electricity is the flow of charge or charges like electrons they carry energy from a source of energy to a component where the energy is released as another type of energy here's a simple circuit we have a cell here this is the symbol for that this is the symbol for a battery that's just several cells connected in line we draw straight lines for the wires which in this case are going to a lamp a light bulb and that lights up of course you have to have complete Loops of components and wires in order for these charges to flow by the way you're going to see me mix up cells and batteries in this video because they're just the same thing really and they do the same job leave an angry comment below if you're really that mad about it so what's going on here in this circuit then the battery has a store of chemical potential energy when connected in a complete circuit this energy is transferred to the electrons which moves through the wires this movement of charge is called a current and we say it always goes from the positive terminal of the battery to the negative don't think about it too much as the electrons pass through the bulb their energy is converted into light and some heat too probably as they're never 100 efficient this light and heat is then transferred to the surroundings including your eyes so you can see it but the electrons don't just disappear once they transfer all the energy to the Bob as this is one being Loop these electrons are pushed back round to the battery by the ones behind them where they're refilled with energy ready for another trip around the circuit this constant flow of electrons transferring energy is what keeps the light bulb on because electrons are so small and there are so darn many of them we don't deal with individual electrons but instead deal in coulombs of electrons or coulombs of charge similar to moles in chemistry it's just a specific number but we don't care what the number in a coulomb is potential difference PD for short also known as voltage tells us how much energy is transferred per coulomb of electrons so if a cell or battery says it's one volt that means that one joule of energy is given to every coulomb of electrons that pass through it if a battery is 6 faults that means six joules is supplied per coulomb instead we measure PD with a voltmeter voltmeters always get added last to a circuit as they're always connected in parallel to the components you want to measure the voltage of in the real world that means the leads or cables from the voltmeter always piggyback into other leads if we put the voltmeter across the battery it should measure 6 volts right because 6 volts is supplied to the electrons in the circuit that's just six joules per coulomb but put it across the bulb and it should still say six volts why because the electrons have to lose all of that six volts worth of energy as they pass through okay it might be -6 volts but we don't care about minuses really when it comes to PD we only care about the number here's the equation for PD PD and volts is equal to energy in joules divided by charging coulombs in simple form V is equal to e over or divided by q q is the symbol for charge but it's measured in C in coulombs you'll see the rearrangement version E equals QV on your formula sheet current on the other hand tells us what the rate of flow of charges essentially how fast is charge flowing through a circuit or a component like any equation for a rate as per usual it's something divided by time so here it's current in amps equals charging coulombs divided by time in seconds or I equals Q divided by T yes we use capital i as the symbol for current not C blame the French for that as they called current intensitated cohant it does mean though that we don't get confused between current and coulombs though so we stick with it you're going to see the rearrange version of this equation on your formula sheet Q equals i t that's I times T we measure current with an ammeter note that it's not amp meter unlike a voltmeter it must go in series that means in line with the component we want to measure the current for components in a circuit have resistance that is they resist the flow of charge or current through them but that's not a bad thing this has to happen in order for them to work work a bulb has resistance which causes energy to be transferred and light to be emitted a resistor of course has resistance too but it just produces heat when current flows through it if we make a circuit with a resistor and change the PD available to it what we find is that an increasing PD results in a greater current flowing in fact doubling one doubles the other so we can say that PD and current or V and I are directly proportional drawing a graph of these two makes a straight line and if we turn the battery round we can get Negative values for both two but still a straight line through the origin this straight line a constant gradient shows that a resistor has constant resistance we say it's ohmic the steeper the gradient of this line the lower the resistance of the resistor as more current is Flowing per volt the equation for resistance is Ohm's Law V equals IR that's PD in volts equals current and amps times resistance in ohms that's the unit for resistance we can get the resistance of a component from an iev graph like this by just picking a point on the line and rearranging Ohm's law so R is equal to V over I for a resistor you'll end up with the same answer no matter what point you pick if you repeat the same experiment for a bulb in place of the resistor however you'll end up with a curved graph like this this shows that the resistance is changing the resistance of the metal filament in the bulb in fact you'll find that any metal has a changing resistance instance if you increase the PD and current they're non-omech at higher PDS the current increases less and less so that means they can't be proportional this shows that the resistance of the metal is increasing with a higher PD and higher current the change in gradient shows us that this is true but we still just take a point on the line and use Ohm's law if we want to find the resistance it's just that it does matter where you pick that point in this case so why does resistance change for a metal well it's because Metals consist of a lattice or grid of ions surrounded by a sea of delocalized electrons that just means they're free and free to move or rather they're fairly free to move because they do collide with the ions as they flow that's why the metal heats up when you pass a current through it the higher the current the more frequent these collisions are and this makes the ions vibrate more and more