Understand concepts of electricity and magnetism

1. Sep 5, 2010

aMajor

Hi guys,
I was answering some questions on my physics homework and I didn't understand why the answer was what it was...

1.) A hollow uncharged metal ball hangs from a silk thread. A glass rod has been charged by rubbing with rayon. When it is brought close to the ball without touching, the ball
-doesn’t change its position butbegins to revolve.
-remains in its originalposition.
-is attracted.
-is repelled.

I said it was attracted because the glass rod is negatively charged and the positive nucleus of the metal ball is attracted to it through induction.

The follow up question to the first question was:
2.) You now touch the ball from Question 2 with the end ofthe charged rod. Which answer best describes what happens next?
-The ball remains in contact with the rod for a long time.
-The ball remains in contact momentarily,then is repelled.
-The ball bounces off and onto the charged end of the rod, as it picks up and loses charge.
-The ball bounces and revolves.

The right answer is option number 2...but I don't understand why. I thought the ball would pick up the charge from the charged rod, but I guess it wouldn't lose charge...

I'm not sure why the ball is repelled from the rod. Can anyone explain?

Thank you very much!!!
-Duylam

2. Sep 7, 2010

Delphi51

Welcome to PF, Duylam!

I would not have used the term "positive nucleus" here. Maybe being picky, but really the electric field from the glass rod charge pushes the electrons on the ball as far away as possible - to the opposite side of the ball. This leaves a net positive charge on the near side of the ball. The attraction is due to the "d" in F = kqq/d² being smaller for the near positive charge than for the far negative charge.

And you have the second answer when you say negative charge is transferred to the sphere. Then the negative sphere is repelled by the negative rod. Of course sufficient charge must be transferred to overcome the attractive force due to the induced charge separation. This is a very tricky business at first when the rod and sphere are very close together, but once repulsion begins and the sphere moves away, the induction effect is reduced. Fascinating stuff and very interesting to see the real thing in the lab. It works best with a small sphere hanging by two threads and a large fixed sphere in place of the rod.