Transcript for:
Electricity Fundamentals

let's get shocked shall we electricity now unit four is split into two parts so electricity and magnetism today we're just focusing on electricity and overall this unit is about charges so what's the charge we discussed the charge in the structure of an atom how like protons and neutrons and electrons protons electrons have positive and negative charge we'll deal with that again so we split this into static electricity portions so what's a charge and what's an electric field that's a very minor topic then we talk about electric circuits we'll start by talking about the properties of a circuit so what quantities do we measure voltage and current and resistance and power and such and then we go into series and parallel connections and multiple components and so on once we're done with that we'll just finish things up by talking about electrical safety because you know electricity can kill you and we do want to keep you safe at least you should know how to keep yourself safe let's start uh before we continue the same friendly reminder this checklist this list of topics uh is from the curriculum i want you to use this as a checklist to see if you understand the topic or not so for example if you say that state that there are positive and negative charges and you understand that I'm sure you do uh you're fine if not you need to revise that topic again okay let's start with static electricity first when we talk about electricity we have to talk about the electric charge a charge is a property of matter that experiences forces near other charges what do I mean this means that all matter in the universe has charge whether it's a proton neutron electron matter me you everything is charged but there are two different types of charges there are positive charges and there are negative charges and they're opposites things can either be positive or negative or neutral neutral objects are neutral because they have both positive and negative charges together so they have a resultant of zero charge charge is measured in a unit called kulum now I want to focus on this for a second the charge of one proton is 1.6 10 ^ of -19 kum and the charge of one electron is 1.610 6 10 the^ of9 kum you don't have to memorize these please don't okay but what I mean is each proton or electron like subatomic particle isn't one charge isn't one kum of charge it's actually like a fraction of a charge cuz later on when I mention something like hey uh the charge is going to split it doesn't mean that we physically break a charge if you have one kum you actually have millions of tiny little particles stuck together and yeah sure these can split and travel 0.1.0 can travel here 0.6 can travel here depends so later on when we say half an ampere 2 aair current splits this is why it can split okay now the relationship between charges is very basic and I'm sure you know like charges repel so positive and positive repel negative and negative repel opposites attract positive and negative attract however neutral objects can also be attracted to charged objects whether it's positive or negative why because inside a neutral object we said you have both positive and negative charge and these rearrange so that the positive is closer so attraction is a bit stronger and the same thing happens here if you have a negatively charged particle next to something neutral the positive and negative rearrange so the positive is closer and the attraction is stronger than the repulsion regardless anything neutral is attracted to anything charged yeah when it comes to materials we have two types of materials good conductors of electricity and bad conductors of electricity which we also call insulators conductors are good conductors because they have freemoving electrons we sometimes call them deoized electrons or free moving ions so it's electrons that are free to move you should know that all metals are freeing electrons so copper iron aluminium you know constant tan nickel whatever it is insulators do not conduct because they do not have free moving electrons somebody forgot the letter T here when typing this sheet aka me well do not have free moving electrons uh things like rubber and plastic and glass and wood you should be familiar with different types of insulators however even though insulators cannot conduct electricity they can be charged like you can make them become charged by rubbing them with another insulator so for example if you rub a metal rod with a cloth a wooden cloth for example a plastic rod not a metal rod a plastic rod with a wooden cloth the friction between the rubbing of the cloth and the plastic rod causes some electrons to be transferred from one object to another the cloth could lose electrons and give it to the rod or vice versa this depends on the materials and we're not going to study which materials lose or gain but you do need to know that if an object loses electrons it becomes positively charged if an object gains electrons it becomes negatively charged okay good right you cannot do this with conductors by the way and this is something that was removed from the syllabus in 2023 uh how do we charge conductors and it's a method called induction but it's not part of our curriculum anymore okay so keep that in mind next what is an electric field now the word field in general means a region a space where something experiences a force so electric field is a region where charges experience a force okay what is the direction of an electric field and how do we draw it we draw the direction of electric field using straight lines coming out of a positively charged point so it's always out of a point or if it's a sphere it's always out of the positively charged sphere or if the point is negative it's drawn into it question is why like why do we draw the field lines like that that's because if I zoom in and I tell you hey if you decide to put a positively charged particle here what's going to happen to it gets repelled what about a positively charged particle here gets repelled so each of these lines shows you the direction of the force on a positive charge at that point which means if you put that same positively charged particle here next to the negative uh point charge it gets attracted right it gets attracted you might say that what what about electrons or like negative charges shouldn't we draw those like we don't have to we know that positive and negative are always opposite to each other so if you have an negatively charged particle here it'll be attracted or here it'll be repelled which means negative charges always move opposite to the direction of the field okay so when I ask you to draw the field lines you always draw them going out of a positive charge or into a negative charge out of a positive or into a negative now one more feature here the space between the lines shows you how strong a field is so if you take a look at this field diagram this is the same field but now it's a metal sphere that's possibly charged or maybe it's a plastic sphere we don't care but the closer the lines are together the stronger the field is and the farther away the lines are from each other the weaker the field is which makes a lot of sense if you think about it because the field has to be stronger near the charge and weaker far away from the charge so if I put two charges one here and one here and I ask you which one experiences more force like they both get repelled but which one experiences more force b experiences more force cuz A is further away and the field here is weaker finally if you put a positive charge in front of a negative charge a plate in front of a plate you end up with an electric field that is equally spaced almost at least this type of field is called a uniform field and we always draw the field lines from positive to negative right a uniform field means the strength of the field is constant everywhere in that space now let me draw one more diagram something that sometimes shows up not always but then imagine that you have a positively charged sphere and a negatively charged sphere and I ask you to draw the field line between them so you draw lines from positive to negative but because they're spheres the field still gets weaker as you move further away so out of the positive into the negative so it's as if I try to join these two fields together sure you can see it now but the more interesting diagram is what if I have a positive charge next to a positive charge sure they should repel but they don't just physically repel even the fields repel so the field lines don't even touch each other out of the positive out of the positive do you know what that means this means that if I zoom in here and I put something perfectly in the middle between them it doesn't move if I put a positive charge exactly in the middle it doesn't move because this space is a melt space there's no field here zero free field okay now let's solve a couple of questions figure 8.1 shows a negatively charged metal sphere draw four lines you just want four to show its electric field and its direction uh let me pull out this line tool and let's draw while I draw I'll answer a question what if I put a negative charge exactly in the middle in that space in that null space nothing as well nothing moves you might say "But it gets attracted." Yeah to which side you don't know if it's perfectly in the middle now because this is a negatively charged sphere you draw the lines going into it piece of advice if I draw a sphere do not draw the field lines going inside the sphere you should draw nothing in here but if this was just a negatively charged dot sometimes called a point charge and I ask you for the same thing yeah you can draw lines exactly through it and it'll look exactly the same okay so be careful if it's a sphere nothing inside the sphere if it's a point charge yeah just go through it that's fine a plastic rod is rubbed with a cloth the rod and cloth becomes charged as electrons move between them the rod becomes negative which diagram shows how the rod becomes negatively charged and shows the final charge on the cloth all right uh now if the rod becomes negative this means it has gained electrons it has gained electrons which means the cloth becomes positive so this gains electrons and this becomes negative and this becomes positive so my choice is B electrons move from the cloth to the rod and this