College

Find the total inductance of the circuit. The coils are sufficiently close such that mutual inductance is present.

Given:
- \( L_1 = 2 \, \text{H} \)
- \( L_2 = 7 \, \text{H} \)
- Mutual inductance \( L_M = 0.5 \, \text{H} \)

Options:
- 8 H
- 9 H
- 18 H
- None of these

Answer :

Given Data: 8 H 9 H 18 H L₁ = 2 H None of these LM = 0.5 H L₂ = 7 H

According to the problem, we have a circuit with two coils and a mutual inductance of LM = 0.5 H, as shown below:

we have the following equations for mutual inductance:

V1 = L₁ di1/dt + M di2/dt

V2 = M di1/dt + L₂ di2/dt

We can rearrange the above two equations as shown below:

di1/dt = [ V1 - M di2/dt ] / L₁

di2/dt = [ V2 - M di1/dt ] / L₂

Differentiating both the above equations with respect to time, we get:

d²i₁/dt² = [-M / L₁] d²i₂/dt²

d²i₂/dt² = [-M / L₂] d²i₁/dt²

Let, the total inductance of the circuit be LT. Then, we can write the equation as follows:

LT d²i₁/dt² = V1 - M di2/dt + LM d²i₂/dt²

LT d²i₂/dt² = V2 - M di1/dt + LM d²i₁/dt²

Now, let's add the above two equations to eliminate d²i/dt² terms:

LT [d²i₁/dt² + d²i₂/dt²] = V1 + V2

We can see that d²i₁/dt² + d²i₂/dt² is the second derivative of the total current with respect to time, i.e., d²i/dt². Therefore, the total inductance of the circuit is given by:

LT = (V1 + V2) / d²i/dt²

We know that for an inductor, the inductance is given by:

L = V / d i/dt

Therefore, we can write the above equation in terms of inductances as follows:

LT = (L₁ + L₂ + 2M + 2LM) / d²i/dt²

Substituting the given values, we get:

LT = (2H + 7H + 2 x 0.5H + 2 x 0.5H) / d²i/dt²

LT = 12 H

Therefore, the total inductance of the circuits is 12 H.

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