A constant voltage of $25 \mathrm{~V}$ is applied to a series $L-R$ circuit at $t=0$, by closing a switch.…

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 $t=0$ ?
  1. $0 \mathrm{~V}, 25 \mathrm{~V}$
  2. $12.5 \mathrm{~V}, 1.25 \mathrm{~V}$
  3. $10 \mathrm{~V}, 15 \mathrm{~V}$
  4. $25 \mathrm{~V}, 0 \mathrm{~V}$

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 $=0 \mathrm{~V}$

Asked in: AP EAMCET 2011

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