Question

This problem explores the behavior of charge on realistic (i.e. non-ideal) insulators. We take as an example a long insulating rod suspended by insulating wires. Assume that the rod is initially electrically neutral. For convenience, we will refer to the left end of the rod as end A, and the right end of the rod as end B. In the answer options for this problem, "weakly attracted/repelled" means "attracted/repelled with a force of magnitude similar to that which would exist between two balls, one of which is charged, and the other acquires a small induced charge". An attractive/repulsive force greater than this should be classified as "strongly attracted/repelled".
A small metal ball is given a negative charge, then brought near (i.e., within a few millimeters) to end A of the rod. What happens to end A of the rod when the ball approaches it closely this first time?

Select the expected behavior.
strongly repelled
strongly attracted
weakly attracted
weakly repelled
neither attracted nor repelled

Now consider what happens when the small metal ball is repeatedly given a negative charge and then brought into contact with end A of the rod

After several contacts with the charged ball, how is the charge on the rod arranged?

Select the best description.
positive charge on end B and negative charge on end A
negative charge spread evenly on both ends
negative charge on end A with end B remaining almost neutral
positive charge on end A with end B remaining almost neutral
none of the above

Part C
How does end A of the rod react when the ball approaches it after it has already made several contacts with the rod, such that a fairly large charge has been deposited at end A?

Select the expected behavior.
strongly repelled
strongly attracted
weakly attracted
weakly repelled
neither attracted nor repelled

Answer 1

Answer:

A) weakly attracted

B) negative charge on end A with B remaining almost neutral

C) strongly repelled

Explanation:

A)

• As the negatively charged ball is brought close to the end A of the road, positive charges on this end try to move away from there, leaving a small negative charge in this end, as they are not free to move as in a conductor.
• As a consequence of this, the end A of the road will be weakly attracted by the negatively charged ball.

B)

• After several contacts between the ball and the end A of the rod, an appreciable quantity of negative charge has been deposited in the end A, repelling positive charges towards the end B.
• However, as the rod is an insulator, positive charges can't travel massively to the end B, so it remains almost neutral.

C)

• After repeated contacts that have deposited a significative negative charge on the end A (which can't distribute evenly as in a conductor), when the ball (negatively charged) is brought close to this end of the rod, as both bodies have the same type of charge, they will be strongly repelled each other.