In an ac circuit, the instantaneous current is zero, when the instantaneous voltage is maximum. In this case, the source may be connected to :
A. pure inductor.
B. pure capacitor.
C. pure resistor.
D. combination of an inductor and capacitor.
Choose the correct answer from the options given below :

Solution

Physics sir · 3 steps

Current zero when voltage peaks means a 90° phase gap, so look for circuits with no resistor.

1. What the condition means. If i = 0 exactly when V is at its peak, current and voltage are a quarter cycle apart. The phase difference is φ = 90°.

   cos φ = R/Z = 0 ⇒ φ = 90°

   Circuit	Phase of current	Current zero at voltage peak?
   A. pure inductor	lags by 90°	yes
   B. pure capacitor	leads by 90°	yes
   C. pure resistor	in phase, φ = 0	no, current is also at its peak
   D. inductor and capacitor	± 90°, since Z = |X_L - X_C| is purely reactive	yes

2. Why LC works. With L and C together and no resistance, the net reactance is X_L - X_C. Whichever one wins, the current is 90° out of phase with the voltage.
3. So. A, B and D fit and C does not. The answer is (c).

The idea to keep. Zero current at peak voltage means zero power factor. That needs R = 0: any purely reactive circuit (L, C, or LC) qualifies, and a resistor never does.

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