Question

A rod is brought near an uncharged pith ball electroscope. What conclusion would you draw about the charge on the rod if the pith ball does not move?
A. Rod is uncharged
B. Rod is charged
C. Its weight is very light
D. Can’t say

Answer 1

Hint:Given Pith ball is a neutral charged body means there are an equal number of protons and electrons in pith ball. This pith ball experiment is used to find the charge nature of the given rod, that is whether the rod is positively, negatively or neutrally charged.

Complete step by step answer:
Given,
Case 1: Assume rod is charged(positive or negative)
When rod is brought near the pith ball the pith ball does not move. The principle here is induction of charges. If the rod is positively charged it will attract all the negative charges in the pith ball and repel all the positive charges in the pith ball. Due to this side closest to rod gets accumulated with negative charge and attraction force between the positive charge of rod and negative charge of the pith ball will make the pith ball to move towards rod. This happens the same even if the rod is negatively charged. But in question the condition is pith ball does not move. So Case 1 is not correct.

Case 2: Rod is uncharged
Now we assume that the rod is uncharged. So the rod will also have an equal number of protons and electrons. When two neutral bodies interact they neither repel or attract each other. So neither the rod nor the pith ball moves.
Case 2 satisfies the given condition.
Therefore Option (A) is correct.

Note: Don’t get confused with the concept of positive, negative and neutral charged bodies in induction.