one object. For example, this is the object. object is moving okay push with 3 Newton and there is also frictional force acting on the object which is 1 Newton right since force is a vector quantity you have to consider positive and negative uh you have to consider the direction so if you want to consider forward as positive direction then the backward will take negative direction just put negative so resultant force F_sub_R normally yeah this is symbol of resultant force you can use this 3 plus -1 so you get 2 Newton that is how we find our resultant force now work and energy work the definition is the product of force and distance but move in the direction of force that also means that if I push you I'm applying force on you you have to move in my direction which is forward if I pull you you have to move in my direction so the distance calculated is in my direction of force the unit of Work is jewels remember or capital J and work is a scalar quantity. Energy in general energy means ability to do work. Right? If you have ability to do work then you have energy. So most of the time when we solve any question right regarding calculation we always say energy is equal to work. Yeah, sometimes this is true, right? It is true in a way. Now there are different energy that is very important in your IGCSA physics. One is gravitational potential energy or in short I call it as G P. The definition of gravitational potential energy is the energy due to the position of the object measured from the ground. So you could see that okay when the object is on the ground because there's no height right that is why the GP is equal to zero. Let me put in the formula first. Formula of the gravitational potential energy is M g H. M represents mass. G represents gravitational acceleration. The value is 9.81 it 1 m/s squared. H represents height from ground and when the object is on the ground there's no height. So that is why GP is equal to zero. But whenever object is being lifted out to a certain height let's say this is a height h then the object will gain GPE. How do you calculate the GPE? By using the equation that I gave you and the unit of GPE is still jewels which is equal to work. In fact all the energies are measured in jewles. Mass must be measured. Oh, let me tell you this. Mass must be measured in kilogram. H must be in meter in order for you to get jewels. No, most of the time some tricky question they give you the mass in gram. Bear in mind, just convert to kilogram. That would do, right? And the gravitational potential energy is also a scalar quantity. Yeah, that is all about gravitational potential energy. The next one is the kinetic energy. the energy due to motion or in short I call it as K. So as long as you're moving as long as the object is moving object has mass and this is moving object has kinetic energy. Formula K is equal to half MV² where this M represents mass and V represents the speed but you have to square it right and the unit is jewles. Mass must be in kilogram. Speed must be in meter per second. Just square it meter squared per second squared. And kinetic energy is also a scalar quantity. That's all. Principle of conservation of energy. Normally they will give you two marks. So states that energy cannot be created nor destroyed. That's one mark. But can change from one form to the other form. That's all. So we can also use the principle of conservation of energy to solve equations. So you just need to remember okay first one when resistive forces like frictional force air resistance okay are ignore then you can use the principle of conservation of energy to solve it. That means you can you just write down total initial energy is equal to total final energy. Total initial energy could be kinetic energy, could be gravitational potential energy. It depends on the questions. Right? The second condition is when resistive forces are not being ignored. What happened? So you have to do a little bit of modification to your principle of conservation or energy. You just write down total initial energy minus okay energy dissipated uh or work done in overcoming the resistive force right uh to overcome resistive force resistive forces then only equal to your total final energy. Remember now efficiency right you know why we calculate efficiency there's not a formula here it's just ratio we calculate efficiency because we want to see our moto right the output given out by the motor is it the one that we expected how much different I put in this amount of energy how much energy that I should get in the end that's efficiency the larger the value of efficiency the better yeah the machine or the motor is right. The equation you can use um efficiency is always equal to the useful output energy per input energy times 100%. So the efficiency the unit of efficiency is in percentage if you times 100%. But if you don't times 100% then your efficiency is in the form of a ratio. But bear in mind in physics we don't let we don't leave our answer in fraction. We put it in terms of two to three significant figures only. Or efficiency can also be calculated by if you don't want to use if the useful output power is not giving uh useful energy is not being given you can use useful output work over input work times 100%. If you are not given work doesn't matter just use efficiency equal useful output power over input power multiplies by 100%. Okay. Now power show that you have the power. This is a symbol of power. Capital P power is defined as work done per unit time or energy per unit time. So the equation of the power formula power is equal to energy over time or power is equal to work done over time. Right? The unit of power is what? What? Yes. What is the unit of power? And power is a scalar quantity. Let me write down. Okay, this is called energy. This is the time and this is the work done. This is the time. Now, pressure. Don't give yourself too much of a pressure. I know IGCSA physics is no joke, but don't give yourself too much of the pressure. Relax. Right. The definition of pressure right is defined as don't ever define pressure as density time g * gravitational acceleration times the height. No no no no pressure one physical quantity one definition and the definition of pressure is force per unit surface area. If you are asked to find the pressure of the solid then we always use this okay for this formula P is equal to F / A. F represents force, A represent surface area, P represent pressure and the unit of pressure could be in Pascal only when force is in Newton, area is in meter sorry meter squared. But okay, therefore you can put unit of pressure as Newton per meter squared as Pascal or Pascal is equal to Newton per meter square. same unit or Newton per cm squared, Newton per mm² depending on the area surface area. Pressure of fluid you know fluids are gas and liquid. So if they give you the density of the fluid then the pressure formula is equal to P equals to row GH. Let me explain. P is the pressure. Row is the density of the gas or the liquid. G is the gravitational acceleration. H. Okay, for liquid, H is the depth, but for gas, H is the height depth. Okay. The depth of a liquid must be measured from the surface of the liquid downwards to where the liquid is not outwards because outwards is gas from the surface of the liquid. Right? Internal energy. So in IGCSC syllabus you are just you need to know the definition of internal energy not the equation don't worry. So internal energy is the energy inside an object right? So let's say you have a glass of water right there particles inside the water liquid molecules and these liquid molecules they are moving right even though it's restricted and they're moving they have kinetic energy and due to the distance between them they have potential energy as well. That is why internal energy is being defined as the sum the total total sum of kinetic energy and potential energy of the random moving particles. Whether you're talking about solid solid no random you are talking about liquid liquid no random but as long as they are moving or gas random moving particles. Now this is how the thermal energy is being transferred right conduction convection and radiation. Bear in mind