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Question:
Obtain the answers (a) to (b) in Exercise 7.13 if the circuit is connected to a high frequency supply (240 V, 10 kHz). Hence, explain the statement that at very high frequency, an inductor in a circuit nearly amounts to an open circuit. How does an inductor behave in a dc circuit after the steady state?
Answer:

Induction of inductor, L = 0.5 Hz

Resistance of the resistor R = 100 Ω

Voltage V = 240 V

Frequency v  = 10 KHz = 104 Hz

Angular frequency  ω = 2πv = 2 x π x 104 rad/s

  1. a) Peak voltage,

Maximum current  I0 = V0/√ R2 + ω2L2

                                       = 240√2/√ (100)2 x (2 π x 104)2 X (0.50)2

                                       = 1.1 X 10-2 A

(b) For phase difference ɸ

tan ɸ = ωL/R =  2 x π x 104 X 0.5/100 = 100 π

ɸ = 89.82° = 89.82 x π/180 rad

so, ωt = 89.82 x π/180

t = 89.82 x π/180 x 2 π x 104 = 25 μs

It can be observed that I0 is very small. Hence at high frequencies, the inductore amounts to an open circuit.

In DC circuit, after a steady state is achieved ω = 0.

Hence inductor L behaves like a pure conducting object

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