Introduction to circuits

Añadido: 2026-05-17

[00:00:02]here's an introduction to circuits so what I've drawn here is a wire and inside that wire there are lots of electrons that are free to move now they're not moving anywhere in particular at the moment they might be jiggling around a bit but if we uh put apply an electric field on this by putting a negative charge at this end and a positive charge at this end those electrons are all going to feel a force away from the negative towards the positive so you're going to get a lot of uh net drift in that direction because of that applied electric field what we're going to do now is essentially what happens in a circuit we're going to take this negative and bump it up against the end the positive and bump it up against the end of a negative and create a circuit and I'm going to draw that with the negative at one end the positive at the other this is the negative terminal and positive terminal of a battery and join it up into what we call a circuit and if we draw it like this we've got basically a y joining up the positive and the negative end now positive is longer the symbol is is longer than the negative basically got one Y which is attaching U the positive and the negative ends of the terminal now we know that electrons flow from the negative end to the positive end so I'm just going to draw in electron flow going that way now in a very strange way uh and it's an accident of History electron flow is actually the opposite direction to what they thought the charge large carriers were moving and so from the beginning of time and still to this day we use something called conventional current which is can be thought of as the way that positive charge carriers would flow if they could of course they don't so it's just the opposite direction of the electron flow now of course circuits don't just have a wire uh around them uh they actually have um if that happened it would essentially be a short circuit and things would get very hot very fast because a lot of energy would be dissipated um just in the wire very quickly large currents would flow and we'll talk about that later so what we're going to do here is put in a symbol for a resistor here and that's just a rectangle or it could be any um thing that we call a load so in this case it's a resistor but we could have another load of any type um such as a lamp or a motor something like that now if we were to measure the potential difference across the battery or the cell a cell um that I've drawn here we would find that the electrons going in here would have a much would have a lower potential energy than the ones coming out here and we know that from the definition of potential difference which is the work done per unit charge we can also uh write that as it's the amount of energy or the change in energy per unit charge when it moves from one point to another so essentially we can think of the electrons gaining potential energy as they um comp the ones coming into this side of the battery compared to the ones coming out they have a higher potential energy they have higher potential to do to um to do work as they move through the circuit and it's just really important to keep that definition in mind that the potential difference across a battery or as we'll see later across um a resistor it's the amount of energy per unit charge per of course again its units are we've seen this before Jew per Kum or volts of course this uh potential difference also indicates the amount of it's a measure of the push there is to push these electrons around the circuit so for a high potential difference that's kind of looking at a a high uh electric field from one end to the other there's a bigger Force per charge to push um electrons uh through this circuit if we assume that this wire is a or almost a uh a perfect conductor we actually get no energy loss as the electrons move through that perfect conductor so when we look at where that energy is actually lost in the system as it goes around the circuit it actually uh is dissipated so turned into another form of energy like heat in the case of the resistor own that that that transformation from potential energy of the electrons to the kinetic energy and uh the um the heat that's generated happens at the resistor only and not during not within the wires so if we go back and make it even more confusing and think about uh conventional current and just imagine for just a moment that we've got positive charges flowing the opposite direction to the electrons we would actually get an increase in potential uh an increase in potential as we go um from the negative terminal of the battery to the positive terminal the potential be the same along here so there'll be no potential difference anywhere in the Y and over here there will be a loss of potential as we go from one side of the resistor to the other so as we move from one side of the battery to the other we get a positive we go from one side of the um resistor to the other we get a negative potential difference and if you think about it carefully the same thing will apply if you um just Define the direction of of electron flow um but we won't worry too much about that now just all you need to know is that you get a potential an increase in potential across the battery and a decrease in potential across a load we could put into this circuit an ameter oops I've just lost part of my circuit here you could put an ameter in here and measure the current now it doesn't matter where we put this ameter we could put one uh here on this part or across this top part of the circuit here it doesn't matter the current or the rate of of charge flow through those wires and in fact through the resistor will be the same no matter where you are in that in that single circuit we're talking about a series circuit here um where there's just one path through through the circuit the current will be the same and the current of course which we have seen before is defined as the rate of flow of charge so essentially the amount of charge in passing a point uh every unit of time so every second so this would be uh kums per second or amps which we give the symbol of a and this is the same throughout the circuit the resistor that I've referred to here which we're going to look at the you know what defines resistance a little bit more in the next video um but a resistor just by wave introduction provides resistance to current flow and that resistor uh provides resistance basically by the nature of the material that's there the uh electrons that are flowing might bump into things more often um so it basically slows everything down by way of collisions with the material that makes up the resistor and if there are more collisions it means that more um energy is being dissipated so a resistor uh dissipates the energy uh that the charge charges have they start off with a potential energy and then they um convert that within the resistor to another form of energy uh usually uh heat in the form of a when we're talking about just a normal resistor so I've just got a quick three-part example to talk to you about just move that up there so our example is in 1 minute we have 15 colums of charge doing 300 jewles of work in a motor it could be um in a resistor or a lamp it doesn't matter so uh the first question that I uh you need to calculate is what is the current flowing in that circuit and of course all we need to do is look at our definition of current it's the amount of charge PL flowing per unit time and our charge is 15 our time is 60 seconds which gives us 0.25 amps so for the second part of the question what is the potential difference across the motor so potential difference is the amount of energy lost across the motor per unit charge so for every charge that goes through there how much energy is lost uh sort of a coolum of charge how much energy is lost so we have 300 jewles of energy because that's how much work is been done divided by the amount of charge which is 15 which of course gives us 20 volts so we know we have a potential difference of 20 volts across that motor the last part is what is the potential difference or the voltage provided by uh the power supply and we know that the voltage provided by the power supply just write PS there power supply is the same as the voltage lost across the motor Because by the time those uh charges get back I'll just go back up to the diagram up here the charges that come out of here lose all their potential energy as they go across this load and then when they come come back to here um they uh are given again that 20 volts in this case it's lost 20 volts across here and so on and so on it goes around and around so all the energy that the Chargers gain across the battery they lose across the resistor and this is a little bit conceptually hard to get a hold of and we we'll talk about it more in class so just down here the potential difference across the battery is the same as across the motor so it's at 20 volts