Friday, February 02, 2018

Induction .... Nailed it!

Induction is one of those things that make students go "Whhaaaatttt???"
You have probably seen and demonstrated a moving wooden meterstick like this:
Usually when I do that demo in regular Physics my students call me a witch. I don't exactly correct them. I wanted an equally "wow!" demo but didn't want to repeat myself in AP Physics C. While searching for something completely different I stumbled upon this small image of an induction experiment.

At first I wasn't sure if I could do it but after some experimenting I found a set up that worked. I set a nail on a styrofoam cup (made some grooves in it so that it would stop the nail from rolling) instead of a metal rod. I couldn't find any metal rods that weren't gross and rusty. I used a rubber rod and wool but any friction kit combination should work.

First I charged the pith ball through induction with the rod. Then I moved the nail point close but not touching the pith ball. I would recharge the rod and bring it close but not touching the nail. The pith ball on the other end would repel. My students were amazed. They watched me do it but they still had to think about which item was charged vs neutral. It led to lots of additional questions, some we were able to answer experimentally, some would probably require some more charge:

1. What happens if you touch the nail with the rod? The nail would then have the same charge as the rod and pith ball. I presume the nail would continue to repel the pith ball even when the rod were removed. We tried this but could not confirm with the small amount of charge we had.

2. If the pith ball is charged won't it be attracted to the neutral nail without the rod inducing a separation of charge? Yes! You'll notice in the video below that I move the nail in after the pith ball is charged. Otherwise I found that the pith ball would pull towards the nail immediately.

3. Would it work with an insulator? I presume so but didn't get a chance to try it.

Below is the video of the demonstration for absent students:

Depending on the strength of your charge source you could set up a wide variety of things like this as discrepant events for your students to puzzle over.