becomes negative this becomes positive easy all right i have another question do we draw arrows or just Oh yeah absolutely when it comes to field lines you must draw arrows otherwise why did we go over explaining drawing the lines because the lines show you the direction of the force and force is a vector and fields are vectors by the way cuz they have direction you have to show the direction with arrows lines going into a negative or out of a positive if it's a positive charge okay you mean we draw four lines or do you need to do more i mean I'm sorry but you're given instructions listen to them he said four lines the examiner said four lines i draw four lines come on all right next up current electricity another giant checklist because it's full of tiny tiny little dots i'm not going to read the full checklist i want to go straight into current and voltage and resistance before I do however before I talk about current or voltage or resistance let's talk about circuits in general the purpose of any electric circuit like this one is to deliver electricity a battery is a source of energy it has chemical energy inside or maybe this is an electric socket so it's just electrical energy direct the point is to deliver the electrical energy from the source of the energy that you have to my target that needs that source in this case it's what it's the light bulb so electric charges travel through the circuit to give that thing energy and it leaves but in order to do so there needs to be a force acting on it for the time being and I'm going to change my mind very quickly now i'm going to assume that positive charges are moving inside a circuit now in any circuit symbol before I move on as well the long line of the battery symbol is positive the short line is negative all right so when I close a switch a switch needs to be closed in order for the circuit to work the term open means the circuit's not going to work the term closed means the circuit is now going to work cuz now it can conduct electricity now the moment I don't know why it's I want to choose the entire thing please thank you there we go the moment you close the switch a positive charge gets pushed out of the positive side of the battery zoom moves through because it gets repelled now because it was pushed out it now has energy it goes through the light bulb which is a target which takes the energy away because it has resistance it slows it down slows down the charge and takes the energy away the charge is still positive but it leaves without energy and goes back to the negative side of the battery to essentially recharge again get some more energy gets pushed out and you end up with a sequence and you end up with a sequence a current that flows so when I say what is a current and I ask you what is a current you say it is the rate of flow of charge or the amount of charge that passes through a point per unit time now charge is measured or sorry charge is measured in kum current is measured in ampere where 1 ampere means 1 kum per second so there's one charge that goes through the circuit per second the equation that defines electric current is I = Q / T where I is current now the reason we use I for current is because I is currently short for electric current intensity so they got the I from intensity q is for charge i don't know why they chose Q so many things in physics use the letter Q and T is for time so current is just how many charges go through per second we can measure current using an ammeter and a meters must always be placed in series okay now there are two types of currents that we have in electricity what we call DC and AC dc is short for direct current okay not the comic books and AC is alternating current well I I like DC more than Marvel though that's my personal preference not the movies the heroes so what do I mean by DC and AC dc is a current that has a fixed direction which means the source has a positive and negative side that are fixed and the value of the current or the voltage is fixed so the current flows from positive to negative and it has a fixed value that's called DC ac however comes from a generator or your electric sockets which initially come from a generator in a inside a power station okay good the thing is the reason it's called AC or an alternating current is because the positive and negative sides of the terminals are never fixed this side could be positive and this side could be negative and in the next 0.2 seconds so if we have like a 50 Hz AC supply this could be positive and this could be negative which means for 0.2 seconds the current flows this way zoom and for the next 0.2 seconds the current flows this way and then zoom and zam zoom and zam and backs and back and forth back and forth back and forth it's very confusing if I plot a graph of how the voltage or the current changes over time it looks like this positive negative the negative sign simply means that the direction has changed it's in the opposite direction it looks like a wave sine wave okay now finally remember how I said there's a mistake that was made back when I described the circuit earlier i said a positive charge travels from the positive side of the battery to the negative side of the battery correct yes that's physically not true because earlier in static electricity we said conductors are good conductors metals are good conductors because they have free moving electrons and electrons are negatively charged so this thing of us saying "Hey the currents flowing from positive to negative is hogwash." But it's stuck why a very long time ago scientists did not know what was free to move inside a metal like they knew charges existed they knew they could be positive or negative but they didn't even know about protons and electrons yet so they decided that charges need to flow from positive to negative why because it's easy come on like positive negative it's it's nice high to low and then decades passed by until finally some other you know good scientists discovered hey guys uh what's actually free to move inside a metal is an electron not a proton not a positive charge but all our rules in electricity are fine they work just fine so what do we do what we did was we stuck to the concept of positive to negative we just called it the conventional current which is the flow from positive to negative and if somebody asks what are the actual particles that flow inside a circuit we go ahead and we you know submit and say oh yeah we're sorry it's actually electrons that travel from negative to positive but we don't call it a current because we don't want to be confused we just call it the flow of electrons or electron flow so in all of our circuits if I ever discuss a current it is always from positive to negative we never use negative to positive unless I specifically ask you about electrons okay that's quantity number one let's go to quantity number two remember circuit delivers energy the flow of those charges is called current the energy that we deliver is called voltage voltage in general simply means the amount of energy or work done per unit charge so voltage is the work done or energy transferred per unit charge now what do I mean by that uh I was just changing the battery for one of my clocks here at home it's a nice small you know Energizer battery i remember a long time ago when Energizer had these really crazy ads with the rabbit and stuff anyway and uh it has a positive and negative sign and written in very small font this is like a a 1.5V what is this if I tell you that a battery has a voltage of 1.5 volts this means that each charge that leaves the battery carries with it 1 and a2 jewels if this is one kum it carries with it one and a half jewels remember the charge delivers energy the charge is not the energy itself this means that each and every single charge will carry one and a half jewels with it this is the capacity of the battery this is how much it can give each charge that goes through it has nothing to do with the maximum amount of energy inside the battery because I can buy another like I have a very old radio i like to use it all right uh it gives me like nice old vibes especially when I don't want to use the internet but these batteries the battery uh the radio uses are huge a big chunky almost like toss at somebody's head and you know give them a fracture kind of big called D batteries or DD I don't remember but it's also 1.5 volts what's the difference they will both give the same amount of energy per charge the difference is how much it lasts so that that changes sure but voltage doesn't specify that voltage just tells you how much energy does each charge take and deliver so when these charges go to the light bulb they give the light bulb 1.5 jewles each okay yeah there are actually two different terms that we use for voltage we have emf which is short for electromotive force and PD which is short for potential difference they both mean energy per charge but the difference is emf is the work done or the energy transferred by a source in moving a unit charge around the whole circuit is the energy per charge of a source because batteries give the charge energy potential difference is the work done per unit charge or the energy per charge lost through a component because the light bulb takes the energy away from the charge so that's why we need two different names for the same quantity because it's either energy gained by the charge so we give the charges energy or we take the energy away from the charge but they both have the same symbol as far as we're concerned it's just V from now on okay sometimes use E for emf but regardless V is fine the unit of measurement is called volt just V as well and we measure it using a voltmeter a voltmeter is this device over here and it must always be connected in parallel meaning the wires have to be placed before and after the thing that you're trying to measure the voltage across okay finally back when we talked about the charge we said that you know this little dinky little charge moves through the circuit blah blah blah blah blah and then it has to slow down it goes through the light bulb and that causes it to give the light bulb energy the wires however are so good at conducting electricity the charges do not lose energy through it so what's the deal the deal is in the final property called resistance what do I mean by resistance resistance is a property of matter that