Question

A positively charged ball A hangs from a string. A non-conducting ball B is brought near ball A. Ball A is seen to be attracted to ball B. 

(a) Give reason why it is NOT possible to determine whether ball B is negatively charged or neutral for sure from the above experiment alone. 

(b) Suggest any ONE additional experiment with ball B required to determine whether ball B is negatively charged or neutral for sure.

Answer 1

(a) The attraction between A and B could be due to the following reasons:

• B is negatively charged and hence A and B attract each other.
• B is neutral. The two balls attract each other due to the polarization of molecules in neutral ball B. 

It is not possible to determine for sure that ball B is negative or neutral from this experiment alone.

(b) Possible additional experiments:

• A known neutral ball can be brought near ball B (without ball A nearby).

If the neutral ball is attracted to ball B, then ball B is negatively charged for sure.

If there is no interaction between the two balls, then ball B is neutral for sure. 

• A known negatively charged ball is brought near ball B (without ball A nearby). 

If ball B repels the negatively charged ball, ball B is negatively charged for sure. 

On the other hand, if ball B is attracted to the negatively charged ball, then ball B is neutral for sure.