Read the following passage and answer the questions.
A student studies the charges on four very tiny plastic beads, P, Q, R and S. All four beads are neutral at the start. She charges each bead by rubbing it with a different material. A sensitive meter then gives the charge on each bead. Her readings are: P, \(+8.0\times10^{-19}\ \mathrm{C}\); Q, \(-4.8\times10^{-19}\ \mathrm{C}\); R, \(+3.2\times10^{-19}\ \mathrm{C}\); and S, \(-1.6\times10^{-19}\ \mathrm{C}\).
Next, she puts all four beads in a small closed box. The beads can touch one another inside the box, but no charge can enter or leave it. She also compares each reading with the magnitude of the charge on an electron, \(1.6\times10^{-19}\ \mathrm{C}\). Her teacher tells her that charged bodies in everyday experiments carry much larger charges, of a few microcoulombs.
The charges on the beads are small whole-number multiples of \(e\). For a body with a charge of a few microcoulombs, the quantisation of charge can be ignored because
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → such a charge is about \(10^{13}\) times \(e\) or more, so steps of \(e\) are too small to notice
Charge is quantised. It changes only in steps of \( e \). Whether these steps can be seen depends on how large the charge is.
Option (1) quantisation holds only for electrons and protons, not for large bodies — Incorrect. Every free charge, large or small, is a whole-number multiple of \( e \). Quantisation holds for large bodies too.
Option (2) such a body gains or loses charge only in steps of \(1\ \mu\mathrm{C}\) — Incorrect. The step is always \( e=1.6\times10^{-19}\ \mathrm{C} \). A microcoulomb is only a unit for measuring charge.
Option (3) the value of \(e\) becomes larger when the charge on a body is large — Incorrect. \( e \) is a fixed basic unit of charge. It does not depend on the charge of the body.
Option (4) such a charge is about \(10^{13}\) times \(e\) or more, so steps of \(e\) are too small to notice — Correct. A charge of \( 1\ \mu\mathrm{C} \) contains about \( 10^{13} \) electronic charges. One step of \( e \) in so large a charge cannot be noticed, so the charge seems continuous.
NCERT (2026-27):
Quantisation of charge – “At the macroscopic level, one deals with charges that are enormous compared to the magnitude of charge e. … say 1 μC, contains something like 1013 times the electronic charge. At this scale, the fact that charge can increase or decrease only in units of e is not very different from saying that charge can take continuous values.”