Question

The time constant of an inductance coil is 4 ms. On connecting a resistor of $60\Omega$ in series with it, the time constant is reduced to 1 ms. The inductance and resistance of the coil are respectively

• A. 20 mH, 40 $\Omega$
• B. 80 mH, 40 $\Omega$
• C. 80 mH, 20 $\Omega$
• D. 40 mH, 20 $\Omega$

Answer 1

The correct option is C

80 mH, 20 $\Omega$

$\tau =\frac{L}{R}=4$ and ${\tau }^1=\frac{L}{R+60}=1$ On solving L = 80mH. And R = 20 ohms