Question:medium

Two identical coils of inductance L joined in series are placed very close to each other such that the winding direction of one coil is exactly opposite to that of the other. The net inductance is

Show Hint

This principle is used in "non-inductive" resistors where the wire is doubled back on itself to cancel induction.
Updated On: May 16, 2026
  • $\frac{L}{2}$
  • 2 L
  • zero
  • L
Show Solution

The Correct Option is C

Solution and Explanation

Step 1: Understanding the Question:
When two coils are in series and placed close to each other, mutual inductance ($M$) comes into play. The net inductance depends on whether the magnetic fields aid or oppose each other.
Step 2: Key Formula or Approach:
For series opposing connection: $L_{net} = L_1 + L_2 - 2M$.
For identical coils placed "very close", assume perfect coupling coefficient $k = 1$, so $M = \sqrt{L_1 L_2}$.
Step 3: Detailed Explanation:
Given:
1. Coils are identical: $L_1 = L_2 = L$.
2. Winding direction is exactly opposite: This is a series opposing case.
3. Placed very close: Coupling is nearly perfect, so $M = \sqrt{L \times L} = L$.
Now, substitute these into the net inductance formula:
\[ L_{net} = L + L - 2L = 2L - 2L = 0 \]
The magnetic fluxes produced by the two coils cancel each other out completely.
Step 4: Final Answer:
The net inductance is zero.
Was this answer helpful?
0

Top Questions on Semiconductor electronics: materials, devices and simple circuits