Question

A lightweight metal ball hangs by a thread. When a charged rod is held near, the ball moves toward the rod, touches the rod, then quickly “flies away” from the rod. Explain this behavior.

Short answer

Different methods of charging is responsible for above behavior.

Step-by-step solution

Basically there are three methods of charging. that is charging by friction charging by conduction and charging by induction. in above behavior we can observe charging by induction and conduction

usually in charging by induction due to external charged material, nearer end of reference material get opposite charge and farther end gets same charge. but due to distance, the effect of nearer end is more pronounced. hence in first part ball due to force of attraction moves toward the charge.

In second part the behavior can be explained by charging by conduction. in  

In charging by conduction, the charged material when in contact with other uncharged material, the uncharged one gets same charges as that of charged material. Thus as the ball move towards the charge it touches the rod. then charging by conduction starts. hence then quickly “flies away” from the rod.