conduction. One more thing before uh we look into conduction, convection and radiation. Bear in mind heat can only be transferred when there is a different between the two objects temperature. Uh whenever there's a temperature different if the surrounding and the object has the same temperature then heat cannot be transferred. Heat is equivalent to thermal energy. When I say heat, it means thermal energy. Thermal energy can only be transferred when the temperature different okay between two objects or object with the surrounding. But if the temperatures of the two objects are the same, then no heat is being transferred. First one, this is how the heat can be transferred. Conduction. Conduction is the transfer of heat from the hotter end to the colder end in the solid. Let's say um you are okay giving one end. This is a metal and then you place a fire here. This is a fire and then you just hold the other end. Since this is a metal, metal will transfer heat from the vibrating atoms and the localized electrons to the other end. Heat must be transferred from the hotter end to the other end. So when you touch the other end, you could feel hot. Ouch. Like that. That is conduction, right? There must be physical contact uh between you and the the other object. Okay, there is this one is a clue just a tip. There is physical contact between two objects. How about that? Convection on the other hand is not valid to solid. Okay, not valid to solid. Remember why? Because convection is the circulation circulation of heat. Circulate of heat from the hotter region to the colder region. So the particles must move in order for the heat to be circulated. Right? So about convection a little bit um water okay particles okay have larger kinetic energy right or they expands volume increases right so density decreases that is why when you heat up a place okay air particle inside the room the hot air will rise, right? Same goes to liquid molecules. Whenever you heat this out, you put fire here. So these liquid molecules that gets the fire first, gets the heat first, they get heated up. So the liquid molecules will expand which makes their distance further away from each other. So density back to density, mass over volume. So when the volume decreases, the density will in uh when the volume increases, the density will decrease. That is why the hotter molecules will rise out and the okay colder particles why I put red color for the colder particles I hope I don't confuse you. Yeah have smaller K. So smaller volume right because the particles are closer because they are colder they don't have enough energy to expand right density is larger so that is why the colder particles will move down outer particle move out circulation of heat occur. Okay radiation is the transfer of heat from the hot region to the colder region without physical contact. For example, you know, every morning you wake up, okay, when you see the sun, you can feel the heat from the sun. But bear in mind between the earth and the sun, there's nothing. It's just a vacuum. This is called radiation. Or you go for a campfire camping. So nighttime you set a campfire. So this is because you are cold, you sit close to the campfire. Are you touching the campfire? No. Right? But you sit close to the campfire, you can feel the heat. So you feel warm, you feel more comfortable. This is radiation. Now boiling vs evaporation. Boiling is the changing of phase. I'll highlight the key words. Changing of phase from liquid to gas at fixed temperature. Evaporation. The definition is changing of phase from liquid to gas as well at any temperatures. Remember boiling okay occurs at boiling point. Since I already write fixed temperature I will replace this with boiling point. Evaporation occurs at any temperatures. Yeah temperatures remember. And then boiling occurs throughout the whole liquid. But evaporation only occurs at the surface of liquid. That's it. Specific heat capacity. Okay, if you see this word specific heat capacity, the first thing, the first formula, I talk about formula first formula. If you don't remember the definition, it doesn't matter. First thing you see the word specific heat capacity straight away put E equals MC deltat T because to calculate the specific heat capacity, there's only one formula which is the one that I stated here. E represents energy or heat, right? thermal energy or heat. Energy or heat. M represents mass must be in kilogram as energy is in jewles. C represents specific heat. Capacity symbol is C. Delta T represents change in temperature. Could be in degrees Celsius, could be in Kelvin, doesn't matter. So if you cannot memorize the definition for the specific heat capacity doesn't matter you can use equation. So you can put C is equal to E over M deltaT. But don't just leave it like that. Put a comma where C is the specific heat capacity, E is the thermal energy, M is the mass and delta T is the change in temperature. That's all that you need to write. Remember that. So if you can remember the definition, just check whether you still remember whether you get the correct definition. So it is a heat supply or thermal energy supply per unit mass to increase its temperature by 1 Kelvin or 1° C doesn't matter right now but you have to remember mass must be in kilogram because the energy is in jewles sometimes the question is too tricky right because in physics lab we don't have bunson burner so there is no apparatus to calculate the thermal thermal energy so normally in physics lab we use heater. heater comes with power rating right but don't worry don't get panic when the power is being given and we use a stopwatch to time the time to heat up the object right so how much time that I used to heat up this substant right so how to find the energy thermal energy can be calculated if you recall oh power is equal to energy over time yes exactly so you could replace the energy equation of the energy as power times time equal to MC deltat T just in case if they don't give you the heat or the thermal energy you can use power if they give you power this is a time so the unit of specific heat capacity is joule per kilogram per degree Celsius right perh wait let me see per kilogram per degree CC yes or Joe per kilog kilogram per kelvin. Uh it depends on the temperature. Now waves waves there are two types of waves that you have to memorize. One is transverse wave. The other one is longitudinal wave. Transverse wave. Okay, I will highlight the key words. Transverse wave means that weakest waves means vibration. Vibration of what? Vibration of the particle which is what transverse perpendicular to the direction of travel of the wave or the direction of propagation of the wave doesn't matter longitudinal wave wave you have to talk about vibration but vibration of what vibration of particle which is parallel to the direction of travel of the wave right this is how I remember transverse transverse give you some tip put 90° you don't like to put the word perpendicular just put 90° what's wrong or you can put right angles not a problem for longitudinal wave I just put a in front along so you want to put parallel just put along vibration of the particles is along the direction of travel of the wave right so the examples of the transverse wave is um light with water wave radio wave, microwave etc. Longitudinal wave example sound wave. Okay. Ultras sound. Yeah. Uh so something related to sound that is called longitudinal wave. So I prefer you to memorize longitudinal wave because less example. So other than sound right everything are transverse wave general equation of the wave V is equal to F lambda. V is called wave speed. F is called the frequency of the wave and lambda is called the wavelength of the wave. Now wavelength right sometimes they give you a graph. Remember when the graph is being given check the x-axis on the graph if the x-axis is distance right so the wavelength you can safely use this graph wavelength is one complete uh the length of the wave the wave okay how fat the wave is that's called wavelength right wavelength is being defined as distance between two successive crest or two successive trough crest and trough Right? Or you start from origin back