Electricity Basics (Ohm's Law) - GCSE & A-level Physics

electricity is one of those things that people find tricky sometimes because it's quite conceptual but by understanding just a few basic ideas it all becomes very clear let's start with a battery and a circuit and an ammeter in it as well now let's say that this battery says six volts what on earth does that mean well technically we could say the voltage supplied is the same thing as something that we call electro-motive force or EMF for short in other words it's the amount of energy supplied to electrons we know that if there wasn't a battery here then electrons wouldn't be flowing around the circuit but there is a battery which gives the electrons energy and that enables them to flow around the circuit but the thing is is that we have billions of these electrons going around the circuit so it's not very good to talk about the energy supplied to an individual electron so we say energy supplied to one Coulomb of electrons Coulomb is the unit of charge and it turns out that one electron has a charge of one point six times ten to the minus nineteen coulombs so you can see then we need not only billions but trillions and trillions these electrons to make it warm Coulomb of charge that's why it's better to talk about coulombs to melt the energy given to one electron by the way we call this eve for short little e so here is that voltage supplied it's the same thing as electromotive force so I probably should say that epsilon there that's our EMF six volts in other words each Coulomb is given six joules of energy what about if I wanted to find out how its energy they lose when they go across this resistor here I'm going to call that R and I need a voltmeter volt meters are always put across a component that way it can measure what we call voltage across resistor that's also known as potential difference PD for short and this measures the energy lost by each Coulomb of charge so the electrons travel from here we do say that current conventional current goes right there conventional current goes from positive to negative but actually electrons flow in the opposite direction but that's just because they decided on conventional current befallen you that it was the electrons that moved so here we have energy supplied to a Coulomb of electrons a Coulomb of charge by the battery here and here our voltmeter is measuring how much energy the electrons a Coulomb of electrons losses between entering and exiting the resistor EMF or voltage or PD is always equals to energy to find it by charge we could call that energy or work done if this battery is supplying the electrons with six volts worth of energy that's six joules per Coulomb and there's nothing else in the circuit that would cause them to lose energy apart from this resistor what reading should we expect to see on this what we to here we should expect this to say six volts as well because they should be losing all of that six spots worth of energy in this resistor here and then flowing back to the other side of the battery ready to be filled up with energy again don't forget that in the circuit we just have one complete loop of electrons that are all flowing together like water being pumped around a pipe system that we're going to find out a little bit later on that this probably isn't going to be six volts if the EMF of the battery is six volts and that's to do with something called internal resistance but will it end that doesn't exist for now so this is a really important equation here voltage PD or EMF is equals to energy per unit charge it's just telling you how many joules a Coulomb of electrons or generally Coulomb charge has so if we have our circuit again here we also had an ammeter in there as well now you should know that the more energy is applied to the electrons the faster they're going to flow so higher EMF I'm going to put brackets there and voltage as well but technically we should call it you know if the higher EMF equals electrons flow faster and that's the same thing as a higher current we can't spell today if you replace this battery with a 12-volt battery you would expect the electrons to flow twice as fast and that is what we see or we should see anyway current we give the letter I we don't give it a letter C because that's reserved for capacitance so current is the rate of flow of charge in other words how fast the electrons are flowing we're not saying how many electrons pass through the circuit every second we talk about how many coulombs so we say coulombs per second so that means that current is equals to charge divided by time so what should we expect to read on this voltmeter here the potential difference across this resistor there should be 12 volts as well because all the electrons should be losing all 12 volts worth of their energy by the time they go through the resistor there so if we draw a graph a voltage against current and we're talking about voltage across this resistor here and we should get a nice straight line like that because if we give the electrons more energy to lose across a resistor that's a higher voltage they should be flowing quicker through the resistor so we have a constant don't wait our gradient is constant that means that V divided by I is a constant for this resistor we call this constant resistance and that's where our equation V equals I R