Andy R

Video transcript

Right. Today I'm going to share something with you. It's a useful trick to help understand circuits. OK, now consider the following question, all right? We've got one cell with one bulb, and that produces normal brightness. Remember, the current flows from the positive terminal here to the negative terminal. Why? Because there's what we call a potential difference across here, therefore across here. When there's a potential difference, current will flow, and in this particular case, the lamp will light up, so it flows this way, and the light shines with normal brightness. All right, now consider the scenario we've got below here. You can see that. Right. Bulbs A and B, are they shining normal, or brighter than normal, or are they dim, or are they off? Let's consider number one. This is fairly straightforward. There's a switch here in the middle, so as a result, the current flows from the positive terminal through both bulbs, and then back to the negative terminal. As a result, we have one cell being shared by two bulbs, so therefore they will be actually dim. But what about part two here? Can you see that the switch has been closed, and as a result, which way is the current going to flow? Is it going to flow this way? Is the current going to flow this way as well? And what about flowing this way? Well, we don't know, but we can use something called the potential in a circuit to sort it out. All right, let's have a look at what we've got down here. All right, let's just move out a little bit. This area here is all to do with actually talks about potential difference, but I want to actually just concentrate on this particular circuit here. This is a two-cell battery. All right, they're in series, and you can see that the positive end is at the here and the negative end is here. What we can say is if each cell is worth 1.5 volts, then the total potential difference, or total potential here, is 3 volts. Okay, so I'm going to put that in. So, 3 volts. And that potential, okay, I'm going to put that narrow there, put a narrow in, is also here as well. That wire connected to this terminal here is also at 3 volts, and because that wire is connected to this wire, it's also at 3 volts. Okay, can you see that? And, in addition, that 3 volts is also connected here and here. So, the potential of these wires, the electric potential, 3 volts. That's 3 joules of energy, the Coulomb charge that passes through them. So, if we look at this particular point, start over here, well, this is the negative terminal, and the negative terminal is said to be at zero volts. There, and because these wires are connected, they have also that zero volts potential. Okay, so what can we say about what is the potential here? It's 3 volts. And what's the electric potential here? It's zero volts. So, therefore, the difference between here and here is 3 volts minus zero volts, which is 3 minus zero, 3 volts across here. And the voltmeter would measure potential difference from here to here of 3 volts. And so, as a result, current will flow through the resistor through to here. Okay, just to go through that again, the positive terminal's at 3 volts. So, therefore, this wire, this wire, and this wire here are also at 3 volts. This wire also is at 3 volts next to voltmeter. The negative terminal's at zero volts. So, this one here is zero volts. This wire is also zero volts. Zero volts here and zero volts here. So, the potential difference across the resistor is 3 volts. Current flows this way. Okay. The voltmeter measures 3 volts, measures potential difference. The difference potential here and here. Okay, now let's consider the problem before and let's apply this idea to that situation. Okay, we're looking at number two. Here we are. So, this is 1.5 volts because it's a single cell, just like a battery in one of your toys or something like that. If I put in text here, I'll put 1.5 volts. And let's put that over to here. All right. And I'm going to put some arrows in and let's see where that 1.5 volts goes. It's there because it's connected to that terminal. It's connected to 1.5 volts. It's also connected to here and here. So, both of those are at 1.5 volts. So, again, 1.5 volts is also here. And 1.5 volts is also here as well. Now, because 1.5 volts is here, it is connected to this wire here and this wire here. Those wires are also at 1.5 volts. Okay. Now, in the negative terminal, and this is at zero volts as before. So, let's put text here and put zero volts to measure the electric potential. Well, it's indicated then certainly. And it's here and it's here. So, it means zero volts, right, is located here and here. I'll move that across a little bit. It's a little bit clearer. Okay. All right. So, then let's have a look at the potential difference across the bulbs. We've got 1.5 volts here, okay, because these wires here are all connected to that positive terminal at 1.5 volts. So, the electric potential here is always at 1.5 volts. And can you see across here between this side of the bulb, B, and this side of the bulb, B here, the potential difference is zero because 1.5 volts there, 1.5 volts there. What's the difference between these two 1.5 volts? It's zero. If we have zero potential difference, no current will flow. And as a result, bulb B is actually off. Okay. So, bulb B is off. What about bulb A? Well, we've got 1.5 volts here and we've got zero volts here, which means that the potential difference across here is 1.5 minus zero volts, which is 1.5 volts. A current will flow. And as a result, because the potential difference across here is 1.5 volts, this cell powers bulb A through normal brightness. So, why don't we put an hour there. Okay. So, normal brightness for bulb A. That is just looking at the potential in the circuit and being able to work out which bulb is on, which bulb is off. Okay. On and off in this particular case, because that switch is closed. Okay. Just to go through that again, we have a cell 1.5 volts, the positive end is 1.5 volts. That means these wires here are all connected to it, and therefore they're all at 1.5 volts in potential, electric potential. The potential difference across B is 1.5 minus 1.5. As a result, there's no potential difference across it because the difference 1.5 minus 1.5 is zero. Therefore, bulb B is off. Bulb A, the negative terminal is said to be at zero volts. We can see all the way down here, it's zero volts. And all the way over here, it's 1.5 volts. So, as a result, the potential difference is 1.5 volts, 1.5 minus zero, 1.5 volts. And therefore, we have an electric current flowing through bulb A. And it's normal brightness because it's 1.5 volts across it. Okay. All right. Hope that makes some sense. Now, can you apply it to these problems? Okay. We'll go through them.