Q4. Assertion: the total charge of an isolated system stays the same. Reason: charge-carrying particles can never be created or destroyed. Are both true, and does the reason explain the assertion?
The assertion is true but the reason is false: charged particles can be created or destroyed (a neutron decays into a proton, an electron and a neutral antineutrino); only the net charge cannot change (NCERT pages 4–5).
Q5. A copper cube of side 1 cm contains about 2.5 × 1024 electrons. What is their total charge? Take e = 1.6 × 10−19 C.
2.5 × 1024 × 1.6 × 10−19 = 4.0 × 105 C of negative charge, balanced by the same positive charge of the nuclei (NCERT page 6 gives the electron count).
Q6. Two identical isolated metal spheres, with no nearby external charges, carry +5 nC and −9 nC. After they touch and separate, what charge does each carry?
The total, −4 nC, is conserved and, by symmetry, shared equally: −2 nC each (NCERT pages 4–5 and 7). The equal split needs identical conducting spheres; spheres of different sizes share the total unequally.
Q7. (a) Explain the meaning of the statement ‘electric charge of a body is quantised’. (b) Why can one ignore quantisation of electric charge when dealing with macroscopic, that is large-scale, charges? (NCERT Exercise 1.4, page 42)
(a) Any charge a body can carry is a whole-number multiple of the basic charge e: q = ne, with n an integer (NCERT page 5). (b) Large-scale charges hold enormous numbers of e: 1 µC is about 1013 of them, so steps of one e are far too small to notice and the charge behaves as continuous (NCERT page 6).