First slide

Inductance

Question

The time constant of a circuit is 10 sec, when a resistance of 10 $Ω$ is connected in series in a previous circuit then time constant becomes 2 second, then the self inductance of the circuit is :

Moderate

A

2.5 H
B

5 H
C

15 H
D

25 H

Solution

Time constant of LR circuit = $\frac{L}{R}$ = 10 …….(i)
When 10 $Ω$ is connected in series
$\frac{L}{R + 10} = 2 . . . . . . . . . . . (ii)$
$\Rightarrow \frac{R + 10}{R} = 5 \Rightarrow R = 2.5 so L = 25$

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