Magnetic field due to currentHard

Question

Three identical coils $C_{1},C_{2}$ and $C_{3}$ are closely placed such that they share a common axis. $C_{2}$ is exactly midway. $C_{1}$ carries current $I$ in anti-clockwise direction while $C_{3}$ carries current $I$ in clockwise direction. An induced current flows through $C_{2}$ will be in clockwise direction when

Options

A.$C_{1}$ and $C_{3}$ move with equal speeds away from $C_{2}$
B.$C_{1}$ moves towards $C_{2}$ and $C_{3}$ moves away from $C_{2}$
C.$C_{1}$ moves away from $C_{2}$ and $C_{3}$ moves towards $C_{2}$
D.$C_{1}$ and $C_{3}$ move with equal speeds towards $C_{2}$

Solution

Magnetic field through the coil is

$$\begin{matrix} \overrightarrow{B} & \ = \left( B_{C_{2}} - B_{C_{1}} \right)\widehat{i} \\ \phi & \ = \left( B_{C_{2}} - B_{C_{1}} \right)A \\ \varepsilon & \ = \frac{- d\phi}{dt} \end{matrix}$$

Find the direction according to Lenz's law

If coil move away then magnetic field decreases & vice versa

Correct Ans. (2)

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