all conductors have they don't it doesn't have a definition you're not going to define it yourself but know this everything has resistance some materials that are really good conductors have very low resistance and if the resistance is very low the current is very high the charges can zoom quickly but something like a light bulb has high resistance which causes the current to what to decrease to move slowly i want you to think of resistance like speed bumps on the road if a car is traveling you know through a road and forgive me this is a beautiful car i know futuristic but if there are speed bumps it slows down a bit the more resistance a component has the bigger the speed bump so it essentially crashes into the wall you lose a lot of energy to go through that speed bump okay so more resistance lower current less resistance higher current the unit of measurement of resistance is called ohm and then we have Ohm's law which is arguably the most important equation in electricity r= V / I or my personal version favorite version of it or rearrangement of it is V= IR all right memorize this it's going to be very important yeah when it comes to the relationship between V and I and R if you take a look I is equal to V / R you need to understand something if you increase the voltage the current increases as well so if this increases this increases makes sense bigger battery faster charges if you increase or decrease sorry the resistance the current also increases because less speed bumps means faster motion or movement so current and resistance are inversely proportional but current and voltage are directly proportional voltage and current right now have no relationship because voltage is something that you decide i put in a battery a 1.5 volt battery i decide to put in a 10 ohm light bulb so just me putting in a bigger battery doesn't change the light bulb both of these are factors that you control and they affect who the current keep that in mind voltage and current resistance don't affect each other at least in simple circuits with just one component for now now let's talk about resistance again resistance is affected by a few things temperature for one but we'll discuss temperature in the next section we're assuming that temperature is constant the second would be the length of a wire and the third would be the cross-sectional area of a wire what do I mean i mean that if I have a wire of a certain length L let's say that's I don't know 10 cm or so and you double it the resistance also doubles the resistance also doubles because the longer the wire the obviously the more the charges have to go through that slow resistance same thing here about with area if this is the cross-sectional area of your wire it's very small you think about it it barely allows a charge to move through but if you have something that's twice as large hey it doesn't just let one charge through it can let two charges through three charges through like it depends on how thick it is so the thicker the wire is the larger the area the less resistance it has it's inversely proportional the smaller the area the thinner the wire the larger the resistance okie dokie very nice by the way I like to write this down as uh just just as for me when I practice r is equal to l / a i know it should be proportional but I just like to do this so I remember that oh yeah r and l are directly proportional r and a are inverse yeah a trick that is very often used in exams is the concept of diameter what do I mean let's let's assume for a second here in this example that's already been solved that you have a wire of a cross-sectional area A and you double the area what happens to the resistance it decreases by a factor of two it gets halfed so if the resistance was 20 now it's going to be 10 because 20 / 2 is 10 cool but if I repeat the same question again and I say "Hey you have a wire of area A and diameter D and resistance of 20 ohms." And then you double the diameter not double the area you double the diameter what's the value of the resistance it's not going to be 10 it's going to be 5 ohms wait how did I get that remember that the area of a circle is pir 2 or pi d^2 over 4 so when you double the diameter so when the diameter is doubled technically the area isn't doubled the area gets doubled squared because you didn't change the area directly you changed the radius or you changed the diameter and that has a square factor in there which means the area is not two times larger it's four times larger so the resistance is not two times smaller it's four times smaller so 20 / 4 gives you what 5 ohms this is a very evil trick like when an examiner wants to make a question harder like if they just want to raise the difficulty a bit they use the word diameter instead of area keep that in mind how do I find the resistance of any component i might ask you to describe an experiment to do so so it's a very simple experiment you get your component whatever it is and you connect it to a supply now this could be a variable supply like the supply itself can change or you can have a variable resistor in there to change the voltage or the current and you have an emter and a voltmeter and you say oh yeah to find resistance I need to measure a uh sorry measure V which is the voltage measure I which is the current and I can calculate resistance using R= V over I perfect measure voltage measure current calculate the resistance but you know what in physics we We don't like to do things once we like to do things multiple times so what we do is we change the voltage like I said sometimes by changing the value of the supply itself changing the value of the supply itself and second right and second you can change it using a variable resistor assume for now that you're changing the voltage directly like you have a set of batteries or like a variable battery that you can click new values into and you start with 0 volt so you get 0 ampere you can't really calculate anything there but at 1 volt you get half an ampere now you get the resistance which is two and then we repeat the experiment several times you change the voltage to two volts that gives you more current you change it to 3 volts that gives you more current you change it to four that gives you more current and so on and you get several values now why do we do this we do this number one so we can get an average because it makes sure my results are reliable and accurate and second what we can also do is plot a graph of current against voltage with voltage on the x axis because it's the primary thing I'm changing and currents on the y axis if you plot these points and it's a resistor like a fixed resistor you should get a nice straight line going through the slope of this line or like the inverse of the slope of this line shows you what happens to the resistance if the slope is constant the resistance is also constant now I want to go over IV graphs of different components meaning we we put a component in that circuit in this circuit that you see right now over here over here and we change the voltage and we get lots of readings when we plot a graph there are three components that you need to remember one the fixed resistor which is just a component with resistance something has fixed resistance if you plot an IV graph it ends up being a straight line which means the resistance is what constant all right but if you put a lamp like that first light bulb that we put up there a filament lamp and you start increasing the voltage you'll notice that the current does increase like let's zoom in a bit let's zoom in a bit you'll notice that the current does increase but it doesn't stay constant for long it actually starts decreasing in slope the question is why why does it like drop down droop down the slope decreases because the higher the voltage the brighter the light bulb gets but light bulbs don't just get brighter they also get hotter so the temperature of the light bulb also increases if a metal heats up its resistance also increases which means what do you think should happen to the current it should decrease but remember you're increasing the voltage so the voltage should increase the current but the resistance should decrease the current so the slope just ends up decreasing instead because the hotter it gets the higher the resistance and therefore the lower the current gets okay if I ask you to sketch this you should sketch this from memory if I ask you to explain why this line slopes this way oh yeah it's because as the voltage increases the light bulb gets brighter and as it gets brighter the temperature increases and as the temperature increases you know the current drops then finally we have a diode i'm kind of spoiling like the next section but a diode is a component that allows the current to only flow in one direction so current always flows in the direction of the arrow of the diode but if the current tries to flow in the opposite direction the diode stops it so it's like a one-way gate current goes right but can't go left so if I plot an IV graph for a diode you'll notice that just a little bit for a small voltage it doesn't conduct because diodes are not made of metals they're made of what we call semiconductors and it's a specific type of semiconductor that needs a little bit of extra voltage to start conducting this voltage is around 0.5 to 0.7 it depends on the material it's a tiny thing but the moment the voltage crosses that value it shoots up like boom it has almost no resistance almost zero which is why it doesn't even have a slope no no no it's almost vertical but we do give it some slope so we can see what's going on the the important feature here is what why is this line horizontal on the other side now we didn't focus on that in the previous graphs because they were all symmetrical but the left side of the graph shows you negative values of voltage and negative values of current negative simply means we changed direction we changed direction all right and obviously in a diode if you change the direction it does not let the current through so the current stays zero that's basically it nothing more okay you need to memorize these all three you should sketch them right away and identify which component the graph is trying to you know show you final quantity something that we call power now this is honestly from unit one so very old quantity not not not nothing new it's just old so what is