Question

When 100 volt dc is applied across a coil, a current of 1A flows through it. When 100 volt ac at 50 cycle s^–1 is applied to the same coil, only 0.5 A current flows. The impedance of the coil is, 

(a) 100Ω 

(b) 200Ω 

(c) 300Ω 

(d) 400Ω

Answer 1

 The correct option (b) 200Ω

Explanation:

V_dc = 100 V, I_dc = 1A, V_ac = V_rms = 100V,

I_ac = I_rms = 0.5A, f = 50Hz

We know R = [(V_dc)/(I_dc)] = (100/1) = 100Ω

Z = [(V_rms)/(I_rms)] = [(100)/(0.5)] = 200 Ω

i.e. impedance of coil is 200Ω.