to origin. Frequency on the other hand when the graph is being given right frequency can be calculated. When a graph is being given check your x-axis if your x-axis is time then you determine not frequency but period first. Right? This is called period. Period is a time taken for one complete oscillation. So frequency can be calculated by one over period. T means period. Done. Now this is a phenomenon one of the phenomenon of wave reflection. Reflection means the change in the direction of propagation of wave when it strikes a reflecting surface. So you need to have a reflecting surface. Okay, let me draw it nicely again. Okay, you need to have a reflecting surface. Okay, this is called reflecting surface in order for the reflection to occur. And then on the reflecting surface you need to draw a dotted line which is perpendicular to the reflecting surface. This line is called normal. What is normal? Normal is an imaginary line which acts perpendicular to reflecting surface. Okay. And then check. Okay, this is the light ray. Light ray that is coming. You have to put arrow, right? Light ray is striking on the reflecting surface. This this ray if this is a light then this is called light ray. But if this is just a wave then you say this is incident wave and then wave that is directed away from the reflecting surface is called reflected wave. Okay. The angle between normal and inc that is why normal is very important. The angle between the normal and the incident wave is called incident angle. Incident angle the angle between normal and reflected wave is called reflected angle. So reflection the law of reflection law of reflection bear this in mind angle of incident must always be equal to the angle of reflection. So if the angle of incident is 50 angle of reflection must be 50 as well. That's all refraction. Okay the key word is that bending of wave. When a wave travel through different media maybe from air to glass, glass to air. Why wave ever bend? It is because the speed of wave is different due to the different densities of different media. So one thing that you should know two things right you should memorize. One is this one uh let's say this is air this is water. This is also air. This is normal. Okay. This is water. So if the wave is coming from air to water, it will be bent refracted towards normal. Right? Okay. When wave travels from less dense, less dense is your air to denser medium which is water with refracted towards normal. So in other word I can say that incident angle refracted angle refracted angle is smaller than the incident angle. But if the wave travel from denser medium to less dense medium then the wave will be refracted away from normal. So I'm going to write that this is called incident angle. This is called refracted angle. So when wave travels from denser medium to less dense medium wave refracted away from normal. Remember normal is very important. So you need to draw towards normal away from normal less than to denser denser to less tense. Remember that memorize this. Okay. Now refractive index or the symbol is small little n. Do you know why we calculate refractive index? Just to see how much bending has already occur when the wave travel from one medium to the other medium. So there are different equation n can be calculated from s i over sin r. i is incident angle. R is a refracted angle. N also can be equal to sin r over sin i. N also can be equal to the speed I don't want to write speed of light. speed of light in vacuum or speed of light in air which is 3 * 10 ^ 8 divided by speed of light in medium okay and n also can be calculated from the real dep okay I will explain this over apparent dep to say that you go to the swimming pool if you're standing at the middle of the swimming pool I'm outside from a swimming swimming pool. I could see you your legs are getting shorter. Why? Because the bending of light, it seems that your legs will be shorter. Or you go to a crystal clear river, you can see all the fish swimming. You could say that, hey, how come the fish, you try to catch the fish with your bare hand, right? Most of the time you cannot get. Why? Because the fish appear to be close to the surface of water, but when you try to catch them, right? Because they swim in the school, right? A lot of them that's why we call that uh we call them as school of fish right but but the real dep right the real location or the position of the fish is deep down in the water right now let me tell you when to use s I over s R this is valid when the wave travels from denser no less dense less dense medium to denser medium. S R over S I. When the wave travels from denser medium to less dense medium. Okay, you might look at all the equations that I stated for refractive index and you get demotivated. But don't get demotivated. Do you know why? I have a very important tip to share with you. You know I love shortcut. I want to share my shortcut with you. So if you can't remember because I'm not good in memorizing stuff as well. If you can't remember doesn't matter. I have just put this aside. You know why? Whenever you calculate let's say you get s I over s R or S R over SI. It doesn't matter if you confuse right. Let's say you stuck in wrongly. Let's say this situation occur here. Um um this one is 20° for example. This one is 50° right? So you use uh refractive index of the water and is equal to s is it s r I wonder. So you just put sin r over sin i right sin uh r is 20 uh i is 50. Okay use your calculator. Where's my calculator? Use your calculator. Just put sin 20 divided by sin 50. Okay you get a value which is 0.446. Right? Refractive index has no unit. So when you get a value of 0.446 you have to know right away you are doing wrong thing. Why? One tip that I want to give here refractive index that you calculated must be more or equal to one. If you get lesser than one like what I did here I get lesser than one. No worries. Okay just cross red we do n is equal to sin 50 over sin 20. Okay let us do sin 50 divided by sin 20 and I got 2.2. Yes, it is more than one. As long as the refractive index that you calculated is more than one. Yes, that's correct. Okay, that's the tip. Defraction. Defraction means spreading of a wave. When a wave travel through a small aperture uh travel through a small opening, yeah, or block etc. Right? I want you to bear in mind when defraction occur, right? Did I give you okay? I will give you one more slide which I'm going to draw it out. When defraction occur okay no change in wavelength I will explain later. No change in frequency of the wave. Frequency remain constant after being defracted. No change in speed of the wave. Why? Because V equal to F lambda. Since this frequency is constant, the wavelength is constant, then the wave speed is also constant. But okay, what is changing? Direction of wave changes. So due to why direction of wave changes because wave when passed through the small opening aperture it will spread out. Spreading that mean move outwards in all direction. So since the wave's direction changes you bear this in mind. velocity changes. Velocity of the wave changes. Okay? Because velocity depends on direction. Yeah. Now I'm going to draw you the wave. This is let's say this is a small opening. This is the larger opening. Okay. Now normally they would draw you okay wave front. Uh they would draw you wave like this. Okay. The distance this is called wave front. Distance between one wave front to the other wave front is called wavelength. So whenever you see this question use your ruler. Okay? Or you see with your naked eyes, right? So if you see that the wavelength is almost similar to the size of the gap. This is the size of the gap. I put it as a. So if the wavelength value is almost similar to the size of the gap then bear in mind maximum defraction occurs is that okay maximum spreading occur right this is how you're supposed to draw it I'm using green color so a lot of pushing spread out right the wave is more circular and then try using your naked eyes again draw back but make sure that the gap between uh the circle okay or uh circular wave is almost similar constant because wavelength cannot change after being defracted and then don't