Read the passage and answer the following questions.
A metal rod held in hand and rubbed with wool will not show any sign of being charged. However, if a metal rod with a wooden or plastic handle is rubbed without touching its metal part, it shows signs of charging. Suppose we connect one end of a copper wire to a neutral pith ball and the other end to a negatively charged plastic rod. We will find that the pith ball acquires a negative charge. If a similar experiment is repeated with a nylon thread or a rubber band, no transfer of charge will take place from the plastic rod to the pith ball.
A gold-leaf electroscope has a vertical metal rod with two thin gold leaves at its lower end. The negatively charged plastic rod is made to touch the metal knob at the top of this rod. What happens to the gold leaves?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → They move apart, because charge flows down the metal rod to the leaves.
The rod of an electroscope is a metal, which is a conductor. Charge given to its knob can move along it to the leaves.
Option (1) They stay together, because the charge stays at the knob. — Incorrect. In a conductor the charge does not stay at one place. It moves down the metal rod.
Option (2) They move apart, because charge flows down the metal rod to the leaves. — Correct. Both leaves get negative charge. Like charges repel, so the leaves spread apart.
Option (3) They move apart only if the touching rod is positively charged. — Incorrect. The leaves get the same kind of charge as the touching body, whatever its sign. A negative rod also makes them diverge.
Option (4) They move apart, because the two leaves get charges of opposite signs. — Incorrect. Both leaves get charge from the same rod, so their charges have the same sign. Opposite charges would pull the leaves together.
NCERT (2026-27):
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. The degree of divergance is an indicator of the amount of charge.”
Electric Charge – “There are two kinds of electrification and we find that (i) like charges repel and (ii) unlike charges attract each other.”