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JEE Advanced · Physics · 21. EMI

In the figure below, the switches S1 and S2 are closed simultaneously at t = 0 and a current starts to flow in the circuit. Both the batteries have the same magnitude of the electromotive force (emf) and the polarities are as indicated in the figure. Ignore mutual inductance between the inductors. The current I in the middle wire reaches its maximum magnitude Imax time t =τ. Which of the following statements is (are) true?

  1. A Imax=V2R
  2. B Imax=V4R
  3. C τ=LRln2
  4. D τ=2LRlln2
Verified Solution

Answer & Solution

Correct Answer

(D) τ=2LRlln2

Step-by-step Solution

Detailed explanation

The current in the middle, I, as shown in the figure above has a maximum value,
imax=i2-i1max Using the equation of current decay in an L-R circuit, for the given values in the figure above,
\(i=\left(i_2-i_1\right)=\frac{V}{R}\left[1-e^{-\left(\frac{R}{2 L}\right) t}\right]-\) \(\frac{V}{R}\left[1-e^{\left(-\frac{R}{L}\right) t}\right] \)
\( \frac{V}{R}\left[e^{\left(-\frac{R}{L}\right) t}-e^{\left(-\frac{R}{2 L}\right) t}\right]\)
For imax, didt=0
VR-RLe-RLt--R2Le-R2Lt=0
e-RLt=12e-R2Lt
e-R2Lt=12
R2Lt=ln2
t=2LRln2 time when I is maximum
imax=VRe-RL2LRln2-e-R2L2LRln2
imax=VR14-12=14VR
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