# Question Video: Understanding Static Electricity Physics • 9th Grade

A polythene rod is rubbed with a cloth duster, giving it a net negative charge. A second polythene rod is rubbed with a duster and the two rods are brought close together. What will happen?

02:13

### Video Transcript

A polythene rod is rubbed with a cloth duster, giving it a net negative charge. A second polythene rod is rubbed with a duster and the two rods are brought close together. What will happen?

So here we have a polythene rod — that’s a plastic rod — and a cloth duster. And at the outset, we can assume that both of these objects are electrically neutral; that is, they have no net electric charge. It’s important to realize though that this doesn’t mean that the rod and the cloth don’t have positive and negative charges in them. All it means is that they balance one another out. In each object, there are the same number of positive charges as negative charges. So on a net or overall level, they’re neutral.

Okay, we’re then told that the rod and cloth are brought in contact and they’re rubbed together. And once that has done, we’re told that overall the rod now has a net negative electric charge. What is that mean? That means that while the cloth and rod were rubbed together, electrons were moving from the cloth onto the rod. Since electrons have a negative electric charge, when they build up on the rod they give it a net negative charge as well. This also means by the way that after this interaction, the cloth has a net positive charge. That’s because it’s lost some electrons to the rod.

But anyway, our focus is on this negatively charged polythene rod. We’re told that with a second originally neutral polythene rod and a second originally neutral cloth duster, a similar process occurs. The two are rubbed together. And we know that because this is the same type of charge transfer process as before, this second polythene rod will also end up with a net negative charge. We’re then told that these two rods are brought close together. And the question is what will happen.

We can see that both of these objects have a net negative electric charge. In other words, they have the same charge, in this case negative. We can recall that when it comes to electric charges interacting together, opposite charges attract and like charges repel one another. We know that both our polythene rods have a net negative electric charge. In other words, they both have like charges.

That tells us that the rods will experience a repulsive force. They’ll try to push one another apart when we try to bring them together. That then is our answer to the question of what will happen when these rods are brought close together. We can say that the two polythene rods will repel each other.