power it's the energy per unit time all right but here's the thing though in a circuit in a circuit a source transfers chemical energy into electrical energy right but then in a component like a light bulb for example it takes the electrical energy and turns it into heat or light a resistor for example just gives out heat a light bulb primarily gives out light a speaker gives out sound and so on so please note that in any cell or battery for example you change chemical energy to electrical but in a resistor you change electrical energy to heat in a light bulb you change electrical energy to light now in electricity because we only measure voltage and current the equation to calculate power is not P= E /T it's actually P= IV or V * I voltage time current okay but what if I want energy what if I want energy now energy is power times time back from unit one and power in electricity is IV or VI time which means energy in electricity can be calculated using VIT or IBT okay i have a very bad pun i'm holding back do you guys want to know the pun oh well I'll hold back i'll hold back yes uh why is it that when you're feeling down or tired doctors always want to give you vitamins because vit gives you energy anyway I'm sorry don't log off now okay let's keep going let's keep going i'm sorry now we have other versions of this power equations and we have other versions p= I 2 R people are dying now and P= V ^2 / R now where did these come from p= I^2 R is actually P= VI and I replaced the V with IIR so you end up with I I R which is I^2 R p= V ^2 / R is actually P= VI again but I = V / R back from Ohm's law it's the same thing so I replaced the I with V / R and I end up with VV / R which is V ^2 / R these formulas are useful if I give you I and R and ask you to get P right away but honestly if you forget them that's fine because if you haven't noticed both of them use the same two formulas P= VI and V= IIR if you memorize these two if you memorize these two you can solve any formula involving I and R or I and V and V and I and P and R and whatever okay however this version of the formula I^2 R will be very useful later in magnetism when we talk about power lost or dissipated as heat in a transformer or when we transmit electricity now let's talk about something new essentially we haven't seen this a lot before because it wasn't there for a long time in the syllabus what is a kilowatt hour now you're like "What i've never seen that." A kilowatt hour is a unit of energy that we use at home it's the amount of energy consumed by 1,000 watts of power in 1 hour what do I mean by that uh let's say I open up my application to pay my you know electricity bill this month and I pull it up i was like "Oh what do you mean I need to pay £1,000?" It's like it's not like God but let's say I have to pay £1,000 what how does the electricity company measure the electrical energy that I use it has that electricity meter right like right next to your door somewhere near your apartment uh but an electricity meter doesn't measure energy in jewels because jewels is actually a very small unit of energy we measure it in kilowatt hours meaning how much energy would 1,00 watts of power like a kettle for example my kettle here at home has a power rating of 1,000 watts my vacuum cleaner is 1,500 watts all right every device has a certain wattage my charger my phone charger is like 20 watts sometimes 30 watts depends on if it's a high-speed charger or not and then I just multiply it by 1 hour because isn't energy power times time kilowatt is power hour is time so you end up with kilowatt hours because the values of energy that we consume at home are super duper large we can't use jewels okay so what is a kilowatt hour it is how much energy is used by 1 kilowatt or 1000 watt in 1 hour how do I get the value in jewels like if I tell you uh this month I used up 37 kilowatt hours for example that's that's nothing but let's say I used 37 kilowatt hours how do I change that to jewels remember energy is power times time this is 37 * kilo means times 1,000 because that's the kilo times what's 1 hour in seconds 60 * 60 this gives my me the answer in what in jewles let me work it out so 37 * 1 * 60 * 60 it's a giant value so standard form 1.33 3 10 ^ of 8 the 3,600 in this uh revision sheet is the time of 1 hour but time 60* 60 this is extremely new to the syllabus we've never used this before so and even in 2023 up to 2024 now like this year is 2025 uh we haven't seen it much yet like maybe once okay so we're going to see that now okay now let's solve a few questions define potential difference potential difference or voltage or emf it doesn't matter it's the same thing it's the amount of energy transferred or work done per unit charge but add one little extra bit if it's emf it's by a source to drive a charge per charge by a source through the whole circuit if it's potential difference it's work done per unit charge through a component or lost through a component all right sometimes these definitions want you to mention the energy change that has happened by the way it honestly depends on how many marks the question is but also wants you to mention the energy change so here's the question potential difference is the energy change from what to what and you don't have to be specific i'm looking for in general it's the energy change from electrical to something else like thermal or light or something so keep in mind if I ask you to define potential difference and I ask you to also include in terms of energy explain it in terms of energy you have to mention the energy types from electric to other forms okay next state the equation which defines emf so what's emf e equ= W / Q or you can say work per unit charge just write it down as an equation next emf of a battery is 9 volts the battery is in a circuit calculate the work done by the battery when it moves a charge of 30 kum so this one's easy work is energy or sorry emf time cube to 9 volts time 30 kum which gives me 270 yes 270 this one's easy direct to the point moving on an electric heater uses a resistance wire of resistance 26 ohms the power dissipated meaning lost so power dissipated means power lost in the resistance wire is what wait hate it when notification just pops up okay the power dissipate in resistance wire is 2,500 W calculate the current in the resistance wire so how do I calculate current look at what you have do not automatically go for V equals IR i have power and I have resistance and I want current short way is to use P = I 2 R which means I 2 is equal to P / R and then I have to square root my answer so square 2500 over 26 which gives you 9 81 hey almost like the value of G right gravity does or if you forget this version you can say oh yeah P equals uh VI so I equals P over V but I don't have V so first you get V= IIR separately right oh sorry my bad want the current right what do we have power and resistance so what can I get say P= I V or I= P over V you have power but you don't have this so you say V equ= IIR now you don't have I M you don't have I but you can say that I * 26 is your V and plug that in here so it's 2500 over I * 26 which gives you I^2 = 2500 over 26 you can do that or you can memorize all the equations and choose the easiest one so 9.81 ampere next the resistance of a wire the resistance wire of a heater has a length of 1.2 m so this is the length and area oh thank god he used the area of 7.9 10 ^7 by the way this is a challenging question okay a new heater is designed using wire of the same material with a length of 1.8 8 ooh the length what increased and a cross-sectional area of 5.8 oh and the area what decreased calculate the resistance of this wire uh well what was the resistance of the old wire 26 so the first wire was 26 ohms so what's the new resistance well first remember resistance is proportional to length and inversely proportional to area if the length increases resistance should increase by how much again this is a challenging question by how much so first I want to see how much did the length increase by the increase in length is 1.8 over 1.2 it's a ratio 1.8 / 1.2 which gives me 1.5 times so the length is 1.5 times larger so the resistance will also be 1.5 times larger what about the area how much did the area decrease by what's the ratio what's the change in the area huh is the change in the area it's 5.8 since that's the new area 10^ 7 over 7.9 the old area i just want to see how many times this is larger or smaller so this is 0.734 times smaller so what's the value of the new resistance okay the old resistance was 26 the length increased 1.5 times so it increases by 1.5 times this is because of the length then the area decreased by 0.73 right decreased by 0.73 so we divide not times by 0.73 that's your area look at this i take the old resistance multiply it by whatever the length has changed by and divide it by whatever the area has changed by you just do the opposite everything will fix itself so 26 * 1.5 divided by 734 this gives me a 53.1 ohm resistance this is not an easy question it's actually quite challenging because you need to m see how much it gets multiplied by increased or decreased or whatnot okay we decrease the area right but because resistance and area are inversely proportional whatever I do it'll be the opposite you know what you could have done earlier you could have just did the opposite ratio so instead of dividing if you want to multiply you do the opposite ratio but the reason I did this is to keep you know my thought process the same what's the change in area it's 0.7 times smaller so how does the resistance change is it multiplied by 0.7 times no it's inverse so it's divided okay next a 2500 W heater is used in a country where electricity costs $0.3 per kilowatt hour calculate the cost of using the heater for 2 days okay uh how do I get the cost well he said it costs $0.3 per kilowatt hour which means I need the number of kilowatt hours but I have power and time so what do I do first step one let's get the energy which is power times time so 2500* 2 days i can't use days it has to be in you know seconds right or if you think about it in hours because I want the power to be in kilowatt and the time to be in hours so look at how I'm changing this this is the power this is the time i don't want it in watts and days i want this in kilowatt so this is going to be 2.5 kilowatt times 2 days is how many hours 