Q4. Assertion: an uncharged zinc plate lit by ultraviolet light becomes positively charged. Reason: the light ejects electrons from the plate. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion: losing negative electrons leaves the plate with a net positive charge (NCERT page 276).
Q5. In Lenard’s tube, why does the current stop as soon as the light is switched off?
The current is carried by electrons that the light ejects from the emitter; with no light, no electrons are ejected and none cross the tube (NCERT page 276).
Q6. Why did Lenard’s tube need to be evacuated, and why is the window made of quartz?
Without air the electrons can cross from emitter to collector without colliding with gas molecules (reasoned). Quartz lets ultraviolet light through to the emitter (NCERT page 277).
Q7. A photocurrent of 4.8 μA flows. How much charge, and how many electrons, reach the collector in one minute?
q = It = 4.8 × 10−6 × 60 ≈ 2.9 × 10−4 C, which is q/e = (2.88 × 10−4)/(1.6 × 10−19) = 1.8 × 1015 electrons (reasoned; e from NCERT page 275).