just stop there because you need to draw arrows to show that the wave is being spread out after passing through the gap. So when the gap is bigger which is larger than the wavelength. Ah this is wavelength right? Size of the gap is large not much of the spreading occur. So not much of the spreading occur. It will just curve at the edges. Wavelength remain constant like this and then just draw arrows to show that the wave is being defracted spread out. So normally drawing uh defraction for the wave they will give you five marks one mark each for the pattern of the wave. So you draw two waves two two different types of patterns and then one mark will be given for the first wave when the lambda is constant wavelength is constant and then for the second wave wavelength is constant as well four marks already and the fifth mark is going to give to the direction that you show that the wave is being spread out after being defracted. Right? Remember now critical angles and total internal reflection right critical angle the definition is refraction of the light wave. Normally we use simple experiment we use light so that we can see refraction of light at 90° on the surface when the light travels from denser medium to less dense medium. Total internal reflection is a phenomenon which the light ray is being reflected inside the denser medium when the light ray is incident from okay or the incident angle of the light ray is larger than the critical angle. Yeah. Of the denser medium. And the light must be traveling from the denser medium. Okay. Let me explain. In order to get your toal internal reflection right, you have to fulfill this condition. First one, the light must travel from denser medium to less dense medium. Water to air, glass to air, right? And the angle of incident of the light ray must be larger than the critical angle of the medium. So okay before that let me discuss a little bit about the simple experiment to determine uh to to determine the critical angle and to prove total internal reflection. Okay let us discuss a simple experiment for total internal reflection. Right. So discuss this is a title discuss a simple experiment for total internal reflection. Now I want you to see this. Let's say this is a water. This is air. This is water. This is normal. And then I need to have a laser box because I want to direct the light. So the laser box must be wrapped up nicely because you place it in the water. So you direct laser box will give out light. Why we use laser? Because laser light is the illumination or the intensity of the laser light is stronger. So we can see the result. Right? I don't know the critical angle of the water. So I start with a small angle. So that is why okay I make sure that my laser box start at small angle. Okay direct light onto okay the air. So let's say angle incident. Yeah angle of incident is smaller than the critical angle. What happens? You could see two rays only occur when the angle of incident of light is smaller than the critical angle of the water. You could see two ray one refraction of the ray the other one a fainter ray being reflected into water. So you can see two ray but if you increase let's say you keep on increasing the angle of incident uh I just ignore that laser box is that okay so I just draw you the angle of incident is increases because I want to determine the critical angle first right if the incident angle is equal to the critical angle this is air this is water right this is what you will see Oh, the light ray very nice because I conducted the experiment the light ray will be refracted at the surface of the water is so nice. This is called 90° and then when you keep on increasing yeah the angle of incident right which is larger than the critical angle incident which is larger than the critical angle all the light ray will be reflected into the water. No light ray is refracted into the air. Ah so this is the condition where the total internal reflection has already occur. Now I will make my explanation when incident angle is smaller than critical angle of water. Some of the ray is refracted. Some of the ray is reflected into water. Second. Okay. When incident angle is equal to critical angle of water. Ray will be refracted at the surface or interface interface of water and air or you say 90° not a problem and then the last point when incident angle is greater I'll highlight the key words later than critical ical angle of water all the ray are reflected into water. So we could say that total internal [Music] reflection occur occurs. Okay. Now the keywords are incident angle smaller critical angle I want to see refracted I want to see reflected incident angle equal to the critical angle refracted at 90° or interface incident angle greater than the critical angle I want to see reflected into denser medium total internal reflection has already occurred that's Now, okay, definition of some terms. Yeah, for your lens, converging lens, bear in mind, focal length, length means distance. Distance between focal point and the center of the lens. Distance, focal point and the center of the lens. Focal point is a point. Okay. Sometimes it is called principal focus. It is the point in which all the rays will be refracted to after passing through converging lens. Principal axis on the other hand is is just an imaginary horizontal line that you draw right passing through the center of the lens. So all the measurements whether the image is getting larger or smaller image is inverted or upright is depending on based on principal axis. Right? Let us see ray diagrams for converging lens. There are three rules that you have to understand. So the first rule is that any ray that is parallel to principal axis will be uh refracted towards focal point or principal ex uh or principal focus. Right? So this is your lens for example. Okay, this is what we call principal focus. Cut through the center of the lens. So since the lens has two curvature. So there are two focal point. Any ray that is parallel uh this is the center of lens parallel to the principal axis the horizontal line will be refracted at principal focus. And whenever you draw a ray diagram put arrows. Right? And the second point any ray that passes through center of lens will uh go through straight no refraction occur. So I draw like this. I draw a smaller one. Principal axis is here. Right? And then these are all your principal focuses. Right? If the ray passing through the center of the lens, it will just go through straight like this. Is that okay? And the third one, any ray that passes through principle uh I just write focal point. Is that okay? Focal point first pass because there are two focal points, right? Pass through the first focal point before the lens. Pass through focal point will be refracted parallel to principle axis. Okay, I'm going to draw that up like this. Okay, like this center of the lens. Just imagine that this is a very nicely drawn center of the lens. And then these are the two focal points. Pass through the first focal point go through straight towards the center. You can extrapolate the graph. Yeah, just draw straight line will be refracted horizontally. Uh these are the three rays that you have to remember. Three rules to draw the ray diagram for uh converging lens. Now, tada. Could you see ray number one? Any ray that is parallel to the principal axis will be refracted towards focal point. Right? Any ray that pass through the center of the lens will just go through straight and any ray that pass through the principal focus first or the focal point first will be refracted horizontally. Right? So the intersection between these three rays will give you the location of the image. So the characteristic of this image characteristic okay let me talk about characteristic this image is real why it is real image okay the one the part okay the curve which the object is facing is called the front part of the uh front part of the lens. So the other side of the lens is called the back back of the lens. So whenever the image is being formed at the back of the lens, the image is where okay like what I say this is our principal axis could you see that from the object going outwards ah so principle from the