2 * 24 48 hours which gives me 2.5 * 2 * 240 kilowatt hours now we have the energy in kilowatt hours number two you need to get the cost if 1 kilowatt hour is $0.3 then 120 kilowatt hours is how many dollars 36 you pay $36 because you kept your heater on continuously for two days who does that who does that okay next a wire has a uniform cross-sectional area which statement is correct okay let's read the resistance of the wire is directly proportional to its cross-sectional area what no it's inversely proportional to the area so no the resistance of wire is directly proportional to cross-section area oh yeah that's the same thing the resistance of a wire is directly proportional to the length yes and inversely proportional to the area yes it was probably C well let's read the last one the resistance is directly proportional to length and inversely proportional to diameter nuhuh but you may say but but it's inversely proportional to area yeah but area is equal to pi d² over 4 if he said to its diameter squared he would be right but it's not inversely proportional to diameter inversely proportional diameter squared so C is still the correct answer next the diagram shows the current voltage graph for a metal wire what can be deduced from the graph like what can we find out from the graph as the voltage increases the wait a second before before I keep going isn't this a straight line straight line means it's a fixed resistor this means the resistance is constant before I even start and the temperature is constant nothing is changing because the voltage and current are increasing uniformly with each other so as the voltage increases the temperature increases no as the voltage increases the temperature decreases no as the voltage increases the resistance increases no as the voltage increases the resistance remains constant yes that's what I want i didn't even need the term as the voltage increase resistance is constant moving on yeah next question a battery is connected to a circuit it's switched on for 1 minute during that time there is a current of 0.4 ampere so hold on i have the time and I have the current the battery supplies a total of 48 jewels i have the energy which row gives the charge and the emf of the battery so I want charge which is Q and emf which is V just voltage now when it comes to Q what do I have i have time and I have current oh that's all I need current is charge over time which means charge is current times time current is 0.4 and the time is 1 minute i can't use minutes right because the term charge or current is ampere which is charge per second so it's time 60 not* 1 so 4 * 60 this gives me 24 kum so the answer is either C or D the emf of the battery now here you have a few options you have energy so option one is to use the definition of voltage which is energy per charge or work per unit charge and since we already got the charge from the previous step you can say it's 48 over 24 which gives you 2 volts or you have energy and current and you want voltage and you have time so you can use E equals VIT which means V is equal to E over I T which is 48 over uh 0.4 * 60 again it's the same answer too so the answer is both options are okay very nice next oh yeah time for the good stuff let's talk about circuits but these were circuits yeah yeah yeah let's talk about bigger circuits let's first talk about all of these components now you don't have to memorize them perfectly 100% it's good that you remember what they look like but you don't have to memorize them all okay okay so as I was saying you don't need to memorize the components not all of them not 100% but it's good to know most of the basic ones there's a lot more than these lots more but at least these are some of the basic ones that you'll see very frequently so starting from the top left hand the cell and the battery are basically the same thing a battery is just multiple cells long line is positive short line is negative positive and negative switch is always drawn open because a closed switch is just a line power supply this could mean anything this could be like a battery it could be a battery pack full of batteries that gives you a voltage it could be your computer's ATX power supply that controls and gives you different voltages regardless it's just two circles and either it could be variable with an arrow through it or not it doesn't matter an AC power supply means a socket it's the same symbol as a power supply with with an AC wave in the middle voltters and emters you know the drill you know what a diode is a light emmitting diode is a diode that emits light so we just draw the diode and two light bulbs sorry two light rays out of it a light bulb is a circle with an X a fixed resistor is a rectangle a variable resistor is a rectangle with an arrow a potential divider is a resistor with an arrow that's coming from the outside we'll see the difference near the end of the session a thermister is a resistor that changes based on temperature so this is basically a thermometer that's placed in it an LDR is a resistor it changes based on light so you have light rays coming in to hit it a heater is like a resistor but with more resistance and more heat so we just draw a few lines in the middle a buzzer is something that produces sound all right a bell also produces sound they both basically have the same symbol it's like a half circle doesn't matter which side is up and the fuse is a safety feature that we'll discuss it's like a resistor in terms of the symbol but there's a line in the middle to represent the wire inside we will see these often okay let's describe some of these in more detail first a variable resistor it's resistance resistor whose resistance can change you change the resistance of a variable resistor by changing the length now the way it works and if we zoom in here this is the structure of it you have a long resistor wire that is connected to one side of the battery but the other side is not the wire itself is connected to what we call a sliding contact it's like a clip that clips on and you can move it back and forth and slide it back and forth this length is the effective length that affects the resistance the longer this length becomes by moving the slider to the right the greater the resistance the smaller this length the lower the resistance so if you move it to the left the resistance decreases if you move it to the right the resistance increases okay now the reason for that if you're curious is because as the current moves from positive to negative it only passes through this section of the wire and then it goes out so if this entire wire was 100 ohms and it passes only through half of the length then it's actually passed through how many ohms 50 you can always get the value of the resistance by checking the ratio of the lengths like are you through half of it a quarter of it a tenth of it 3/4 and so on is this concept used for light switches where you control the brightness yes absolutely but we'll see that light switch control thing in the end cuz that's actually a potential divider but it does use a variable resistor in it okay now the purpose of it is to control the current next what's a thermister from its name it's a thermal therm resistor depends on heat if the temperature of a thermister increases its resistance decreases it doesn't increase like the light bulb because thermisters are made of what we call semiconductors they're not metallic conductors all right metals when heat when they heat up sure their resistance does increase that's true but non-metallic conductors which are called semiconductors are basically silicon and doped with boron and nitrite they're non-metallic conductors when they gain energy usually in the form of heat in this case they get more freemoving electrons they become better conductors which means they resist less and when they cool down they lose that energy they lose those freemoving electrons they go back to their atoms So the resistance increases so the temperature and resistance are inversely proportional so be careful if it's a metal when the temperature increases the resistance increases when it's a non-metal a thermister is the only nonmetal we'll care about for now when the temperature increases the resistance decreases its cousin or its brother from another mother is the LDR it basically is a resistor that's affected by light basically when the light shining on that resistor increases its resistance decreases as well it's inversely proportional and when the light on it decreases meaning you put it in the dark its resistance increases for the exact same reasons as what as a thermist it's made of a non-metallic conductor so when it gains energy doesn't get hot gains energy from the light sunlight light bulb doesn't matter it gets more freeoving electrons and it becomes a better conductor its resistance decreases yeah if I plot a graph of how the resistance changes with temperature or light this is how we sketch it this is how we sketch anything that's inversely proportional not a straight line going down but a curve with a negative slope okay a curve finally the diode what is a diode we described this earlier it is a component that only allows the current to flow in one direction meaning if you take a look at this here the current flows from positive to negative if the current is going to flow in the direction of the diode it passes through lights up the circuit as if it's not there it has no resistance but if you connect the diode in reverse and if the current is going to flow in the opposite direction it doesn't even leave the supply again again cuz some some some seasons think this look at look at this it does not go through the circuit the charges don't go through the circuit meet a diode and go like there's a diode in reverse we need to stop now no no there's no current in the first place there's no current in the first place zero current diode stops now a diode has primarily one function to convert AC to DC and there are two ways to do it the lazy way which is called the halfwave rectifier method okay which basically means you take the normal AC that you're used to alternating current and you pass it through one diode when the current flows this way it goes through yay and when the current tries to go in the opposite direction it doesn't because that would make it reverse that reverse compared to the diode so it doesn't go through zero