A positively charged glass rod is brought near a light, uncharged metal-coated ball hanging from a silk thread, close enough for the ball to swing over and touch the rod. What happens to the ball?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → It is attracted, touches the rod and is then repelled from it.
The metal coating makes the ball a light conductor. Its electrons are free to move under the field of the rod. It can also take up charge from the rod when the two touch.
Option (1) It is attracted, touches the rod and stays stuck to it. — Incorrect. The ball could stay on the rod only if the two still attracted each other. After contact, both carry positive charge and repel.
Option (2) It is repelled from the rod without touching it. — Incorrect. Before contact the ball is uncharged, and a charged rod attracts a light uncharged body. Repulsion needs like charges, and the ball gets these only after touching the rod.
Option (3) It is not affected by the rod. — Incorrect. The field of the rod acts on the free electrons of the coating, so the neutral ball does feel a net force, towards the rod.
Option (4) It is attracted, touches the rod and is then repelled from it. — Correct. The motion has two stages:
Before contact, the rod draws the free electrons to the near side of the ball and leaves the far side positive. The nearer side is pulled more strongly than the far side is pushed, so the ball is attracted and swings to the rod.
On contact, charge flows between them. The ball gives up some electrons to the positive rod and is left positively charged. Like charges repel, so the ball is pushed away from the rod.
NCERT (2026-27):
Electric Charge – “… Many such pairs of materials were known which on rubbing could attract light objects like straw, pith balls and bits of papers.”
Electric Charge – “… When a charged object touches the metal knob at the top of the rod, charge flows on to the leaves and they diverge.”
Electric Charge – “… There are two kinds of electrification and we find that (i) like charges repel and (ii) unlike charges attract each other.”