which in turn makes it harder for the electrons to flow the resistance has increased now as an aside AQA have royally messed up lately in their exams whereby they've asked the question what would happen to a resistor if the temperature increased to which the mark scheme says that its resistance would increase it would act like a metal they are wrong resistors are specially made from specific materials such that their resistance state is constant even if the temperature changes if that wasn't the case we wouldn't get this straight line for a resistor and we might as well just use Metals instead silly AQA now there is another component called a diode it will give you this graph the circus symbol might give you a clue as to why this is a diode only lets current flow through in One Direction we say that in One Direction the resistance is very high and it's very low in the other which is why the current increase is suddenly at around 1 volt an LED is a light emitting diode similar symbol just with a couple of extra bits these are what most lighting electronics are these days rather than filament labs they act in the same way as a diode so they give you the same graph but they just happen to emit light as well we can do another practical on Resistance by measuring V and I for a length of metal wire connected to a circuit with crocodile Clips to calculate resistance of the wire using Ohm's law then we can move One Clip further up the wire to see how the length of this wire affects resistance you should end up with a straight line through the origin showing that resistance and length of wire are directly proportional series and parallel circuits this is where things get a bit tricky remembering what happens to current PD and resistance when we have components in series or in parallel here's a simpler series circuit we can make really just two resistors in line with the battery what you need to remember is that for components in series total PD is shared between them current is the same for all of them and the total resistance is just the sum of all resistances that just means added up let's deal with that first point if these resistors are the same let's say 10 ohms each then that 6 volts total PD from the battery must be shared between them so if we put a voltmeter across each of these they'd both read three volts it wouldn't matter what resistance these resistors are they could be a million ohms each if they're the same then that total PD is shared equally by the time the electrons leave the second resistor they have to have lost all six volts worth of energy ready to go back to the battery to be refilled by the way we can also call this setup a potential divider circuit as the total potential total PD is being shared if the resistors don't have the same resistance then we can use the second point to help us that is the current is the same for both let's say the first resistor is 20 ohms using 4 volts of the total 6 volts available we know two things out of v i and r so let's use Ohm's law to find out the third fret current in this case I rearranging Ohm's law we get I is equal to V over R so that's four divided by 20 0.2 amps same for the second resistor two is there also a second thing we know about the other resistor why yes there is remembering the first rule up here we know that if the first resistor is using 4 volts of the total six volts available well then the other resistor must be using up two volts we could then use Ohm's law again to find its resistance 10 ohms the rule of thumb is this the greater the resistance the greater the share of the total PD it gets we can also use Ohm's law for a whole circuit we just need to make sure that we're dealing with the total PD total current and total resistance the rules for parallel circuits are the opposite the PD is the same for every Branch current is shared between each branch and the more resistors you add in parallel the lower the total resistance this by the way is because you're given the current more roots to move through the circuit which means more current can flow so these two resistors are connected to the six volt battery in parallel you know straight away that the PD for both has to be 6 volts voltage isn't shared in parallel circuits if however we say 0.5 amps total current is flowing through the battery and 0.2 amps of that is flowing through the top resistor that must mean that there's 0.3 amps flowing through the bottom resistor if you're not in a rush why not pause the video and see if you can calculate these two resistances by the way if you want a little bit more help on this then have a look at my video how to answer any electricity question it's not only metals that can change resistance we can have a thermistor and you have a circuit that responds to changes in temperature a thermistor's resistance decreases if the temperature increases so in essence it does the opposite to a metal in this case if the temperature increased the resistance of the thermistor would go down as does its share of the total PD that means the PD measured by this voltmeter will increase this could be the basis of a temperature sensor for your central heating for example an ldr is a light dependent resistor very similar to a thermistor but resistance goes down with increased light intensity not temperature so this circuit could be on the top of a street lamp light intensity goes down resistance of the ldr goes up as does its share with the voltage this could then be connected in some way to the light bulb so it turns on as it gets dark we know that power is the rate of energy transferred so energy divided by time however when it comes to electricity we can also calculate it with this equation too P equals VI power equals voltage PD times current moreover if we substitute Ohm's law into this we swap the V for ir and we end up with the alternative equation P equals I times I times r or P equals I squared r the electricity that comes out of a battery is DC or direct current that's current that only Flows In One Direction AQA these days have an obsession with calling it direct PD which is pointless but means the same thing direct PD is a potential difference that is only in One Direction and this results in direct current Mains electricity that comes out of your socket is AC alternating current resulting from an alternating PD in the circuits in your home the neutral wire stays at a potential of zero volts while the Live Wire well its potential varies but it averages out to an equivalent of 230 volts so we say this is Mains voltage or Mains PD this alternating PD causes current to go back and forth fourth at a frequency of 50 hertz 50 times a second