What is this balloon? I rub it against my cat. Stick to the wall. Okay, Tilly. Hold still. I have to do an experiment on you. Sorry. You're probably thinking the answer is static electricity, and you would be right. You've learned about that in lots of previous science classes. This idea that when you have two opposite charges, one positive and one negative, they're going to attract one another. They're going to provide an attractive force. That makes sense when I rub this balloon against my cat. The cat and the balloon are both originally neutral. When I rub the balloon on the cat, some of the electrons from the cat's fur move into the balloon. This is called charging by friction. You'll notice that the electrons don't really evenly spread out through the balloon because the balloon is a good electrical insulator. That means it can hold electrons, but the electrons can't move freely around the balloon. It is important to note at this point that it's the electrons that are moving, not the protons. The protons are inside of the nucleus of the atom that makes it the cat and the balloon. And the nucleus and type of atom is determined by how many protons are in it. So if I was gaining and losing protons from my cat or balloon, I'd have totally different elements making them up. I'd have some sort of like crazy radioactive cat. You see that when you're charging through friction, the cat gains one nature of charge, and the balloon the opposite. Charging two neutral objects by friction always produces equal magnitude, but opposite nature of charge in the objects. Now that my balloon is charged, I can touch it to something else and make the electrons move from the charged balloon to this neutral screwdriver. In this case, the electrons are going to move from the balloon to the screwdriver through the process of conduction. In conduction, there has to be contact, and electrons actually move from one object to another. You'll notice here we're using a metal screwdriver. During conduction in a screwdriver, the electrons spread out evenly along the metal. That's because metal is a good conductor. But what about the balloon in the wall? Here we have that negatively charged balloon from the original experiment, and I was able to stick it to the wall, but the wall is neutrally charged, so why do negatives become attracted to neutral objects? Conduction is clearly taking place, because the wall and the balloon are touching, but that would mean both the wall and the balloon are negatively charged. So why are they attracted? The secret is in what's happening inside the wall. As the negatively charged balloon comes close to the wall, it applies a repulsive force to the electrons in the neutral wall. Now these electrons can't move very far, because the wall is an insulator, but they can move a little. And as those electrons move away from the balloon, the protons left over in the wall, still on moving, are now closer to the balloon, so we get this attractive force. And there are many more protons on the edge of the wall closest to the balloon, than there are the few electrons that moved over through conduction, so what we have is an overall net attractive force.