## Mastering Physics Solutions: Charging an Insulator

Charging an Insulator

Part A = weakly attracted
Part B = negative charge on end A with end B remaining almost neutral
Part C = strongly repelled

Solutions Below:

Part A

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?

• strongly repelled
• strongly attracted
• weakly attracted
• neither attracted nor repelled

When the ball moves towards end A, a positive charge is induced at end A and a negative charge at end B. Since charge flow is inhibited in an insulator, the induced charges are small:

weakly attracted

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

Part B

After a great many contacts with the charged ball, how is any charge on the rod arranged (when the charged ball is far away)?

• 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

The ball will deposit a negative charge at end A. But since charge flow is inhibited in an insulator, the negative charge will more or less stay there and not spread out over the entire rod:

negative charge on end A with end B remaining neutral

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?

• strongly repelled
• strongly attracted
• weakly attracted
• weakly repelled
• neither attracted nor repelled

Since the rod acquired a negative charge at end A, the ball will be repelled. The problem says there was a fairly large charge deposited, so the repulsion will be strong:

strongly repelled

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