principal axis where is the image image is formed below the principal axis that is why the image is called inverted because object is formed at the top right on top of the principal axis and clearly When you measure the height of the object and compare with the height of the image, image is diminished or getting smaller. Uh the opposite to diminish is enlarged. Opposite to inverted is called upright. Ah upright. If the image is formed up here then you can call it as upright. The image of real is virtual. Image is virtual. When the image is formed on the same side as the object uh somewhere here that's virtual. Now dispersion right dispersion is the spreading of light into white light into seven colors when white light passing passes through a prism. So you use a prism to spread the white light into rainbow. That's dispersion. Now electromagnetic spectrum. So you have to memorize the sequence. Whether you memorize in terms of increasing frequency or increasing wavelength is totally up to you. Bear in mind all the electromagnetic wave travels with the speed of light. in air and vacuum. So I memorized it in terms of increasing frequency. So I'm going to write down here. First one is radio wave, um microwave, infrared, visible light, seven colors. um ultraviolet, x-rays and gamma ray, right? So this is in terms of increasing frequency. Okay, of course not just memorize in terms of increasing frequency. You need to know their function as well. What is what are the usefulness of this electromagnetic spectrum? Radio wave right is is used in radio station broadcasting radio station. Radio station you listen to the radio. Yeah, it is actually from the radio wave right. And then you could also use uh microwave. It is being used for cooking. Oh sorry radio wave. Okay. Cell phone radio station. Microwave is for cooking. And then satellite communication. Do you know what is satellite communication? Let's say you want to watch um football, right? Um football that happen in some of the country. You want to watch live, right? You can, but you just need to tune your time, right? So this is called satellite communication because it let it it allows due to the microwave you're allowed to watch live show without going flying to that country. Uh so this is satellite communication we use micro wave remember that infrared is being used in the TV remote control. Uh so when you press the TV remote control you can see red light that's infrared or you can use in toy car uh toy car toys remote control visible light maybe traffic light the light that you can see uh red green yellow ultraviolet sterilization sterilization of the medical equipment medical equipment x-ray you know used for x-ray gamma ray is used for cancer treatment. Now loudness vs pitch this is related to soundwave. Loudness is related to amplitude. Pitch is related to frequency. So amplitude increases loudness increases and vice versa. pitch increases that means uh frequency increases pitch increases and vice versa I draw a graph so if you compare right okay this is displacement yaxis is always displacement x-axis could be time right could be distance doesn't matter so I want you to compare the black wave that I'm going to draw compare with Yellow wave see this. So I will say that yellow wave has lower okay or it is softer because the yellow wave has lower amplitude. Amplitude you look at the y- axis. Pitch on the other hand okay you compare green wave. Pitch the x- axis must be time. Green wave okay amplitude is the same. Yeah. Okay. And then the other wave is maybe blue wave. Can you see blue wave has a higher pitch. Why? Because frequency is being defined as number of wave that form in 1 second. Number of complete oscillations in one second. That is frequency. If you can form more number of waves that means the frequency is higher. So the blue wave has a higher pitch. That's how you see right. Acho remember Acho is the reflection of sound wave formula. V is equal to 2 * the distance divided by time. V is the wave speed soundwave speed. Okay. D is distance and T is the time. Bear in mind why I put two times the distance. Memorize this. Because the sound wave must travel to and be reflected back to you in order for you to listen to echo. That is why two times the distance. Now move to electricity. The definition of electric charge. Yeah. Charge definition is equal to the product of current and time. formula Q8 design remember Q is the charge I is the current T is the time yeah unit coolum or capital C charge okay two charges positive and negative bear in I only negative charge can move only electron can move right. current. Okay, cancel current. Definition of current is rate of flow of charge. Same thing. Okay, I'm using different color. Current is equal to charge over time. Unit ampere or capital A. And bear in mind current flows from positive terminal to negative. Whereas electrons on a negative charge electron flows from negative to positive. Remember that DC VS AC DC is called direct current. So the definition of direct current is the current that flows in one direction only graph. I have two graphs here. So you can see that. Okay. This is current. This is time. Current and this is time. So the graph could be all the currents are positive or all the currents are negative. This is called DC. Uh DC yes direct current. AC on the other hand is called alternating current. Alternating current is the current that reverses is direction every half a cycle. Graph only one graph I have right. This is time. This is the current. So the graph up and down sometimes you get positive value of the current sometimes you get negative value of the current. So this is called AC. Now electromodifi force or emf vs potential difference or PD emf right is defined as energy supplied per unit charge in moving the charge through a complete circuit. So equation formula yes formula emf is equal to energy per charge unit volt. Okay. And the sources of EMF let me give you power supply battery cell etc. PD the definition is work done per unit charge in moving the charge from one point to another. This is called PD and formula PD or normally we put V. V is equal to work over charge. Unit is the same bolt or capital V. Sources of PD could be resistor, lamb, um, LDR, light dependent resistor, thermister, etc. Now Ohm's law Ohm's law states that current is directly proportional to potential difference. warm up. When the temperature of the conductor is constant, if the temperature is not constant, Ohm's law is no longer valid. Is that okay? So graph whether you plot IV graph or VI graph, it doesn't matter. So as long as V I is ampere start from origin it is a straight line graph. Okay from the graph I remember I told you when the graph is being given you can find two information one is the gradient the other one is the area under the graph but not valid to Ohm's law. Remember Ohm's law resistance right resistant is not the gradient of the graph you have to delete that right away. Resistance is the ratio of potential difference to current. Remember never gradient. Don't ever find gradient. So you find gradient is the same as the ratio for a straight line graph. But what if the graph is not straight but the graph is curve like the graph of filament lamb. So you have to bear in mind ratio just see the value of potential difference get the value of the current and then just divide that would do for resistance. Yeah. So Ohm's law formula V equals to IIR where V represents potential difference PD I represents current and R represents resistance unit of resistance is ohm unit of current is ampere unit of PD is V that is Ohm's law now electrical energy so I split into two parts in order to make your life easier here for your calculation. So if you see energy supply normally this is the equation that we use E = to VI T. Energy loss normally we use E = I² RT or E = V² / R * D. It doesn't matter which equation you use. Make sure that you get enough information before you use Yeah. Uh if you have enough information, you can use the first one. If you have lack of information, maybe you should consider to use the second or the third one. E represents energy. V represents PD. I represents current. T repres represents