so this video is a summary of the AQA electricity topic and what we have in electrical circuits are a load of small electrical components and what we need to do is we need to draw out circuits and to help us with this we'd have some circuit symbols so some of these might be common from before but there's a few extra ones things like light dependent resistors diodes and so on that it's worth just spending the time memorizing what all of these stand for and the reason for this is that allows us to draw circuits really simply now in the simplest case we might have a power supply so a cell we might have an ammeter and you always put the ammeter in series and this is connected to maybe a lamp and when the circuit is connected it starts shining we can also use a voltmeter to measure the potential difference across a component so it's always worth remembering that a meters go into series and volt meters go in parallel so what we have here is our a really really simple circuit now for this one here what we can think about is the rate of flow of charge particles around the circuit that's what trail is transferring energy from the cell in this case to the lamp and we've got to remember that electric current is a rate of flow of electric charge now one of the equations we can use is that Q is equal to I times T now somewhat confusingly we use Q as a symbol for charge and charge is measured in coulombs we use I for the current this is like the intensity of current and we measure the current in the unit of the amp and then time we measure in second and really what electric current is is a flow of electric charge and in a circuit like this where we have maybe a DC supply so it's a direct current we've got the flow of electrons in the wires now we can also think of this electric current which is going through a component it depends upon the potential difference across it V and also the resistance of that component effectively we're gonna have a bigger current flowing if you've got a bigger potential difference but if you have a bigger resistance that reduces the current and what we have here is V stands for our potential difference and this is measured in volts our if the resistance measured in ohms and then once again the current is measured in amps sometimes more useful equation is V equals I times R and you can apply this to any part of the circuit if you know the potential difference and the resistance you can then work out the current that is flowing but when it comes to resistance we need to look at the resistance of certain components and we can set up a simple circuit a bit like this and what we have is a power supply we have a variable resistor and this allows it listen allows us to change the current and a potential difference across a component and we can do this for the following things and we need to look at the characteristics of resistors lamps and diodes now if we have a resistor what we find is when we plot our values of current and potential difference we get a straight line that goes through the origin and what this means is that as you have a greater potential difference you've got a greater current that flows and that's because the resistance is constant in actual fact this is a great example of something called an ohmic conductor and provided this stays at the same temperature the current is going to be proportional to the potential difference if however you have a filament lamp this isn't an ohmic conductor and what we find is we get a graph that looks a bit like this well that means is as this gets hotter and hotter and gives out more light the resistance increases so this one here you've got a changing resistance for this light bulb if you have a diode on the other hand the whole point in a diode is it works a bit like a one-way valve and only less the current flow in one direction so we get a graph like this effectively if you've got a negative potential difference the value of current is zero but when you have a high potential difference loads of current can flow through and this really allows us to control the way that current flows in a circuit now there are two other components that we can investigate one of them is a light dependent resistor and if we look at the resistance and the amount of light intensity with an LDR when we increase the light intensity the resistance decreases and the same kind of thing happened when we have a thermistor so a thermistor is a thermal resistor once again if we have the resistance on this axis of the graph and we look at the temperature on this axis as we heat this up the resistance goes down and these two components can be used for all sorts of applications where maybe these can be used to send some things if it gets too hot or gets too dark we can use this to turn on other parts of an electrical circuit now there are two main categories of circuits we've got series circuits and parallel circuits in a series circuit we have one thing after another in a parallel we have separate loops and there's different way for the electricity to flow now in a series circuit if we were to look at the value of the current down here in this part of the circuit we define it it's the same value as the current here and the same value as the cart up there so in a series circuit what happens is I 1 is equal to I 2 which equals I 3 we have the same current around the whole circuit in a parallel circuit if we were to look at I 1 which is flowing down here and we looked at I 2 and we looked at I 3 so that's the current through each resistor what we find is that the current splits at this Junction and what we find here is that I 1 is equal to I 2 plus I 3 so in a parallel circuit we have a different current in each part of the loop but if we were to look at the potential difference say we had a potential difference of V 1 across the cell and we compared that to V 2 and V 3 that's the potential difference across these two resistors what we find is that this potential