A2 Daily A Level Physics question
Which statement must be true immediately after closing the switch when a 9.0 V ideal battery is connected through a 100 Ω resistor to two identical, initially uncharged 100 µF capacitors that are either in parallel or in series?
Answer
The correct answer is B.
Correct: B — The initial current is 0.090 A in either arrangement, because uncharged capacitors initially behave like a wire. At t = 0+, the capacitor voltage is zero, so the full 9.0 V is across the 100 Ω resistor, giving I = 9.0/100 = 0.090 A, independent of how the capacitors are connected. A The current is not set by the equivalent capacitance at t = 0+, but by V and R; a larger capacitance changes the time scale, not the initial current, so 0.180 A is incorrect. C This contains two errors: series does not add capacitances (it reduces the equivalent), and the initial current does not depend on capacitance; 0.180 A is therefore wrong. D No energy stored initially does not imply zero current; in fact, the initial current is maximum because the uncharged capacitors initially present nearly zero voltage across them.