Q4. Assertion: an isolated copper wire at equilibrium, with no battery or other emf, carries no current, though its free electrons move fast. Reason: after each collision an electron moves off in a random direction, so the electrons have no preferred direction of motion. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion (NCERT pages 82–83).
Q5. A copper wire 1.0 m long has a cross-section of 1.0 × 10−7 m2. With 8.5 × 1028 free electrons per cubic metre, how many free electrons does it hold?
Volume = 1.0 × 1.0 × 10−7 = 1.0 × 10−7 m3, so the number is 8.5 × 1028 × 1.0 × 10−7 = 8.5 × 1021 (density from NCERT Example 3.1, page 87).
Q6. Taking one conduction electron per copper atom, as NCERT does, about how many free electrons are there in 1.0 g of copper (atomic mass 63.5 u)?
6.0 × 1023/63.5 ≈ 9.4 × 1021 atoms, so about 9.4 × 1021 free electrons (NCERT Example 3.1, page 87).