difference is split between the components so v1 is equal to v2 plus v3 in this circuit however if we've got V 1 here V 2 and V 3 we find that the potential difference across each loop of this parallel circuit is the same so here we find that V 1 is equal to V 2 which is equal to V 3 so potential difference around any loop in a parallel circuit is going to be the same now when it comes to looking at resistance if we have resistor R 1 and R 2 in a series circuit their total resistance is gonna be equal to r1 plus r2 effectively the more resistors we have in the circuit the bigger the total resistance but in a parallel circuit if we had r1 and r2 now in parallel there's effectively more ways for the current to flow so the more resistors you have here the lower their total combined resistance now you don't need to calculate it but you need to know that the total resistance is going to be less than the value of r1 and also that total resistance is gonna be less than the value of r2 so now we know a bit more about the way that current and potential difference occur in series and parallel circuits we know we've got some of the other equations we can also look at the electrical power so this is the amount of energy transferred per second and what we can say is that the power is equal to the potential difference times the current now again we measure our potential difference V in volts our current in amps and we measure our power in watts that's the amount of joules per second data transferred so effectively if you've got a component with a bigger current flowing through it and a bigger potential difference it's going to transfer more energy per second but sometimes we might not know the potential difference and the other equation we can use is V is equal to I squared R so we can also work out the power capital P is equal to the current squared times the resistance now again you've got to remember that energy is going to be equal to power times time and another way of looking at the energy transferred is equal to cubed V so this is Q which is a charge in coulombs potential difference in volts and this then gives our energy transferred in joules so there's a whole load of different equations that you need to be familiar with it you can then apply to circuits now so far we've been looking at direct current which flows one way around the circuit but then there's another type of circuit which uses alternating current this is the kind of thing that you might have in a household appliance where you plug something into the mains and the UK mains is 230 volts and it's got a frequency which is means there's 50 cycles per second if we were to look at this on a graph this means that sometimes you've got a positive value and sometimes it's negative and this means that the current flow is one way to any other that than the other way than the other than the other than the other really really quickly now this is used in normal kind of household appliances and what we often have is this three core cable so we have three different wires in the core of that cable so the brown wire provides the alternating potential difference from the supply the neutral wire completes a circuit and it's the difference between the potential difference here that causes a factory that current to flow and then finally you've got this green and yellow wire which is called the earth wire and this is a safety wire that means if there's a Fault in the system this carries us there any electricity back to the earth rather than going through your body as it says over here so you need to remember some of these facts about an alternating supply especially the household supply that we have in the UK now the other thing that means that AC is really useful is it allows us allows us to transfer energy over large distances or transfer electricity of large distances by using the National Grid now where can I put this just down here so the National Grid is a system of cables and transformers that link power stations to consumers and what you have in a transformer is you have two coils of wire the primary and the secondary and these are wrapped around an iron core now when you have an alternating supply in one it causes there to be an alternating supply in the other and this allows us to massively increase the potential difference but also decrease the current and this reduces energy losses in our power supply so you need to know a bit about the National Grid and how we have step-up transformers which increase a voltage or we also have step-down transformers which decrease the voltage and finally we have static electricity now this is often caused if you have an insulator and maybe a rub it against another insulator what happens then is that some of the electrons are transferred from one to the other so that means if some electrons are transferred from this to this by the rubbing action this thing becomes more negative while this thing here now has an overall positive charge and if you then separate these two things these two things are going to have a force acting on each other so in this case there's going to be an attractive force but if they had the same charge they'd then be repulsive and if we had maybe a charged sphere and maybe this one had a positive charge we can look at the region around it and in the region around it where other charged objects experience the force we say that this is an electric field and if we were to draw the field lines around this positive charge here they'd all be pointing away from it actually that's a direction that if you had a positive charge and you put it here which way would it move well this would be repelled and what we find is that the electric charge is stronger near the surface so that's it that's a Qui electricity we've got different circuit components and how they work in both Sue's and parallel circuits loads of equations as well as information that you need to understand about an alternating supply you