but this is a very lazy way of doing it because you've essentially gotten a pair of scissors and you've cut out half of your energy you only allowed one side to go through but not the other the negative doesn't go through so how do we fix this if we want to purely convert AC to DC we use what is called a full wave rectifier or a bridge it is essentially four diodes connected in a diamond pattern two of them look up and they're parallel to each other and two of them look down when they're also parallel to each other two of them look up and two of them look down you do not have to memorize this by the way but you do know have to you do have to know if a diode is drawn correctly or a bridge is drawn correctly or not remember what I'm saying two of them are parallel to each other looking down two of them are looking a up i don't care which is up and down though like these could be the ones up so these have to be the ones down but if I draw the diodes like this like each one is in a different direction this is wrong so that's the first thing you need to identify the second is knowing how to trace the current through a diode like knowing how to draw how the currents flowing through the diodes you don't have to like I said memorize the diagram but you have to look at how it goes through so I'm going to draw two currents one in red when the top side of the supply is positive and the bottom is negative and one in blue which is when the top is negative and the bottom is positive like when the current flips and look how it reaches the other side like the other terminal so let's start with the red current goes up goes right goes down reaches a junction it cannot go left so it goes right reaches the next junction it cannot go left so it goes right so this side is positive and this side is negative and the current goes through no problem if you want to see the current going back this is how it goes back okay how about the blue side let's follow the blue line now the blue means the bottom terminal is positive so it goes down goes to the right reaches a junction left or right left is blocked off so it goes right through D2 then it meets the next junction left or right left is completely blocked off so it goes right oh this is still positive so what reaches the output this is negative so flows this way and on the way back when it reaches the negative it flips you essentially take the negative version of this graph and you flip it that's all you do this is called a full wave rectifier so again if I tell you you only use one diode it's a jump and a zero a jump and a zero if you use a full wave rectifier four dodes it's jump no negatives no use finally what is an LED or a light emmitting diode it's just a diode that shines bright gives out light when a current goes through it that's it so it's basically a diode but it's got the extra function of glowing or like emitting light let's get into the highlight of this particular chapter series and parallel connections this is the part that confuses some people and gives them trouble but you need to remember something whatever I say in series will be the opposite of what happens in parallel all right and then you need to memorize exactly what happens to the current the resistance and the voltage the current the resistance and the voltage sometimes it increases sometimes it decreases sometimes it splits sometimes it doesn't sometimes it's the same sometimes it's not like you need to memorize how each method of connecting components together changes these three factors so starting off with series connections a series connection means you connect the two resistors along the same wire so if I were to connect them to let's say a 10V supply and there's a current I going through it it's number one the current is the same you have the same current going through both because it's only one wire that's the current second when you connect resistors in series their combined resistance increases and you calculate it using R1 plus R2 plus R3 plus you just add them so if this was uh 10 ohms and 10 ohms the total would be 20 ohms finally but most importantly what happens to the voltage the voltage of the supply in series gets split now the voltage of the supply doesn't change but the voltage that you try to deliver gets split the potential difference splits if I were to calculate the voltage across the first resistor I would first need to get the current so I'd say I = V / R which is 10 / 20 that's the total voltage and the total resistance which is 0.5 amp and then if I ask you to get the voltage only across the first resistor V1 you would say I R1 which is 0.5 that's the current time 10 which is 5 this means that this resistor only gets 5 volts from the supply which means the other one also gets 5 volts from the supply the voltage the total voltage that you have gets split gets divided it's not always equal it depends on the ratio of the resistors for example if this wasn't 10 and 10 what if this was a Let me delete all of this what if this was a uh let's just use the same supply a 10 ohm resistor and uh 90 ohm resistor going for the extreme the first resistor will only get one volt and the second resistor only gets nine gets the rest because the voltage gets split according to the ratio of the resistors one will increase the other has to decrease so that the total voltage stays the same it's 10 volts okay that's if they're in series what if the resistors are in parallel well everything I said gets flipped over huh in series I said the current is the same like it doesn't change right but in parallel uh you have two branches for the current to go through it's not just one loop it's two loops actually so the current that flows through the circuit splits does that mean it gets smaller on the contrary the combined resistance of resistors in parallel doesn't increase it actually decreases because if you think about it connecting resistors in parallel on top of each other is like making a wire thicker you allow more charges to go through at the same time so connecting resistors in parallel gives the charges more space to go through which increases the current so the current gets very large and the resistance A decreases how do we calculate it using this rule R equals product over sum or R2 R1 * R2 over R1 + R2 so for them example if this was 10 ohms and 10 ohms their combined resistance wouldn't be 20 it would be 10 * 10 over 10 + 10 which gives me 5 the combined resistance of resistors in parallel decreases and it has to be lower than the smallest resistor so if I have 10 and 10 has to be less than 10 if I have 10 and 20 still less than 10 if I have 10 ohms and 2 ohms has to be less than 2 ohms the resistance just decreases drops hard finally this is also voltage is arguably very important why because in series if you remember the voltage gets split in parallel it doesn't in parallel all the components get the same voltage this means that this 10V supply can deliver 10 volts completely to the first resistor and 10 volts completely to the second everything in parallel gets the same voltage and that's because each like resistor gets its own charge like this is a charge carrying 10 volts goes through just this here's another charge carrying another 10 volts it only goes through R2 in series the same poor charge has to do what if you had a charge carrying 10 volts oh it has to go through both R1 and R2 so it's forced to split its voltage but in parallel each one gets its own voltage because it gets its own charge full voltage very good now obviously parallel has a few advantages over series what do I mean at home most of your light bulbs and switches and sockets are all connected in parallel for three reasons number one everything in parallel gets the full voltage so at home if my voltage is 220 volts this light bulbs gets 220 this light bulb gets 220 this light bulb also gets 220 everything lights up right sure you end up paying more money capitalism but that sounded Scottish now uh everything gets the same voltage but here's the another advantage you can switch them on and off separately or independently meaning you can close the switch here and open this switch and close this switch and you have two light bulbs on one light bulb off it's up to you and then if one of them breaks the others still work that's the advantage of parallel like having them on separate branches they don't affect each other if everything in your house was connected in series if everything in your house is connected in series if one light bulb breaks everything goes off everything turns off okay however there is something else that we do connect in series is very important cells or batteries you can have more than one cell in series by the way like if I put a 6V cell in this example next to a 3V cell the voltage that you deliver is not six or three it's nine they get added up so when you connect cells in series you just add their voltages together to anybody observant you look to the right side over here and say uh why is this four volts not not like eight or something why is it four not eight because look at this first cell it's positive and negative are pointing to the left so the current is going to flow to the left the second cells positive and negative point to the right which means it's in the opposite direction these are in the same direction these both try to push the current in the same direction so what do they do they cancel out instead of adding them you subtract them so if the cells are in the same direction you add them if the cells are in opposite direction you subtract yeah can you put an AC supply next to a battery i mean you can it's not a good idea though finally let's talk about the potential divider a potential divider is just a very fancy way of saying two resistors in series but the function here is to actually split the voltage like look at this example if this was a 10 ohm and this is a 30 ohm resistor the voltage will split with a ratio of 1 to 3 so 2.5 volt to 7.5 volt do you know what that means this means if you connect another light bulb only parallel to the first resistor it gets 2.5 volts you got you get to choose what the voltage is across it no no no no but what if I connect another light bulb here this will get what 7 1/2 volts because the voltage was split first according to the ratio of the resistors and then you ended up connecting this component not parallel to the entire