BBC Bitesize - GCSE Physics (Single Science) - Current, voltage and resistance - Revision 4
Revise calculating current, measuring potential difference and energy transfer. Find out about charge, resistance and ohms law with BBC Bitesize. Did you know that electrical current is affected by the voltage and resistance in a circuit? In this lesson, we'll use Ohm's law, which tells us. Alternating current (AC) circuits carry energy due to the coordinated vibrations of neighboring electrons. While DC circuits require single electrons to (slowly!).
AC current diagram Think about it this way: Yet a tremendous wave like a tsunami is capable of sinking ships and destroying docks purely due the combined force exerted by the tremendous mass of water that it carries. Electric circuits are very much the same waythe individual electrons travel remarkably slowly through the circuit, yet there are so many of them that they can do all sorts of useful things.
How do electrons in alternating current circuits carry energy? Alternating current AC circuits carry energy due to the coordinated vibrations of neighboring electrons. While DC circuits require single electrons to slowly! The mechanism for this is pretty clever: This shoving is periodic: When the power source pulls the backmost electron back to his original position, the guy in front of him is then able to scoot back a little bit as well, and so on and so forth until the electrons throughout the wire are back in their original spots.
So you can visualize an AC circuit as a bunch of electrons spaced evenly apart, where the guys in the back periodically shove the guys in front of them, creating ripples that travel through the entire line until reaching the device that is connected to the power source. And the unit here is the ohm, is the ohm, which is denoted with the Greek letter omega.
- Current and Resistance
- Current and resistance
So now that we've defined these things and we have our metaphor, let's actually look at an electric circuit. So first, let me construct a battery.
Current, voltage and resistance
So this is my battery. And the convention is my negative terminal is the shorter line here. So I could say that's the negative terminal, that is the positive terminal. Associated with that battery, I could have some voltage. And just to make this tangible, let's say the voltage is equal to 16 volts across this battery. And so one way to think about it is the potential energy per unit charge, let's say we have electrons here at the negative terminal, the potential energy per coulomb here is 16 volts.
These electrons, if they have a path, would go to the positive terminal. And so we can provide a path. Let me draw it like this. At first, I'm gonna not make the path available to the electrons, I'm gonna have an open circuit here.
I'm gonna make this path for the electrons. And so as long as our circuit is open like this, this is actually analogous to the closed pipe. The electrons, there is no way for them to get to the positive terminal. But if we were to close the circuit right over here, if we were to close it, then all of a sudden, the electrons could begin to flow through this circuit in an analogous way to the way that the water would flow down this pipe.
Now when you see a schematic diagram like this, when you just see these lines, those usually denote something that has no resistance. But that's very theoretical. In practice, even a very simple wire that's a good conductor would have some resistance. And the way that we denote resistance is with a jagged line. And so let me draw resistance here. So that is how we denote it in a circuit diagram.
Now let's say the resistance here is eight ohms.
Introduction to circuits and Ohm's law
So my question to you is, given the voltage and given the resistance, what will be the current through this circuit? What is the rate at which charge will flow past a point in this circuit? Pause this video and try to figure it out. Well, to answer that question, you just have to go to Ohm's law.
Current and Resistance
We wanna solve for current, we know the voltage, we know the resistance. So the current in this example is going to be our voltage which is 16 volts, divided by our resistance which is eight ohms.
And so this is going to be 16 divided by eight is equal to two and the units for our current, which is charge per unit time, coulombs per second, you could say two coulombs per second, or you could say amperes.
And we can denote amperes with a capital A. We talked about these electrons flowing, and you're gonna have two coulombs worth of electrons flowing per second past any point on this circuit.
And it's true at any point, same reason that we saw over here. Even though it's wider up here and it's narrower here, because of this bottleneck, the same amount of water that flows through this part of the pipe in a second would have to be the same amount that flows through that part of the pipe in a second. And that's why for this circuit, for this very simple circuit, the current that you would measure at that point, this point, and this point, would all be the same.
But there is a quirk. Pause this video and think about what do you think would be the direction for the current? Well, if you knew about electrons and what was going on, you would say, well, the electrons are flowing in this direction. And so for this electric current, I would say that it was flowing in, I would denote the current going like that. Well, it turns out that the convention we use is the opposite of that.
And that's really a historical quirk.
When Benjamin Franklin was first studying circuits, he did not know about electrons. They would be discovered roughly years later.