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JEE Advanced · Physics · 21. EMI

The inductors of two LR circuits are placed next to each other, as shown in the figure. The values of the self-inductance of the inductors, resistances, mutual-inductance and applied voltages are specified in the given circuit. After both the switches are closed simultaneously, the total work done by the batteries against the induced EMF in the inductors by the time the currents reach their steady-state values is_______mJ.

  1. A 11
  2. B 22
  3. C 33
  4. D 55
Verified Solution

Answer & Solution

Correct Answer

(D) 55

Step-by-step Solution

Detailed explanation


\(\varepsilon_1=L_1 \frac{d i_1}{d t}+M \frac{d i_2}{d t} \)
\( d W_1=\varepsilon_1 i_1 d t \)
\( =L_1\left(d i_1\right) i_1+M\left(d i_2\right) i_1 \)
\( d W_2=L_2\left(d i_2\right) i_2+M\left(d i_1\right) i_2 \)
\( d W_1+d W_2=L_1 i_1\left(d i_1\right)+L_2 i_2\left(d i_2\right)+\) \(M d\left(i_1 \cdot i_2\right) \)
\( W=\frac{1}{2} L_1 i_1^2+\frac{1}{2} L_2 i_2^2+M_1 i_2 \)
\( =55 mJ\)
Note : If mutual induction is taken in opposite direction then W=35 mJ
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