If wattless current flows in the AC circuit, then the circuit is :

Solution

Physics sir · 3 steps

Wattless means zero average power, and that needs a phase gap of exactly 90° between V and I.

1. Average power. In an AC circuit the average power is P = Vᵣₘₛ Iᵣₘₛ cos φ, where φ is the phase difference between voltage and current.
2. Wattless condition. Current is called wattless when it flows but uses no power on average. That needs cos φ = 0, so φ = 90°.

   Circuit	Phase difference φ	cos φ
   Purely resistive	0°	1, maximum power
   Purely inductive	90° (current lags)	0, wattless
   LCR series	between -90° and 90°, never exactly ± 90° while R > 0	R/Z, not zero
   RC series	between 0° and 90°	R/Z, not zero

3. So. Only the purely inductive circuit has φ = 90°, so the current in it is wattless. The answer is (b).

The idea to keep. Any resistance in the circuit makes cos φ = R/Z nonzero, so power is used. Wattless current needs a pure L or pure C circuit, where φ = 90°.

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