Force Between A and D: Repulsive?

[hotmail590]

There is a ball (A) is carrying a uniformly distributed unknown charge(wich may be zero) and an uncharged copper ball (D). A positive test charge (T) experiences highly attraction with ball (A) and (D). What is the nature of the force between balls A and D if they are brought very close together?


Choices are

attractive
repulsive
no force



What I think the answer should be is repulsive because since (T) is strongly attacted to both balls, that means both balls must have a same charge; therefore repel when brought close together.

However it says in the problem that the copper ball (D) is uncharged. What does that mean? It has equal amount of electrons and protons?

Also what does copper come to play here? I know it is an extremely good conductor of electricity but does this fact play an important role in this question?


Thank you for your help.

 

[Tide]

You know what sign the charge is on A. Since D is uncharged but nevertheless attracted to the test charge you must infer that the test charge induces charge on D. It is reasonable to conclude that were the test charge negative you would get the same result.

What would you then conclude about A interacting with D?

 

[hotmail590]

Then I would believe that both A and D are negatively charged and when placed near each other they will repel?

 

[Tide]

D is not charged!

 

[Mk]

A positive test charge (T) experiences highly attraction with ball (A) and (D).

Meaning D is negative?!??

 

[vanesch]

What you are missing here is that the TOTAL charge of a ball can be zero, but that negative charges move to the left of the ball, and positive charges move to the right of the ball. This can be caused (induced!) by a nearby charge, who will attract opposite charges near it, and (hence) have opposing charges far away from it. Because the force goes in 1/r^2, the near charges are "felt" stronger than the far away charges, which induces a net force between the ball and the inducing charge. In this case, it will be an attractive force.

 

[Mk]

There is a ball (A) is carrying a uniformly distributed unknown charge

I thought this meant all the balls? But now I realize it doesn't have to.