circuit but parallel only to the first resistor so it only gets the voltage of the first resistor if you take a look at the second one you connect it parallel to the second resistor so it only gets the voltage of the second resistor so if I were to turn this circuit on the light bulb on the right would be bright and the one on the left would be dim a very similar circuit to this is what we call a variable potential divider this is a much cleaner version because if I wanted to change the voltage like increase and decrease the voltage directly I don't need a potential divider with two resistors and then I have to split the voltage and then if I want to change the ratio I have to change the ratio of the resistors i can just connect a variable resistor completely to the supply which means this entire wire gets the full 10 volts but then you connect your slider or sliding contact in parallel to the light bulb that you want so if the sliding contact is in the middle it's not parallel to the full wire it's parallel to what half of it so it doesn't get 10 volts the entire wire gets 10 sure but if you're only connected to half of it you get what 5 volts what if you move this parallel here to the very end you get the full 10 volts cuz you're now parallel to the entire wire what if you move it to the left you get nothing because you're parallel to nothing look you're parallel to an empty wire like it's nothing this is called the variable potential divider and whoever asked me earlier uh is this how light bulbs or like variable light bulbs that you have at home work this is how they work a variable potential divider is so much cleaner than just putting a variable resistor in series with a light bulb put it in parallel instead this allows you to change the voltage directly from zero to full to 10 volts zero being zero being the slider near the side that's connected to the battery and the maximum being the other side the other end okay so the purpose of it isn't to control current now it's to control what voltage across component as one final rule a variable potential divider can follow this rule v_sub_1 over V2= R1 / R2 meaning the ratio of the voltage is the same as the ratio of the resistors we sometimes use this when solving questions it does help do I always need it not really but it does help okay let's solve a few questions starting off with this March question over here a circuit consists of a DC supply a lamp and a thermister draw a circuit diagram of these components connected in series so let's go a DC supply you can either draw just a supply and just write DC on it or like positive and negative or you can draw a battery or a cell that's fine a lamp so light bulb and a thermister are all in series do I need anything else nope that's it okay next explain what happens in the circuit you have drawn when the temperature of the thermister increases ooh ooh okay when the temperature increases the resistance of the thermister decreases and if the resistance of the thermister decreases the current will increase but there's a light bulb in there so the light bulb becomes what bright look at the way I sequenced my logic i started off with what happens when the temperature increases when the temperature increases the resistance decreases of the thermister like I I'll be even of the theister but that causes what that causes the resistance of the circuit to decrease the total resistance increases and the total current increases right so the current increases that's honestly enough honestly that's enough but you know what i just want to push through so the light bulb is bright or glow bright it's only two marks so honestly I'm just looking for resistance decreases so the current increases that's that's genuinely what I'm looking for but I wanted to express exactly what's happening inside because if it was worth more marks that's great we end up getting more marks next question ooh I see an AC supply oh what's this huh is this a single diode or a full wave rectifier like without even reading the question is this a single diode or a full wave rectifier just one diode like look at this he has the AC supply connected to one diode and he's like "What's the voltage across this resistor if it was the full wave rectifier it would be a bunny hop going up down up down even if it's negative." But it's not it's just one diode so Cuts off half of the voltage next figure 7.1 shows a circuit that contains a battery a switch and a voltmeter and three 40 ohm resistors oh this is important you didn't draw the values you just mentioned them here they're all 40 ohm resistors r1 and R2 and R3 describe the switch is opened and R1 R2 form a potential divider explain what is meant by the term potential divider so what is a potential divider now the function of a potential divider is to do what is to split the voltage or divide the voltage between two resistors right proportional to the ratio depending on the ratio that is what a potential divider is and that's the function of it at the same time so a potential divider is just what a circuit that divides the voltage across two resistors in series according to their ratio or according to the value of the resistance or proportional to their values next up remember all of this all of this and the switch is still what open which means this is open which means R3 is not part of the circuit yet cuz the current's going to flow here through R1 and R2 and then come back it's not going through R3 yet the reading on the voltmeter is 7.5 which means this is 7.5 volts what is the emf of the battery actually that's easy didn't he say they're all 40 ohm resistors this is 40 this is 40 this is 40 but this doesn't count if I have two resistors that are equal in value in series doesn't the voltage split equally between them doesn't the voltage split equally between them so if this gets 7.5 this also gets what 7.5 which means the total voltage is what 15 7.5 + 7.5 is 15 i could tell all of that just because he said "Hey they're all 40 ohm resistors," which means they're all the same resistance okay which means the voltage splits equally so if the first one got 7 and 1/2 the second one gets 7 and a half if the first one got like 200 volts the second should also get 200 volts however now the switch is closed oh boy what's the resistance of the complete circuit yeah I'm going to redraw the circuit here but instead of vertically because sometimes mentally in your head it's hard to imagine I'll draw it horizontally like I'm just going to rotate this 90° here's R1 and then there is R2 and then like R3 is above that which means what which means you have two resistors in parallel first which are then in series with this dude over here so step one I need to get these two together first in parallel and after I get their combined resistance I add it to the series one so let's go the parallel version first it's product over sum r1 * R2 over R1 + R2 which is 40 * 40 over 40 + 40 this will give me 20 and this is something you can memorize by the way if you have two resistors in parallel and they have the same value their combined resistance is half of them so 40 and 40 give you 20 30 and 30 give you 15 five and five gives you two and a half but then I need to add it to the series resistor so the 20 ohms the combined 40 and 40 gives you 20 plus the 40 for that separate resistor this gives you a total of 60 ohms knowing what to sequence and when is important calculate the reading on the voltmeter when the switch is now closed ooh oh wait wait wait hold on hold on let me erase this or maybe I didn't need to yeah I didn't need to didn't we just say that these two together are worth 20 ohms and now it's technically in series with this which is 40 ohms will the 15 volts of the supply now split equally no it won't r1 which is 40 will get more than R2 r1 which is the 40 ohms will get more than R2 and R3 because they're together they're 20 ohms so when he asks you for the reading on the voltmeter he means what's the voltage across this now that this is 40 and 20 it's going to be what 10 volts and you can use two methods to get this you can either use Ohm's law which is V= IR let's try that one out first or you can use the ratio example remember how I used ratios r1 / R2 V1 over V2 let's try this out let's try Ohm's law first step one I need the current so I= V / R the total voltage is 15 volts so the total resistance is what 60 oh uh wait I didn't I didn't write this okay 60 this gives me a current of 4.25 then you use Ohm's law again but to get the voltage of V1 which is the voltage across R1 so it's 0.25 25 * 40 which gives you what 10 yay or you can use a ratio example but ratio of what exactly i don't have V2 like I don't know what this voltage is but I do have the total voltage so here's another version of this ratio example thing we can say V1 over V total equals R1 / R total it still works if you compare it to the total not just to the one next to it so V1 over what's the total voltage 15 equals R1 which is the 40 ohms that you want over the total resistance which is 60 so V1 is equal to 40 / 60 * 15 this gives you 10 pick your poison doesn't matter they're both okay they're both okay all right next question oh wait do you want to take a picture of this why did it disappear the app's being weird okay never mind you can go back and pause and get the picture there next question i see three cells in series i see R1 and R2 and a diode in R3 and a voltmeter the three cells have zero resistance r1 R2 R3 are identical meaning they're the same the reading on the voltmeter is six the voltmeter reading is parallel to all three which means the total voltage or EMF here is 6 volts when the diode is conducting it has zero resistance and zero potential difference across it he's just saying the diode doesn't affect anything yet determine the emf of one cell if three cells have 6 volts this means one cell has what two volts cuz 2 + 2 + 2 gives you six then determine the ratio of the potential difference across R2 to the potential difference across R3 that's a slightly challenging question it's actually hard but it's based on what we just did earlier remember if this is R1 and this is R2 and this is now R3 which is in series to it just like the previous 40 40 when he says hey what is the ratio of the voltage of R2 to R3 uh sorry this is R2 i don't know why I wrote that as R3 and this is R3 how is the voltage going to get split