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.
In the copper wire experiment, the plastic rod has a charge of \(-5\ \mathrm{nC}\) at the start. Then \(1.25\times10^{10}\) electrons move from the rod to the pith ball through the wire. What is the charge left on the rod? (Ignore any charge on the wire. Take \(e=1.6\times10^{-19}\ \mathrm{C}\).)
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \(-3\ \mathrm{nC}\)
The rod, the wire and the pith ball form an isolated system, so their total charge stays the same. The negative charge gained by the pith ball is lost by the rod.
Charge carried by the electrons that move:
\( \begin{aligned} q &= ne \\ &= 1.25\times10^{10}\times1.6\times10^{-19} \\ &= 2\times10^{-9}\ \mathrm{C}=2\ \mathrm{nC} \end{aligned} \)
These electrons carry a charge of \( -2\ \mathrm{nC} \) to the pith ball. Charge left on the rod:
\( \begin{aligned} q_{\text{rod}} &= -5-(-2) \\ &= -3\ \mathrm{nC} \end{aligned} \)
NCERT (2026-27):
Charge is conserved – “Within an isolated system consisting of many charged bodies, due to interactions among the bodies, charges may get redistributed but it is found that the total charge of the isolated system is always conserved.”