Question

Two identical induction coils each of inductance $L$ joined in series are placed very close to each other such that the winding direction of one is exactly opposite to that of the other, what is net inductance?

• A. $L^{2}$
• B. $2L$
• C. $\frac{L}{2}$
• D. Zero

Answer 1

Answer: (D)

Zero

Answer 2

When the two coils are joined in series such that the winding of one is opposite to the other, then the emf produced in first coil is $180^{\circ}$ out of phase of the emf produced in second coil. Thus, emf produced in first coil is negative and the emf produced in second coil is positive, so, net inductance is $L=L_{1}+L_{2}=L+L$ $\Rightarrow L=-\frac{\phi}{i}+\frac{\phi}{i}=0$