A constant voltage of $25 \mathrm{~V}$ is applied to a series $L-R$ circuit at $t=0$, by closing a switch. What is the potential difference across the resistor and the inductor at time $t=0$ ?

• Correct answer
  A

  $0 \mathrm{~V}, 25 \mathrm{~V}$
• B

  $12.5 \mathrm{~V}, 1.25 \mathrm{~V}$
• C

  $10 \mathrm{~V}, 15 \mathrm{~V}$
• D

  $25 \mathrm{~V}, 0 \mathrm{~V}$

Solution

At, $t=0$, switch closes
Current $i=i_0\left[1-e^{-t R / L}\right]$ So, at $t=0, i=0$ So, potential across inductor $=25 \mathrm{~V}$ and across resistor $=0 \mathrm{~V}$

Hence, the correct answer is option: 'A'.