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

Concepts and reason

The concepts required to solve the given question is Electrostatic induction.

Initially, find what happens to the end A of the rod when the ball approaches it closely for the first time. Later, find the arrangement of the charges on the rod. Finally, how does the end A of the rod react when the charged ball approaches the end A and then find the reaction of end A when the charged ball approaches it.

Fundamentals

Electrostatic induction is a redistribution of electric charges on an object due to influence of nearby charges. According to the sign of charge on the charged body, positive charge develops one end and negative on the other side.

A charged object has unequal number of negative and positive charge. When a charged object is brought in contact with uncharged conductor, then the transfer of electron take place for electrostatic equilibrium. The process of charging an uncharged object by bringing in contact with another charged body is called charging by conduction.

(A)

When the negatively charged small metal, ball is brought near the end of the rod, all the positive charges in the in the rod tends to move towards end A whereas the all the negative charges inside the rod tends to move inside the rod tend to move towards the end B. Thus, the ball is not strongly repelled, weakly repelled, and weakly attracted.

When the negatively charged small metal, ball is brought near the end of the rod, all the positive charges in the in the rod tends to move towards end A whereas the all the negative charges inside the rod tends to move inside the rod tend to move towards the end B. As a result of which the small metal ball gets strongly attracted when it closely approaches the end A for the first time.

(B)

When the negatively charged metal ball comes in contact with the rod, the positive charges from the rod gets conducted to the metal ball and gets neutralized. But there is a continuous supply of negative charges to the metal ball. Thus, there is not positive charge on the end B and negative charge on the end A, there is no negative charge on end A with end B remaining neutral, and there is positive charge on end A with end B remaining neutral.

When the negatively charged metal ball comes in contact with the rod, the positive charges from the rod gets conducted to the metal ball and gets neutralized. But there is a continuous supply of negative charges to the metal ball. Thus, there is not positive charge on the end B and negative charge on the end A, there is no negative charge on end A with end B remaining neutral, and there is positive charge on end A with end B remaining neutral. Eventually, all the positive charges from the rod gets neutralized and after the neutralization of all the positive goes from the rod, negative charges start to conduct from the metal ball towards the rod.

(C)

The many number of contacts of the negatively charged metal ball with the rod, the negatively charges spread evenly on both ends of the rod. Thus, end A of the rod neither strongly attracted, weakly attracted, weakly repelled nor It is neither attracted nor repelled when the charged ball approaches it after a great many previous contacts with end A.

The many number of contacts of the negatively charged metal ball with the rod, the negatively charges spread evenly on both ends of the rod. Hence, when the negatively charged metal ball approaches end A, it gets strongly repelled by it.

Ans: Part A

The small metal ball gets weakly attracted when it closely approaches the end A for the first time.