Q4. A glass rod rubbed with silk is brought near a plastic rod rubbed with fur. Do they attract or repel, and why?
They attract: the glass rod is positive and the plastic rod negative, and unlike charges attract (NCERT page 2).
Q5. After rubbing, a piece of silk carries a charge of −3.2 × 10−9 C. How many electrons did it gain? Take e = 1.6 × 10−19 C.
n = (3.2 × 10−9)/(1.6 × 10−19) = 2.0 × 1010 electrons, and the glass rod it rubbed has lost the same number (NCERT pages 3 and 5).
Q6. Assertion: after a glass rod is touched to the silk it was rubbed with, the two no longer attract light objects. Reason: the unlike charges on the rod and the silk neutralise each other’s effect. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion (NCERT page 2).
Q7. When a glass rod is rubbed with a silk cloth, charges appear on both. A similar phenomenon is observed with many other pairs of bodies. Explain how this observation is consistent with the law of conservation of charge (NCERT Exercise 1.5, page 42).
Rubbing creates no charge: electrons move from the glass to the silk. The rod is left positive and the silk becomes negative by exactly the same amount, so the total stays zero, as before rubbing (NCERT pages 3–4).
Q8. A polythene piece rubbed with wool has a negative charge of 3 × 10−7 C. (a) Estimate the number of electrons transferred, and say from which to which. (b) Is there a transfer of mass from wool to polythene? (NCERT Exercise 1.11, page 42)
(a) n = q/e = (3 × 10−7)/(1.6 × 10−19) ≈ 1.9 × 1012 electrons, from the wool to the polythene, since the polythene became negative. (b) Yes, a tiny one: the electrons carry their mass, about 1.9 × 1012 × 9.11 × 10−31 kg ≈ 1.7 × 10−18 kg, too small for an ordinary balance to detect (electron mass as in NCERT Example 1.3, page 9).