Static electricity | Physics | Khan Academy

One of my favorite things to do with a balloon is to rub it on my wife's hair because it makes the hair stick to the balloon. Isn't that pretty cool? Why does it happen? And now, if I bring the balloon close to a few pieces of paper, look! The pieces of paper jump, and they try to stick to the balloon. What's going on over here? Let's find out!

Let's start by looking at the situation before I rub the balloon on my wife's head. At this point, I haven't started the rubbing yet.

Okay, we know that everything is made up of atoms, and atoms have an equal number of positive protons and negative electrons, making them neutral. That's why most things around us are usually mostly neutral. So, this balloon is mostly neutral right now, and my wife's hair is also mostly neutral right now.

But what happens when I start rubbing? Well, for that, let's zoom in a little bit. If you were to zoom in, these are the atoms of the balloon, and these are the atoms of the hair. Rubbing, you know what it does. When you rub things, it has a tendency to transfer electrons from one material to another. Over here, it turns out that because of rubbing, electrons from the hair get transferred into the balloon or to the atoms of the balloon. That's the effect of rubbing.

Now you may be wondering, well, why do the electrons go from the hair to the balloon and not back? Well, it turns out that different materials have different tendencies. Hair has a bigger tendency to lose electrons, and rubber, which this balloon is made of, has a high tendency to gain electrons. So, it depends on the material.

It turns out that for hair and rubber, electrons will just tend to move from hair to the rubber or to this particular balloon.

Okay, another question you may be wondering about is, why electrons? Why not say protons or neutrons? Well, you might recall that protons and neutrons are very strongly held inside the nucleus in the core of the atom due to the strong force of attraction. They will not be participating over here. It's only the electrons, which are on the outside of the nucleus, that interact. Because of that, it's the electrons that get transferred.

Anyways, because of the electron transfer, look at what has happened! The balloon has gained a lot of electrons. Therefore, it's no longer neutral. It's no longer neutral because it has gained a lot of electrons. It now has a negative charge. Similarly, the hair is no longer neutral because it has lost a lot of electrons. It has excess protons now, which means it is positively charged.

By the way, when things get charged like this, we say that they have static electricity, or they have static charge. We call it static charge because, in most materials like rubber, hair, wool, or glass, charges usually don't move. They don't move much; therefore, they are called static.

Such materials in which charges don't move inside of them, we call them insulators. In contrast, if you take metals, charges will move inside the material, so for them, we call them conductors. We'll talk about conductors and moving charges in future videos.

But anyways, because charges are not moving, we say this is static electricity. So, we can now see one method of generating static electricity that is by rubbing things. You mostly do this for insulators anyway, because my hair is positively charged, and the balloon is negatively charged, they will attract each other. That's why when we move the balloon away, the hair starts standing because it is getting attracted to the balloon. Beautiful, isn't it?

Now, I'll tell you what blows my mind away. If you take a strand of hair like this, it is being pulled up by the balloon, right? But remember that this same strand of hair is also being pulled down by the entire Earth due to the force of gravity. The entire Earth is pulling down on it, and yet, a mere tiny balloon with a little bit of charge on it can pull up and overcome the force of gravity. This just shows how strong electrostatic force is compared to gravity. It's orders of magnitude stronger than the force of gravity.

Another thing to note is that because now all the hair strands are positively charged, they tend to move away from each other. As you can kind of see over here, it's not very dramatic because it's not a lot of charge, but if I could somehow get a lot of charge, one of the ways to do that is by using a machine called a Van de Graaff generator. It builds up a lot of charge. When you touch it, the charges get transferred into your body, and then your body gets a lot of static charge.

And look, as a result of that, look! The hair just stands up like that! It does so because they're all repelling each other. Static electricity is pretty cool.

But now comes a curious question: why did the balloon attract those tiny pieces of paper? Yes, the balloon was negatively charged, but the pieces of paper were not charged. I didn't charge them at all! Then why did they get attracted?

Let's see if we can model them. So, here's the situation. We have the negatively charged balloon, and we have these uncharged pieces of paper, meaning neutral paper. Why would it attract it?

Well, for that, let's zoom in a little bit. If you were to zoom in, this is how we can model the atoms inside the paper. It's pretty much the same thing that I've drawn over here. We have the positive core, all the protons over here, and we have the electrons around over here.

Now let's think about the forces. Okay, this negative charge will attract the positive, but it will repel the negative. Since you have the same amount of positive and negative, pretty much everything should cancel out, and therefore we would expect absolutely no force of attraction or repulsion over here, right?

But here's the thing. Because the negative charge of the balloon is repelling the electrons, and the electrons can move within the atom, they will not move out. They will not move within the material, but they can still move kind of within the atom. Because they're being repelled by the balloon, these electrons will slightly move farther away from the balloon.

As a result, look at what happens to our atom. Let me show that over here. Okay, here we go! Look at what happens to our atom. The negative charge and the positive charge kind of gets separated. The positive charge will end up becoming slightly closer to the balloon, while the negative charge ends up moving away from it because of the repulsion.

We give a name to this separation. We say that our atoms are now polarized. Why do we call it polar? I think there's a 'z' in there. I think we use a 'z,' yeah! There’s a 'z' over here.

Okay, anyways, why do we say it's polarized? Well, there’s a word pole over here. Pole refers to the two poles of electricity: positive and negative. So it's polarized, meaning we are separating the charges still within the atom. The atom is not broken or anything, but it's slightly separated, slightly polarized.

As a result of that, now look what happens! Because the positive charge is slightly closer to the balloon, the force of attraction is slightly bigger. Remember, when charges are closer, the forces are stronger. Electrostatic forces are stronger! So the attractive force is slightly bigger than the repulsive force, and therefore, there will now be, on this atom, a net force towards the balloon.

You can think that there's now a thin layer of positive charge deposited over here. Because that positive charge is closer to the balloon, there will now be a net force acting on it. That's the reason why it gets attracted to the balloon.

And again, because the pieces of paper are pretty light, that force can overcome the force of gravity. So look! We have examples in which things are sticking. In one example, it's sticking because they have the opposite charge, but in the other example, the paper particles are sticking not because they have the opposite charge. They're not positively charged; they're uncharged, but because of polarization as well.

Things can stick! Small, light things can stick to charged objects.

Well, one last question we could be having is: why doesn't static electricity last very long? Right? I mean, you can have fun for some time, but then it will very quickly go away, and the reason for that is whatever electrons you lost or gained, well, eventually, you will lose them back or you will gain them back.

Remember, there is so much stuff around us. There's so much air, there's water all around us, so it's very easy to get back our electrons or lose the extra electrons that we have. Sometimes, when a lot of charge gets built up, you can also lose it as a spark. A spark is when charges get built up so much that the force of attraction is so much that charges literally jump. Electrons literally jump from one object to another. They move through the air—that's when we get sparks.

Now, most of the time, these sparks are mild, and they're annoying. They're not usually very harmful, but nature has a way of taking things to a whole new level. When sparks get super duper strong, they become lightning.

But I'm getting ahead of myself over here. The takeaway of this video is: static electricity is a lot of fun! So do try these at home!