Question

In series R-L-C AC circuit,

• A. Current is always behind inductor voltage
• B. Current is always behind source voltage
• C. Voltage across resistance is always a head of source voltage
• D. Voltage across capacitor is always behind current

Answer 1

Answer: (A), (D)

The correct options are
A Current is always behind inductor voltage
D Voltage across capacitor is always behind current


Say, current in the circuit is given by,
$i=i_0\sin \omega t$.
Then the voltage across the inductor is given by,
$V_L=L\frac{di}{dt}=L\omega \cos \omega t=\omega L\sin (\omega t+{90}^{\circ })$
Therefore, the voltage across the inductor leads the current by ${90}^{\circ }$.
Similarly,
$V_C=\frac{1}{C}\int idt=-\frac{1}{\omega C}\cos \omega t=\frac{1}{\omega L}\sin (\omega t-90)$
Therefore, the voltage across the capacitor is behind the current by ${90}^{\circ }$.

The phase difference between the current and the source voltage depends on $\omega$. So, current can be ahead or behind the source voltage.