if you hooked up a battery and Mains electricity to an oscilloscope we'd see these two traces to see how the PD changes over time or doesn't change in the case of DC of course in a socket the wire with blue insulation around it is the neutral wire while Brown is the Live Wire the third yellow and green wire is the earth wire and that's connected to the pin at the top it's not necessary to complete the circuit and there should be no current flowing through it normally it's a safety wire that's connected to the outside of metal appliances like kettles or toasters so if anything goes wrong with the other wires inside of the kettle current will flow through it to the ground instead of through a person if they touch it which would give them an electric shock also in a plug a fuse is attached to the live wire which is designed to melt or blow if the current exceeds a certain number of amps usually 3 5 or 13 amps if something goes wrong in an appliance the current May well Spike so the fuse will blow before too much damage can be done to it or the user you may need to use P equals VI to calculate the normal operating current for an appliance to deduce what fuse should be used in the plug let's say that a microwave draws 800 watts of power from the mains what fuse would it need well we know power is 800 watts we know PD or voltage is 230 volts because it's Mains so we use P equals IV to find the current rearrange it current is equal to P divided by V but that's 800 divided by 230 that gives us 3.5 amps we can't use a 3 amp fuse otherwise it would just blow under normal operation so we go for the next one up a 5 amp fuse a 13 amp fuse would work as well but the current would have to increase to that before it blows and that could be more dangerous electricity is applied to homes and businesses by the National Grid a network of power stations cables and more that transmit it across the country the current produced by a power station is quite large so much so that if it went straight into the overhead cables you see above you when you're out and about a huge amount of energy would be lost as heat due to the resistance of the cables to reduce this energy lost Transformers are used triple people you'll need to know exactly how they work for paper 2 but for now we all just need to know what they achieve a Step up Transformer outside the power station increases the transmission voltage to over a hundred thousand volts as P equals VI and power stays roughly the same in this process if PD goes up current must decrease as a result this decrease in current means less energy and power is lost due to Heating and we can see this from the other power equation P equals I squared r lower the eye lower the P lost of course having such a high voltage going into homes would be dangerous and unnecessary so we have a Step Down Transformer nearby to reduce it down back to a more safe 230 volts the reason one goes up while the other one goes down is because electrical power is equal to voltage or PD times current V times I in an Ideal World the power in and out of a transformer should be the same that would mean that it's 100 efficient so V and I are inversely proportional we can therefore say that V times I for the primary coil is equal to V times I for the secondary coil this is the basic makeup of a transformer the primary coil is connected to the power station in this case the secondary coil is connected to the overhead cables there are more turns on the secondary coil which means it's a Step up Transformer the voltage will increase the current will decrease the cars are wrapped around a soft iron core get this into your head right now though there is or should be no electricity or current in the core instead the electricity is wirelessly transmitted from one coil to the other how is this well it's because the alternating current in the primary coil produces its own magnetic field and the iron core acts like a guide for it we use iron by the way as it's easily magnetized and demagnetized it works well as a guide this magnetic field then induces a voltage and current in the secondary coil in order for a current to be induced though a wire must experience a change in the magnetic field which is why we must use AC if we use DC in the primary chord it would make a magnetic field but it would be static which cannot induce a current in the secondary coil the ratio of turns in the coils is equal to the ratio of the voltages if the secondary coil has double the turns it has double the voltage and therefore half the current so we can say NP divided by NS equals VP divided by vs you can also flip the whole thing when it comes to rearranging it to find vs or NS A Step Down Transformer at the other end of the cables steps the voltage back down to a safer PD of 230 volts which means it must have fewer turns on the secondary coil if insulating materials that is materials that aren't good conductors are robbed against each other electrons are transferred from one to the other the object electrons are removed from is left positively charged as electrons are negative themselves and the object they're added to is now negatively charged oppositely charged on objects attract each other positive and negative if they have like charge that just means the same charge I.E both positive or both Negative they repel each other instead if you touch a Van de Graaff generator electrons are taken from every part of your body including your hair leaving all of you positively charged your positive head repels your positive hairs and the hairs also repel each other too two objects with different size charges produce an electric field between them we can't see this field but we can represent it by drawing lines the arrows on the line show the direction of the field and that's always positive to negative they show the direction of electrostatic force exerted on a positive charge if we put one in the field the space between if we put a negative charge in there instead it would move in the opposite direction to the field lines which makes sense because it would be attracted to the positive object even a single object that is charged creates a field for example this is what the field around the Van de Graaff generator would look like this is called a radial field by the way as the lines are diverging getting further apart but the further you go from the bore this shows that the strength of the electric field gets weaker with distance leave a thumbs up if you found this helpful I've also made videos covering whole papers to help you in your revision click on the card to go to the playlist for your board or have a look on my channel for more see you next time