comes from that is Ohm's law we can think of resistance as how hard it is for currents flow through a components so the higher the resistance the high risk for current to flow in other words you need more energy a higher voltage or high EMF to push the electrons through if I was to draw a resistor I had a higher resistance on here then they would have a steeper gradient but for an individual resistor we should have a straight line on a V I graph what is the unit for resistance we could use volts per amp because of course the unit for voltages involves the unit for current to this amps but it has its own special unit and that is the ohm there's a capital Omega which is the last letter of the Greek alphabet now we talked about energy being lost across a resistor where does the energy actually go in a resistor electrical energy turns into heat simple as that whenever you have a resistor and you have electrons flowing through it they lose their energy they still flow out to the battery to be refilled with energy again but they lose their energy as they go through this lost to heat resistors heat up if you wanted to find out the energy lost every second this is the same thing as power power is just the amount of energy the number of joules lost every second you can write out the unit joules per second or I can write it like this that's going to be V times I voltage times current but we also know that V equals IR so if we substitute this into here we end up with the power dissipated in a resistor is equals to the current squared times the resistance that we said just now and a VI graph for a resistor nice and straight that means that we have a constant R constant resistance so therefore we say it ohmic but not everything acts like this what about if we have bulb instead right lamp so we draw it in a circuit crossing a circle now over lab what we see happening is it starts off straight but then it starts to curve upwards like that what's happening the gradient is increasing with the current so the resistance is increasing and this is because the filament in the bulb is heating up and that means that the particles in the metallic structure of the filament are actually vibrating more which means that the electrons are colliding with the ions more which makes it harder for it to flow so this happens with pretty much any piece of metal we see the resistance increasing when you get to higher currents incidentally this shows that a resistor isn't made out of metal subtly made out of a semiconductor what else do we have we have thermistors we trawl that like this and ldr this a light dependent resistor now with LDLs and thermistors they will actually be ohmic at the same temperature or line level respectively so if the thermistor is at the same temperature then it will be ohmic if an LDR is has the same amount of light falling on it then it will be Omega however that does change with temperature and light level if you expose a thermistor to a higher temperature this graph actually goes down what's happened is resistance has decreased thing with an LDR if you give it more light its resistance decreases as well so when we have a low temperature or low light a thermistor or an LDR resistance will be high whenever you have a high temp or high light as a low resistance this is going to be really important later on when it comes to potential dividers and we have one more component now what's weird is that we actually draw it the other way around in this case we have voltage on the bottom and we have currents on the y-axis there notice that we've got positive and negative voltage this is for a diet we draw a diode like this incidentally if this is an LED then it would be the same as the by ode but if we just draw a diet it's just the arrow underlined their symbol for a diode should imply how it's special special because if you try and send current one way through it so you apply a voltage across it in one direction it will have zero current current will not flow in one direction and then as soon as you hit the right voltage in the right direction then current will flow very easily so here we have very high resistance in one direction if you have very low in the other none of these components here mind which way you put them around in a circuit but diodes are very specific if you put the wrong way around in a circuit this here usually is about 1 volt when current starts to flow so that's our VI graphs for all of the components that we need to know for GCSE and a-level so I hope that helps if you got any questions or if you feel like missed anything out then please put in a comment down below and I'll see you next time

Channel: Science Shorts

Views: 149,135

Rating: 4.9655643 out of 5

Keywords: current, technique, soft, shorts, charge, 12th, 9th, light, equations, high school, english, flow, past, 11th, school, lecture, 1-9, resistance, gradient, crash course, exam, voltage, 10th, fun, potential difference, thermistor, electrons, resistor, spoken, tutorial, lesson, amps, 9-1, p.d., mcat, investigation, grade, sats, edexcel, naked, ocr, filament, unintended, graphs, lamp, science, explained, diploma, aqa, experiment, temperature, homeschooling, revision, diode, questions, ohm's, ldr, coulombs, volts, v=ir, emf, paper, practical, british

Id: NWCi2jUh2o8

Length: 12min 46sec (766 seconds)

Published: Tue Mar 28 2017