Assertion (A) : The quantisation effect of charge can be observed only at the microscopic level.
Reason (R) : The step size of electric charge is \( 1.6\times10^{-19}\,\mathrm{C} \).
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Both (A) and (R) are true and (R) is the correct explanation of (A).
Assertion (A) The quantisation effect of charge can be observed only at the microscopic level. — True. At the microscopic level the charges involved are a few tens or hundreds of \( e \), so they can be counted and appear in discrete lumps. At the macroscopic level the quantisation of charge has no practical consequence.
Reason (R) The step size of electric charge is \( 1.6\times10^{-19}\,\mathrm{C} \). — True. Every charge is an integral multiple of the basic charge \( e=1.6\times10^{-19}\,\mathrm{C} \), so a charge can increase or decrease only in steps of \( e \).
Why R explains A?
(A) says where quantisation can be seen. (R) gives the size of one step of charge, and this size decides where the steps show up.
One step is very small. So a charge of \( 1\,\mu\mathrm{C} \) contains about \( 10^{13} \) electronic charges. A change of one step in it looks the same as a continuous change. Hence (R) is the correct explanation of (A).
NCERT (2026-27):
Quantisation of Charge – “At the macroscopic level, one deals with charges that are enormous compared to the magnitude of charge e. Since e = 1.6 × 10–19 C, a charge of magnitude, 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. Thus, at the macroscopic level, the quantisation of charge has no practical consequence and can be ignored. However, at the microscopic level, where the charges involved are of the order of a few tens or hundreds of e, i.e., they can be counted, they appear in discrete lumps and quantisation of charge cannot be ignored.”