Electric current | Physics | Khan Academy

Electricity that lights up a bulb looks very different than lightning strikes, but they're actually more similar than one might think because they both have electric current. So, let's understand what electric current is, how they are produced, and also get to understand a little bit about lightning.

So, what exactly is electric current? Well, think of electric current as a flow of net charge through any given area. Here's what I mean by flow of net charge: imagine you have a tiny cross-sectional area through which you have an equal amount of positive charges flowing to the right and left in any given time. Now, notice there is a flow, but there is no net flow, and therefore over here we say there is zero current.

Another interesting example is what if you have an equal amount of positive and negative charges flowing in the same direction at the same time, let's say through again a given cross-sectional area. Again, notice there is a flow of charges, but the total flow over here—the total charge that's flowing—is zero. So, the net charge is still zero, and therefore there is no electric current over here.

Okay, what about now? Now we do have electric current! Now we have a net positive charge flowing to the right. Over here, there is an electric current. Now we do have net negative charges flowing to the right. We do have an electric current. Okay, so it's a flow of net charge, but how do you measure it? Well, we measure it as the amount of charges flowing through any given cross-sectional area per second.

So, you can think of it as kums per second—how many kums are flowing per second—and the kums per second is also called, it's also called amperes, okay? Capital A, amperes. Just to give you typical numbers: your air conditioners, heaters—they draw about 10 to 15 amps of current. Your ceiling fans, tube lights, television sets: less than that, about one or two amps. And your smaller circuits, like you know the toy circuits and stuff, they would be even lesser. It would be a fraction of ampers.

But what about lightning? Lightning can have tens of thousands of ampers in them. Okay, how do we set up an electric current? How do we get an electric current? Well, for an electric current, we need a voltage. Just like how if you need to make a ball roll, you need to have a height difference which produces a gravitational potential difference across the end of, say, a plank. Similarly, if you need to set up currents through a wire, you need to have an electric potential difference across the ends of it.

When you have an electric potential difference, you can get a current. But you also need to make sure that there are some charges, that there are charges that are free to move in your material. Not all materials have that. For example, glass or plastic—they don't have free charges because if you look inside them, well, you can model them and say that you know what, the electrons inside these atoms are very tightly bound, so there are no free electrons to move. There are no charges to move. So, if you put a voltage across them, you'll probably get no current over here.

We call such materials insulators: glass, wood, plastic—these are examples of insulators. On the other hand, if you take metals of which wires are made, then you'll find that the outermost electrons are not tightly bound. As a result, they are free to move around the material. We call them free electrons, and since you have free charges available for motion, we call these materials conductors. Because if you put a voltage across them, well, these electrons can move and contribute to current. So, you need a voltage across a conducting medium for electric current.

Okay, but how do you get a voltage in the first place? Well, in small circuits, you probably already know, voltage is given by a battery. One end of the battery is at a higher potential; another end of the battery is at lower potential. When you connect it to a circuit, it provides the potential difference. But in larger circuits, for example, the circuits in our houses, well, the potential difference is provided by large electric generators in our power stations.

By the way, while drawing a battery in our circuit, we use a circuit symbol that looks like this: the longer line represents the positive terminal, and the shorter, thick line represents a negative terminal. So if you just draw this, we don't have to draw a big battery over here.

Anyways, even though we have a battery in this circuit right now, we don't have a current. We don't have a potential difference across this bulb. Why? Well, you can see over here, that's because the circuit is not closed. We say because there is some air in between; air is an excellent insulator, and therefore there's not going to be any current over here.

In order for there to be a current, we need to close the circuit, meaning we need to connect this gap. And that's where the switch is. This is a switch. So, if I close the switch like this, now the circuit is complete. Now there will be a potential difference across the ends of the bulb, and now there will be a current over here. I'm going to open the switch; there is no electric current. The circuit is broken. Close the switch; there's going to be an electric current now.

Because I compared charges moving through a ball rolling down, we might model it by thinking that, hey, when the charges are not, when there is no voltage, all the charges are at rest. Say the electrons over here are at rest. And when I do complete the circuit, the electrons are now nicely moving. But that's not a very accurate way to think about it; that's not a good model. Instead, a better model is if you were to peek inside the wire, we find that the electrons are randomly moving, bumping into stuff because they have a lot of energy, even when there is no voltage. So, they're not at rest; they're in fact moving at very high speeds.

But what happens when you close the switch? When we close the circuit, look, there is a potential difference, and therefore, there's an electric field setup in the wire. That electric field starts pushing on the electrons, and look, you can now see the electrons are slowly drifting to the left. It's that drifting motion that constitutes the current.

And what causes them to drift to the left? Again, there are some analogies which say that electrons push on each other, making them drift. But that's again not very accurate. A better way to think about it is that the battery produces the electric field. There's an electric field set up inside the wire; it's that electric field that is causing—that is pushing the electrons, making them drift to the left.

But wait a second! Why did I show that the electrons are drifting to the left over here? Let's think about it. So one way to think about it is, we could say that, hey, electrons are being attracted by the positive terminal of the battery and being repelled by the negative terminal of the battery, making the electrons go this way. But a question that could arise is, in the wire, that means the electrons are going from a lower potential to a higher potential—like going uphill. How does that make any sense? That was a point of confusion for a long time, so let's talk about it a little bit.

