Question

A circuit is using AC source of 120 $V$/ 60 $Hz$ and the first inductor $L_1$ is 10 mH it the splits in two with inductors $L_2$ 20 mH and $L_3$ 40 mH in series and they are parallel to $L_4$ which is 20 mH and then it comes together back to the power source.
Calculate the equivalent inductance.

• A. 15 mH
• B. 20 mH
• C. 25 mH
• D. 30 mH

Answer 1

Answer: (C)

25 mH

The circuit diagram will be as shown in figure.

First, the inductors $L_2$ and $L_3$ are in series. So equivalent inductance for these is $L_{23}=L_2+L_3=20+40=60mH$

Then $L_{23}$ and $L_4$ are in parallel. So equivalent inductance for these is

$L_{234}=\frac{1}{1/L_{23}+1/L_4}=\frac{1}{1/60+1/20}=\frac{60}{4}=15mH$

Now $L_1$ and $L_{234}$ are in series. So the equivalent inductance of the circuit is

$L_{eq}=L_1+L_{234}=10+15=25mH$