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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