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Two inductors, each of inductance L, are connected in parallel. One more inductor of value 5 mH is connected in series with this configuration, and then the effective inductance is 15 mH. The value of L is ______ mH.

Asked in GUJCET 2018 · Coil geometry, cores and combinations

Answer: (4) 20

Step-by-step solution

Given: L in parallel with L, and that pair in series with 5 mH; total 15 mH.

Idea: with no mutual inductance, inductors combine like resistors.

Two equal inductors in parallel: Lₚ=(L· L)/(L+L)=L/2.

In series with 5 mH: L/2+5=15, so L/2=10 mH.

So L=20 mH.

Why the other options are wrong

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