now you might say I don't have values that's fine assume anything assume anything if this is 10 ohms and 10 ohms and 10 ohms what's the combined resistance of R1 to R2 r1 and R2 five which means this is technically 5 to 10 and what does he want he wants the ratio of the voltage the ratio of the voltage is the ratio of the current so 5 to 10 is 1:2 so you can say it's 1:2 or 0.5 that's fine too it's not an easy question to get it needs you to have seen questions like this before and the fact that you I can just get you know two resistors that are identical it's half the voltage so this gets half this gets one and then it splits what's the ratio 1:2 next all the cells are reversed meaning the current now wants to flow this way wait if the current wants to flow this way it cannot flow through R1 anymore because the diode stops it so the currents only flowing through R3 and R2 which are now technically in what in series r1 doesn't exist now because the current can't go through it the current can't go through it right hey wait a second if if R1 goes away what happens to the total resistance it was in parallel now it's only in series remember when we added things in parallel the resistance would decrease if you remove the parallel portion the resistance will what decrease sorry increase the resistance increases you know because you lost that parallel connection it's now just series so the current decreases that's very sad then finally determine the new ratio of the voltage of R2 to R3 now this one should be easy if you only have R2 and R3 in parallel in series sorry and they're both the same resistance the voltage will be split what equally which as a ratio means it's what 1 one it splits the voltage equally so what's the new ratio 1 one okay this was a tough question by the way like using ratios and concepts it's a tough question and I picked it on purpose the final part of this unit or at least this section of electricity and we're done let's talk about safety now obviously obviously electricity can kill us how well it can give us an electric shock that's bad and it can set us on fire not not us directly but can set everything else on fire like okay can set everything else on fire so in order to keep ourselves safe we need a few basic things at home like insulating wires using a fuse and so on so let's first take a look at what's the electricity at home like an electric socket or electricity at home is called main's electricity or a main circuit it consists of two main wires and a third safety feature the two main wires are the live wire and the neutral wire there are no positive and negative wires why because electricity at home is always set to 220 volts AC electricity at home is 220 sometimes 240 like it fluctuates a little bit but it's AC which means it doesn't have a positive or a negative the live wire is the super high voltage wire that's always fluctuating and the neutral wire is just there to complete the circuit this is usually at zero volts this means that if I get a piece of metal and I shove it into just one of these openings in the socket if I shove it into the live wire I'm a cooked goose and if I shove it into the neutral wire I'm aok okay you and your luck i don't know which one is live and neutral we never label them electricians and designers never label these depends on who connects the wires in the walls now the earth wire is actually a safety feature not all the country not all countries have this this is a safety feature that protects you from getting electrically shocked if a metal case becomes live but I'll discuss this over here like uh we'll talk about the earth wire down here so as I said what are the hazards you can get electrically shocked or you can cause an electric fire what causes these things insulation if it's damaged if you overheat the cables cuz that damages the insulation cuz wires get too hot it could melt the insulation and it could catch on fire damp conditions means if it's wet obviously you don't charge your phone in the bathroom if you do something's wrong with you uh too much current because too much current will cause the cables and the sockets to overheat which causes what electric fires okay so what safety features do we have at home to protect us from these two main problems too much current is the reason why things overheat or things burn and like you know sockets get damaged and everything else and to protect you from that we have a fuse a fuse is just a very thin wire by the way very thin wire that's always placed on the live side of a circuit this wire is designed to melt if the current gets too high so let's say for example you have a 5 ampere lamp it uses 5 ampers normally and you close the switch and it draws in 5 amp that's great it's going to work fine but if the current increases it'll burn like the lamp will burn out replace the lamp with any device you want okay so what we do is we put a fuse over here a fuse has a rating let's say 6 amp meaning if the current reaches 6 ampers it will melt and when I say melt I don't want you to imagine it slowly melting away no no no it just pops right away just snaps breaks immediately okay so when you close the switch and the current that's flowing through is less than 6 amp it's like the normal five no problem fuses like a wire no problem but if you close the switch and for some reason maybe there's an electric surge maybe somebody's fooled around with the switches at home or the wires whatever it is and the current increases for any reason it goes through what first the fuse and it melts the fuse this like pops it so now there is no current that reaches the light bulb so the light bulb turns off but you kept it safe so what we do is if that happens we remove the old fuse we throw it away we plug in a new fuse and then we flip things on again switch things on again how do I choose a correct rating for a fuse you always go for like one to three ampairs higher than what the device needs so for example if I tell you I have an air conditioner that needs 30 amps to work well I had better put a what what fuse should I use maybe a 32 or a 33 ampere right similar to a fuse is a circuit breaker or a trip switch this is just like the fuse it protects the circuit if the current gets too high if it gets too high for any reason but it doesn't melt this time it uses an electromagnet we have an electromagnet over here next to what we call an iron switch cuz iron is a magnetic substance as long as the current is low the magnet doesn't turn on because it's connected to the live wire as well but if the current gets too high the current goes through the magnet first it turns on it magnetizes and it attracts the switch plop opening it and therefore your circuit doesn't work or your entire house doesn't turn on it's exactly the same function as a fuse it stops the current if it's gets too high but this time by opening what a switch using a magnet we always choose a rating that's 1 to 3 amps higher than what you need okay and we also always connect it to the live wire because if you connect it to the neutral wire this is you cutting out the current on the way out of the circuit not on the way in so the current's already gone in burnt your house down and as the current's going out it's like hey you want to stop me now sure go ahead your house is already on fire though okay that's the circuit breaker now final question what's the earth wire the earth wire or sometimes we call it the ground wire is a safety feature that's not in all devices and not in all countries either because it's only useful if the device you're talking about has a metal case so think of a washing machine or a computer a PC or a fridge anything with metal as its outer case the wires inside the washing machine for example have live and neutral connections and if as you connect them one of the live wires accidentally touches the case and you touch the case as well the currents going to flow in not to the washing machine but to you so you're a nice target and you light up now I don't want you to light up okay that's bad so what we do is we connect a new wire to the metal case that's not part of the circuit and we connect it to the ground or the earth when I say ground or earth I actually mean a big metal rod inside a building it has to be a conductor we just call it earththing because it's not part of the circuit so instead of the current going through you that's not a good idea the current goes through what the earth instead cuz which has a lower resistance you as a human being made of flesh and blood and bones and muscles and everything else you've got some resistance by the way but the earth wire which is made of copper often or even aluminium has very low resistance the current's going to prefer that over you not even a current wants you but that's good otherwise I don't want any tiger now you might be thinking but on its own this means I'm losing electricity all the time well because the earth wire has very low resistance it causes the current to spike increase a bit increase a lot which causes the fuse to trip and it's gone so what does it do what does an earthwire do it protects you from getting electrically shocked if a metal case is live okay if something doesn't have a metal case you don't need a live wire and if you have that earth wire and a fuse the fuse will glow okay let's solve the last couple of questions here so which diagram shows a symbol for a fuse yay d obviously and an electric kettle has a metal casing the cable for the kettle contains a wire that is connected to the earth of the plug which danger does this guard against from the cable getting too hot no from the casing of the kettle becoming live absolutely because if the casing is live I get shocked i've gotten shocked before it's not a nice feeling at all the casing of the kettle becoming wet no it's supposed to be wet the casing of the kettle overheating no that's not the issue the issue is you getting electrically shocked so be and you'll notice there are no more questions this last bit this last chapter essentially doesn't show up a lot it's mostly a core part of the syllabus not an extended part so it barely shows up but you know what shows up in a multiple choice question or two or like maybe a one or two mark question paper four it could so it's important to remember and that ladies and gentlemen is unit for at least the first half electricity x time is magnetus