Q4. Assertion: in a metal wire carrying a current to the right, the free electrons drift to the left. Reason: the current is reckoned by the net charge carried forward, q = q − q, so negative charge moving left counts as current to the right. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion (NCERT Eq. (3.1), pages 81–82; NCERT Section 3.3, page 82, says the current in solid conductors is carried by electrons).
Q5. The charge passing a cross-section is Q = 5t + 2t coulomb, with t in seconds. Find the current at t = 2.0 s.
I = dQ/dt = 5 + 6t, so at 2.0 s, I = 5 + 24 = 29 A (NCERT Eq. (3.2), page 82).
Q6. Suppose a lightning stroke carries a steady 20 000 A for 1.0 ms. How much charge does it carry to the ground?
q = It = 20 000 × 1.0 × 10−3 = 20 C (NCERT Eq. (3.1), page 82). NCERT notes that lightning currents are of the order of tens of thousands of amperes and are not steady (pages 81–82).