time. Time must be in second. Current ampere. PD is invoked. Energy must be in jewles. Remember all kinds energies they are measured in jewles. Right? I current R represents resistance must be in ohm. Time second. Right? Just memorize this equation. They're not being given. Power on the other hand, you can use P= to I V or power loss P= to I² R or P = V ^² / R. So I represent current like usual. P represents power. Power must be measured in what? V represents PD must be in volt. Current ampere power is what? R is called resistance. Resistance must be in ohm. That's all. Now theister theister this is a symbol of theristister. Okay. So what is the function of the thermister? Working principle. Yeah. Thermister depends theister depends on temperature. So when temperature increases increases resistance of thermister decreases opposite. Uh let me write it down. Decreases. When temperature decreases, resistance of thermister increases. Right? graph when you draw resistance against the temperature or the temperature again resistance doesn't matter okay inversely proportional right highlight the keywords temperature increase resistant decrease temperature decrease resistant will increase that's all LDR stands for light dependent resistor. So symbol so arrow to show that the light hits it. So it is made up of semiconductor cmium sulfide normally. So the working principle when uh light intensity light intensity means brightness intensity is high resistant of LDR is low. When light intensity is low, resistance of LDR is high. Graph resistance against intensity inversely proportional. Okay. So, I'm highlighting the keywords. Intensity is high, resistance low. Intensity low, resistance high. That's all. Series circuit. Series circuit. You have to imagine that we are so happy we are holding hands side by side. Series circuit is like this. Resistor one, resistor two, resistor three. They are side by side. Current has only one path to flow. Right? This is a total potential difference. Total voltage. Voltage one across resistor one, V2 across resistor 2 and V3 across resistor 3. So bear in mind in series I use red color. Current is the same. Okay. I is equal to the same I that flowing through R1 is the same current that flowing through R2 is the same current that flowing through R3. Right? But the total potential difference total voltage or the emf is equal to the sum of all the PDS across the resistor. So using Ohm's law V equal to IIR. So V total is equal to I R total I R1 + I R2 + I R3. Now my favorite part is here. Cancel all the I you left behind R total is equal to R1 + R2 + R3. Could you see that when all the resistors are connected in series, you just sum up all their resistances. So the total resistor will increase. Parallel circuit. Okay, I need to draw that. Parallel circuit is like this. Okay, behind go behind each other. Okay, R1, R2, and R3. So you see I want you to see this. This is a junction coming out. This is a current total junction. Current will split to okay uh different branches here. Junction is like like you you you are driving in a car. So junction is where the car splits. Some of the car will go to the right, some of the cars will go to the left. Right? Same goes to this circuit. Bear in mind EMF okay is the same as PDS. Yeah. So the emf is constant. Now I'm going to prove that. So since the total current yeah total current I total is equal to I1 + I2 + I3. So you use Ohm's law I is equal to V / R. So that is why you get V / R total is equal to V / R1 is plus V / R2 + V / R3. Now my favorite part cancel all the V. So you get 1 / R total is equal to 1 / R1 + 1 / R2 + 1 / R3. So bear in mind when the resistors are connected in parallel it is one divided uh by the resistance plus one divided by the other resistance plus one divided by other resistant you add up and then just invert it. Now potential divider so bear in mind potential divider can only be used in series circuit. Why series circuit? Because current has only one path to flow. What if the circuit given is in parallel? Don't worry. Just simplified it to become a series circuit. So this is a diagram of the potential divider. So series. Yeah. These two resistors are being connected uh in series. I call this as R1. I call this as R2. Okay. This is a voltmeter. I connect to R2. So I call this as a V output output voltage. This is called total voltage. Is that okay? Now okay the ratio the formula if you want to know formula we don't use current because the current passing through R1 and R2 is the same current. So I don't compare current. I use the ratio. Right? Potential divider is using the ratio. So I use I want to find the V output V across R2 divided by the ratio. Total voltage which resistor is connected to V output is R2. So over R total. So V output modified over V total is equal to R2 over R1 + R2 because R1 and R2 are being connected in series. Uh this is the ratio if you want to know. Now fuse right fuse okay I write down the symbol first this is a symbol of the fuse horizontal line cut through right the function of the fuse to protect electrical appliances so okay you want to know how the fuse protect the electrical appliances let's say I have a fuse fuse come with a fuse rating Inside the fuse that is a thin wire. Let's say the fuse rating. Okay. What is fuse rating? Fuse rating means maximum current that is allowed to flow through the fuse. Okay. Let's say the fuse rating of this fuse is 2 aere. So this fuse only allows 2 ampere to flow through it. Let's say we have a current of 2.5 aere. So if there's a current of 2.5 aere which is more than the fuse rating what happen? This wire the thin wire inside the fuse will be melted. Melted and then cut off the circuit. melted right is like switch open and the whole circuit is being cut right. So this fuse is committing suicide to save the electrical appliances. That is how it is protecting the electrical appliances. Now we move on to magnetic field. The definition of magnetic field is a region in which a moving charge remember is about moving charge because current must flow in a conductor. Current must flow meaning to say that the charge must move throughout the whole circuit. When a moving charge uh is placed in a region in which a moving charge experiences a force. Okay. Moving charge experiences a force when experiences a force that's all when placed in that region right magnetic field okay flows from north okay pole to south pole. Remember that. Okay, that's all is about magnetic field. Magnetic force bear in mind magnetic force arises due to interaction between two magnetic fields. Okay, for example, I draw let's say you have magnet two magnet opposite pole. So this is a magnetic field that flow from north to south, right? And then you have a current carries the current from A to B. As long as there is a current flows through this conductor, conductor will have magnetic field around it. The interaction between these two magnetic field gives rise to the magnetic force and the direction of the magnetic force can be determined by using your flamming's left hand rule. Fly my cat. F represents the direction of the force. First finger represents the direction of magnetic field from north to south. Second finger represents the direction of the current from positive to negative. Okay. Fleming's left hand rule and Fleming's right hand rule. When to use this? for Fleming's left hand rule. First of all, current must flows in conductor. There must be a current flows through the conductor. Got it? Flammings normally used to find force. Used to find direction of force. Okay. Can be used in DC motor normally. Fleming's right hand rule right. No current in conductor. No current flows through the conductor. Normally used to find emf induced or induced current. Direction of the induced current is the same as the direction of induced emf. Right? Same thing. Find my cat right hand. This is my right hand. north to south. Thumb represents the force right or the motion. And then this se second finger represents the direction of the induced emf or the direction of the induced current or current induced normally used in generator AC generator. Now emf induction states that okay the emf em inductions means