Identify the valid statements relevant to the given circuit at the instant when the key is closed.
A. There will be no current through resistor R.
B. There will be maximum current in the connecting wires.
C. Potential difference between the capacitor plates A and B is minimum.
D. Charge on the capacitor plates is minimum.
Choose the correct answer from the options given below:

Solution

Physics sir · 3 steps

At the instant the key closes, the uncharged capacitor acts like a wire, so all 5 V sits across R.

1. Uncharged at start. Before the key is closed the capacitor has no charge. Charge cannot appear instantly, so at t = 0 we still have q = 0 and V_(AB) = q/C = 0.
2. Full voltage on R. With no voltage across the capacitor, the whole 5 V appears across R. The current is I = 5/R, its largest value. As the capacitor charges, q and V_(AB) rise and the current dies away.

   q = CE(1 - e^(-t/RC)), I = E/R e^(-t/RC)

   Statement	At t = 0	Valid?
   A: no current through R	I = 5/R flows through R	No
   B: maximum current in wires	Series circuit, so the wires carry the same maximum 5/R	Yes
   C: V_(AB) is minimum	V_(AB) = 0	Yes
   D: charge on plates is minimum	q = 0	Yes

3. So. B, C and D are valid and A is not, so the answer is (d).

The idea to keep. Charging RC circuit: at t = 0 the capacitor acts as a short (zero charge, zero voltage, maximum current). At t → ∞ it acts as an open circuit (full charge, zero current).

Answer from the NTA final answer key. Solution by PrepWiser.