Physics Review Question

Unit: Electricity
Year: 1991 Question#: 25
Question: When a rod is brought near a neutral electroscope, the leaves diverge. Which statement best describes the charge on the rod?

(1 ) It must be positive
(2 ) It must be negative
(3 ) It may be neutral
(4 ) It may be positive or negative

What is this question really asking?


Explanation by: Derek N
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Answer 1
It must be positive

INCORRECT

Positive charges cannot be transferred but they could pull the negative charges upward, leaving the leaves at the bottom positve so they would repel. We cannot be sure that this is what is happening because a negatively charged object would force negatives down into the leaves causing them to repel. return to top























Answer 2
It must be negative

INCORRECT

A negatively charged object would force negatives down into the leaves causing them to repel. But we can not be sure this is the cause because a positively charged object could pull the negative charges upward, leaving the leaves at the bottom positve so they would repel. We cannot be sure which is happening. return to top



























Answer 3
It may be neutral

INCORRECT

If it was neutral, nothing would have happened. return to top


























Answer 4
It may be positive or negative.

CORRECT

We cannot be certain that it is positive or negative. So we must say that it may be either positive or negative return to top


























What's this question really asking?
What do we know about the charge on the rod if it causes the leaves on the electroscope to diverge when brought near it (but not touching it)?
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