electromagnetic induction. EMF means electromodif voltage induced is directly proportional to the change in magnetic field. As long as there's a change in the magnetic field, voltage or the current could be induced. That is electromagnetic induction. AC generator. So AC generator use split ah sleep rings not split ring sorry sleep rings commutator. Sleep rings commutator. Let me draw it. Okay. It's like this. north, south, and then I have a coil in between. Uh I might not be good in drawing. I hope you don't mind. Okay. One is uh okay to this and the other one is to the other one. And then normally we connect to maybe a light bulb here to see whether uh the light bulb is lighting up. Uh this uh this is carbon brush. Okay. Okay. The function of the carbon brush is to act as the circuit completer to connect the slip rings and the light bulb or filament. You see each and every side of the coil has its own ring. That is why you have two rings there. Right? Graph of okay let me explain. Did I explain here? Okay, I will explain here later. Yeah. How the current is being induced in the AC generator. Now I draw the graph for the AC generator. Okay. This is called slip rings. Graph of the AC generator is like this, right? Okay. You need to know how to draw. Okay. Output voltage or the induced voltage. Yeah. Okay. This is a time. Okay. Uh like that. Right. When the coil is in this position, right, it cuts maximum magnetic field. So it will induce maximum output voltage zero output voltage is when the coil is at vertical direction up and down the coil is at vertical direction because right in between the two sides of the coil is nothing it's just air so no cutting of magnetic field occur therefore emf cannot be induced let us see how the current or the PD is being induced uh in the coil okay can I draw something first is this generator Right? It is a AC generator. But you I want you to see here first of all, okay, you just imagine that this is a magnetic field. Okay, north pole. Okay, going into a solenoid or the coil. Same thing can be explained to the AC generator. Yeah, just bear in mind. And then I have V ah voltmeter. So magnet has its own magnetic field. It's just that we cannot see the magnetic field of the magnet is invisible. Remember that. So magnetic has magnetic field that flows from end pole to south pole and then when the magnet is inserted into the solenoid or the coil right. So when magnet moves towards coil. Magnetic field of magnet is being cut by coil which causes a change in magnetic field. A change in magnetic field induces current uh induces voltage or emf in coil. That is why voltmeter deflects. Is that okay? Magnetic has magnetic field and magnetic moves towards coil. magnetic field cut by coil change in magnetic field. So change in the magnetic field can induce the current or the PD voltage. Now transformer this is transformer. Okay, bear in mind. Okay, the one that is being connected to AC supply is called primary coil. The one that is being connected to maybe voltmeter or b or lamp, this is called secondary coil. Right? The core of the transformer is made of soft iron core. Why soft iron core? Because soft iron core is a magnetic material. It could right because when you conduct the experiment in the air some of the magnetic field that flow out from the primary coil could be right attracted to other magnetic fields nearby. So that is why soft iron iron attract back the magnetic field to make sure that the magnetic field from the primary coil will go to the secondary coil. Right? But the coil is made from copper. Why? Because copper is a very good conductor. Copper has a very high conductivity, right? It can conduct electricity, less heat generated, right? Now, let me explain the working principle of the transformer. AC, okay? Uh produces varying magnetic field. Okay. Okay. Varying magnetic field. from primary coil. Mary coil is being cut by secondary coil which causes a change in magnetic field. Therefore, emf is induced in secondary coil. So if you talk about the light bulb, light bulb will light up. So the formula for transformer NS over NP is equal to VS over VP. Remember NS is the number of coil in the secondary secondary coil. Number of turns, right, in the secondary coil. NP is the number of turns in the primary coil. VS is the secondary voltage or VP is the primary voltage. So one more thing if transformer uh if the question mentioned transformer is 100% perfect right efficient then you do this. Okay, bear this in mind. Total input power of the transformer is equal to total output power. Total input power is I primary V primary power is I VI is equal to total output I secondary V secondary. Yeah. Current secondary current and secondary voltage. That's all. Now alpha particle scattering experiment there are three results that you have to explain. First one, most use the word. Yeah, key words. Most of alpha particles pass through gold boil. Okay. Undelected. I will give the reason. These are just the result. I will give reason at the back. Right. The second point sum of alpha particles were being deflected at angles, different angles. And the third result, a few alpha particles bounce back to original half. Right? So you have to talk about okay most some a few right. So reason let me give you the reason why most of the alpha particles just pass through undelected right it is because atom is mostly empty space right there's nothing this just vacuum right so that is why most of the alpha particle just passed through undelected some of the alpha particles were being deflected at angles why because nucleus is positively charged. Okay. And okay, alpha particles are positively charged. Light charges repel. So when the alpha when alpha particles Come closer to nucleus more okay deflection occur number uh the third one okay why a few alpha particles bound back bounce back to original path that means reflect back right it is because nucleus is very small and dense. You have to mention dense. Yeah. Because alpha particle is a big particle, right? If the nucleus is less dense as compared to alpha particle, then the nucleus will be bounced off, right? uh alpha particles that have head on head to head head on collision with nucleus it uh okay were reflected at 180° go back to their original path now isotopes this is a little bit like chemistry Isotopes means the nucleides, okay? Which have the same proton number but different neutron number or nucleon number. It's not a problem. And bear in mind isotope exhibit the same chemical properties. Why? Because they have the same number of electrons. It is the number of electrons that gives rise to the chemical properties that give the result to the chemical properties. But doesn't means that the isotopes have the same physical properties. Maybe this isotope is solid. The other isotope of eight is liquid. Uh that is what we call physical property. Nuclear fusion and nuclear fusion, right? Fusion means that okay, two light nuclei combined to form a larger and more stable. Okay. uh nucleus by releasing energy. Example, formation of stars, right? Nuclear fusion, right? A heavy unstable nucleus split into smaller fragments by releasing energy. example the one that we use in nuclear power our station right to generate electricity and the energy is quite promising h now alpha beta and gamma decay bear in mind alpha is called helium nucleus so 42 H if you remember right alpha decay that means let's say a zx a repres represents nuclear number. Z represents proton number. If this element, this nucleide, right, under goes alpha decay, then you take the nuclear number minus 4 uh no division, sorry, nucleon number a -4, z - 2, right? Uh you form another element plus alpha. That's it. Right? Beta is called fastm moving electron fast moving electron 01 uh beta right so if this nuclei under go beta minus 0 so you or you just put a here doesn't matter a and then z + one yeah this is y + 0 -1 beta because the nuclear number on the left hand side must be balanced by the nuclear number on the right hand side. Proton number on the left hand side must be balanced by the proton numbers on the right hand side. The sum of the proton numbers on the right hand side. Gamma is called