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Frostbite Theater

Here's a Question! - Can it Roll?

An empty soda can is placed on its side so that it's free to roll. A plastic pen is given a static charge and is placed close to the can. What happens to the can?

Announcer: Frostbite Theater presents... Cold Cuts! No baloney!

Joanna and Steve: Just science!

Joanna: Hi! I'm Joanna!

Steve: And I'm Steve!

Joanna: Here's a question for you...

An empty soda can is placed on its side so that it's free to roll.

Steve: I rub a plastic pen in my hair, giving the pen a static charge.

Joanna: What happens if the pen is placed close to the can?

Steve: Does nothing happen?

Joanna: Does the can roll towards the pen?

Steve: Does the can roll away from the pen?

Joanna: Or, does it depend if the pen is negatively charged or positively charged?

Pause the video now if you'd like to think it over before we show you what happens.

Steve: Clearly, the can is attracted to the pen, but why does an electrically charged object attract an object with no charge?

Joanna: While the can is electrically neutral, it's made from atoms and atoms contain positively charged protons and negatively charged electrons. It's just that the can has equal numbers of protons and electrons.

Steve: But... the can is made from metal and metal's a good conductor. That means the can's electrons are mostly free to move around, so we can rearrange how the can's electrons are distributed.

Joanna: Let's say that the pen is positively charged. If we bring it close to the can, electrons will be attracted to the positive charge and move to that side of the can. You end up with positive next to negative, which attracts!

Steve: What if the pen were negatively charged? Would still attract the can? Yes. It's just that the arrangement of charges on the can would be reversed, with a positively charged area close to the pen.

Joanna: But, what about the other side of the can? If the can is neutral and we're making one side of the can negative, then the other side has to be positive. If one side of the can is attracted to the pen, isn't the other side repelled?

Steve: Yes. Yes it is. But, the electric field made by the pen isn't uniform. The further you are from the pen, the weaker it is. So, the force of attraction between the pen and the near surface of the can is stronger than the force of repulsion between the pen and the far surface of the can. Attraction wins out because it's closer.

Joanna: Thanks for watching. I hope you'll join us again soon for another question!

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