Okay, if I have a big positive charge and next to it, I keep a very tiny positive charge at rest, let's say, and I let go of it, then we know it gets repelled and it gains kinetic energy in this direction. Now, because energy is conserved, we could ask, where did that kinetic energy come from?

We say, ah, there, it must have come from potential energy. So as it goes from here to here, the system must lose potential energy, and therefore we can now say that, hey, this point represents a high potential region. This point represents a low potential region, and this represents the downhill direction for the charges.

As you go from here to here, its potential energy starts getting converted into kinetic energy, kind of like what happens to this ball rolling down. But what about negative charges? Well, negative charges will be exactly the opposite; they will get attracted by this positive charge, so they will gain kinetic energy this way. And for negative charges, it's the exact opposite: as they go from here to here, this is the direction in which they are losing potential energy and gaining kinetic energy.

So, this must be high; this must be low. This should represent the direction of the downhill. But now the problem is, which direction should we say is down for the charges? Well, we could say, hey, for positive charge, this is down, and for negative charge, this is up. But we decided, no, no, no! Let's just use one of these as a reference, and we'll just consider one direction as our actual down. So we decided, hey, whatever happens for positive charge, let's use positive charge as our reference.

Whichever direction positive charge naturally tends to go, we'll call that direction as our down for charges—right down in potential. Because of that reference, by definition, positive charges go down the electric potential; negative charges look to end up going up the electric potential—not because they're literally going to a higher potential energy region, no, no— they’re also going towards a lower potential energy region. It's just a reference because a reference point for high and low is chosen from the perspective of a positive charge.

Because of that reference, negative charges end up going up the potential; they have a natural tendency to go up the potential. Does that make sense? And therefore, electrons, which are negative charges, have a natural tendency to go up the electric potential.

Now, the final question we could have is the direction of the current. What is the direction of the current over here? Well, we could say, hey, whichever direction the charges are drifting, well, that itself could be the direction of the current—that's the most natural way to think about it, right? So, electrons are drifting this way, so let's say that that is the current. But again, there's a problem because we have positive and negative charges.

Remember that example where we had both positive and negative charges—equal positive and negative charges flowing through an area giving me zero current because the net charge over here is zero? Well, if I said that, hey, you know, whichever direction charges are moving, let's just call that direction as the current, then I have a problem because I could say that, hey, positive charge is giving me a current this way, negative charge is also giving me a current this way. But I know the total current must be zero, so that doesn't work.

Because, you know, these two, if I add up, I don't get zero; I should get a net current to the right. But that's not true; I know that the current should be zero. Again, to solve for that, we decided, hey, you know what? Whichever direction positive charges are moving, we'll say that is the direction of the current. And for the negative charges, we'll say the opposite is the direction of the current.

So, we said if the negative charges are moving to the right, we will say the direction of the current is to the left. And now look, now the total current becomes zero because you have right and left current canceling out. Now it makes sense! So the convention for direction—for choosing the direction of the current is whichever direction positive charges are going; that is the direction of the current.

If you have negative charges, opposite to whichever direction negative charges are going—that will be the direction of the current. Okay, now because in wires it's the electrons that are always drifting, those are the ones that constitute the current, and electrons are negatively charged particles, our convention for the current would be not the direction of the electron flow, but in the opposite direction of the electron flow. It would be this way!

So, the conventional direction of the current, notice, is in the opposite direction of the electron flow. And I'll tell you what can be frustrating! Because in most cases, we'll be dealing with electron flows, this will be frustrating; because in most cases, our conventional current will be in the opposite direction of the actual motion of the charges—the actual drifting motion of the charges. But it's unfortunate that electrons, which are the major charge carriers in most of the circuits, end up being negatively charged particles, and our positive charge is a reference for us.

So, it might slightly feel awkward initially, but you'll get used to it; don't worry too much! This now finally brings us to lightning. What exactly is lightning? Well, lightning is also an electric current, meaning a flow of charges. But how does it happen? And more importantly, lightning is a flow of charges through air. But air is an insulator, and we saw that insulators do not conduct electricity, so what's going on over here?

Well, we're not getting into too much detail, but it turns out that clouds usually have charges separated. The top of it is usually positively charged, and the bottom is negatively charged. Now, because the bottom is closer to the Earth, the negative charges push electrons of the Earth away from it because negative and negative repel, and as the electrons get repelled away, the surface of the ground will be mostly positively charged.

Now, during a thunderstorm, the charges build up because the air is an insulator; because there's no current over here, the charges can build up, and as a result, the potential difference becomes incredibly high. It can reach millions of volts! Now eventually what happens is that the electrons from the atoms of the air molecules, like oxygen, nitrogen, and all of those stuff, can actually get ripped apart.

And we'll not get into again the details of how that happens, but you can now imagine if electrons start getting ripped apart. Now we start having charges. Once we have charged particles in between, we have a conducting channel, and once we have that conducting channel, the charges can sort of get dumped into the Earth, and that's basically what we call a lightning.

Now, this lightning produces a lot of heat; that's one of the reasons it glows, and you can see it. But that heat also causes rapid expansions in the air, making the air vibrate. These vibrations eventually reach our ears after some time, and we call that thunder. So, look, lightning is an electric current, and guess what? Sparking that happens sometimes—those annoying sparks we get whenever we get charged up and we're trying to reach out to a doorknob, for example—it's very similar to what happens in lightning; it's a miniature version of lightning.