An ac voltage is applied across a single circuit element. For which element does the current lead the voltage?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Capacitor
Take the applied voltage as \( v=v_m\sin\omega t \). The phase of the current relative to this voltage depends on the single element in the circuit.
Option (1) Inductor — Incorrect. In a purely inductive circuit \( i=i_m\sin(\omega t-\pi/2) \), so the current reaches its maximum a quarter period after the voltage. It lags the voltage by \( \pi/2 \).
Option (2) Capacitor — Correct. The current is the rate at which charge flows onto the plates, \( i=dq/dt \), and it comes out as \( i=i_m\sin(\omega t+\pi/2) \). So the current reaches its maximum a quarter period before the voltage. It leads the voltage by \( \pi/2 \).
Option (3) Resistor — Incorrect. Across a resistor \( i=(v_m/R)\sin\omega t \), so the current and the voltage are in phase and neither leads the other.
NCERT (2026-27):
AC Voltage Applied to a Capacitor – “… in the case of a capacitor, the current leads the voltage by π/2.”