electro magnetic okay wave. So a z okay gamma you still form the same one but it is different element due to the energy release. Oh this is the symbol of gamma by the way. Now halflife definition is the time taken for a radio active nucide to decay to half of its original activity or original count rate doesn't matter or original number right it is I need you to I need to see the time decay to half original value right the unit of half life could be in second in days in years in weeks right in minutes etc it is a time it is a unit of time okay now let us move on to the last topic here space physics X right it is newly introduced in 2023. So planet this is just like general knowledge it is a large object orbiting a star clearing orbit and nearly spherical in shape. So when we do calculation of the orbit of a planet we always assume it to be circular orbit right to simplify our calculation. Satellite it is an object in orbit around a planet. It may be natural like the natural satellite of earth is moon or artificial GPS satellite right. Asteroids and meteoroids are the rocky objects which is smaller than the planets cannot be considered as a planet and most of the asteroids are found in the asteroid belt between the orbits of Mars and Jupiter. So faces of the moon you have to memorize. Right? Right. New moon especially the ones that are highlighted. Right. So here I'm using yellow color. New moon, right? New moon is when moon is between the sun and the earth. Full moon is when the earth right is between the sun and the moon. Memorize this right. Waning um I think waxing crescent are more popular. Waxing crescent. Memorize right. Okay. So and then wax waxing gibbers are also okay. vaccine uh vaccine gibbus is also very popular right so uh if you have time might as well memorize all because we don't know whether they will give you third quarter first quarter waning crescent waning gibbus right uh so might as well memorize all but new moon and full moon are very very important right so one light year one lightyear the definition is the distance that is being traveled by the light in one year. One year you have 365 days. Remember, right? You have to convert to second, right? How to convert to second? 1 day 365 days. 1 day you have 24 hours. 1 hour 60 minute. 1 minute 60 seconds. Remember that if they want it in second, if the speed of light, okay, uh so is given. Okay. Normally we use okay speed of light is equal to distance over time. So one light year is distance. Speed of light time time. Speed of light is normally 3 * 10^ 8 m/s. So that is why you have to convert this 24 let me do it 3 * 10 ^ 8 * 365 * 24 * so one light year is about 9.46 4 6 * 10 ^ of 15 m. Now life cycle of a star. So life cycle of a star you have to consider whether this is a small star or mediumsiz star like our sun or this is a giant star or the super giant star. Right? So if it is a small mediumsiz star like our sun you have to talk about nebula. So what is nebula? Nebula is a cloud right? a large cloud of dust and gas, right? So they come together due to the gravitational force pulling them together, right? Over a certain period of time, of course, it would take million years, it could form protoar. Protoar is like a large globe. You have a fixed shape already. The star has already has a fixed shape. The problem is that okay, shape is not a problem. The problem is that there is no process of fusion occur in the protostar right. So that is why okay that is protoar and then keep on attracting okay because why no process of fusion occur in the protostar because the temperature and the pressure is not high enough. Temperature and the pressure are not high enough to carry out the process of fusion. So of course okay keep on attracting all the particles all the dust in the universe eventually form a main sequin or we call it as stable star. Main sequent can also be called stable star right ah stable star why it is called stable star this is very important right because the process of fusion right fusion remember just now I define fusion I say fusion give out a large amount of energy outwards right outward pressure outwards okay pressure by fusion is balanced by inward pressure due to gravity gravity uh gravitational force right you know in the process of fusion eventually runs out because the hydrogen is the one that carry out the process of fusion to form helium but you know nothing lasts forever hydrogen eventually runs out the process of fusion cannot occur so that is why the only force that acts on the star after main sequence Right? It is the gravitational force. So this gravitational force right will pull the star inwards. Ah so the gravitational force will pull the star inwards. The star become smaller. So get hotter. Now the process of fusion can occur. But due to the lack of hydrogen, the process of fusion will carry out by the other particles such as helium to form a larger element. Uh which we call it as red giant and after red giant the ending of the star it would turn into white dwarf right. Uh so why we call it as white dwarf it because it gives out a lot of light because if initially they are small size star so eventually they become white dwarf and then they give out a lot of lights. Uh that is why it is in terms of white but eventually once they run out of light they would turn black. Uh we call it as black dwarf right and okay life cycle of a big or super massive star start from main sequence form a red super giant right red super giant is super super large and supernova will occur supernova is the explosion right uh explosion of the big red giant super giant star that give out a large amount of energy across the universe right uh so the core of the supernova. After that the core could form neutron star or black hole depending on the size of the star initially. If it is super duper massive then it will form black hole. Black hole has a very large gravitational force basically if the light passing through it will be able to attract the light and nothing can escape from it. Yeah. So Hubbers's law states that the speed at which the galaxy receding receding that mean galaxies are moving away they're directly proportional to the distance between the galaxy and the earth. Right? Her constant is the constant of proportionality due to the Hubbers's law. Hubbers's law say the speed of the galaxy moving away is directly proportional to the distance between the galaxy and earth. Right? To get rid of this proportionality put H no harbor constant right harbor constant will give you harbor constant is equal to speed of galaxy divided by the distance. So it gives the rate of expansion of the universe. Right? Rate shift. Rate shift is the increase in the wavelength. Red color has the longest wavelength which is between 600 nanometer to 800 nanometer. So that is why you could see night time. You try this under the night sky. Look at the star. Wow, that's romantic, right? Look at the star. Do you see the color of the star is close to yellow color, orange color that is close to red region, right? Because the stars are moving away from us. Red shift is one of the solid evidence to the big bang theory. So red shift is the increase in wavelength shift towards the red end of the spectrum of light from the galaxies that are moving away from us. Right? They will give out light in terms of red color indicating expansion of the universe. Big bang theory, right? CNBR. CNBR stands for cosmic microwave background radiation. So what is CNBR? CNBR is a weak radiation detected in all direction uniformly. Right? It is actually considered leftover energy from the big bang. So it is the key evidence right for the big bang theory as well. So explosion supernova occur give out radiation that is very uniform in all direction in terms of microwave region. Okay. So that is all about this final boost for your coming IGCSE physics paper for. So bear in mind, you are ready to show the examiner. Tell yourself, I am not just ready, but I'm also so solid. Let's finish strong. You got this. If you found this useful, drop a like, subscribe for more physics help, and I see you in the next one, and good luck. Very, very best of luck in your coming IGCSE physics paper form.