# Question 1.SGPYQ.27: If, at t = 0^+ , the voltage across the coil is 120 V, the v......

If, at $t=0^{+}$ , the voltage across the coil is 120 V, the value of resistance R is

(a) 0 Ω        (b) 20 Ω

(c) 40 Ω      (d) 60 Ω

Step-by-Step
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$i_{ L }\left(0^{-}\right)=\frac{\text { Total voltage }}{\text { Sum of resistance }}$

$=\frac{120}{20+40}=2 A$            [Position 1]

In the inductor, the current does not change simultaneously.
Therefore,

$i_{ L }\left(0^{+}\right)=i_{ L }\left(0^{-}\right)=2 A$      [Position 2].

Voltage across the inductor at $t=0^{+}$

$V_{ L }\left(0^{+}\right)=120 V$

By applying KVL,

120 = 2(40 + R + 20) ⇒ R = 0 Ω

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