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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

The correct answer is
$C_1$ moves towards $C_2$ and $C_3$ moves away from $C_2$

Analyzing Magnetic Fields and Flux Change

Coil $C_1$ carries current $I$ in an anti-clockwise direction. Using the right-hand rule, this generates a magnetic field along the common axis directed, let's say, towards the right (positive direction). Coil $C_3$ carries current $I$ in a clockwise direction. This generates a magnetic field along the axis directed towards the left (negative direction).

At the location of coil $C_2$, which is midway, the net magnetic field $B_{net}$ is the difference between the field contributions from $C_1$ ($B_1$) and $C_3$ ($B_3$). We can represent this as $B_{net} = B_1 - B_3$, where $B_1$ and $B_3$ are the magnitudes of the magnetic fields generated by $C_1$ and $C_3$ respectively at $C_2$. The net magnetic flux through $C_2$ is proportional to $B_{net}$.

Evaluating Motion Conditions (Option C)

Let's analyze the effect of the motion described in Option C: $C_1$ moves towards $C_2$ and $C_3$ moves away from $C_2$.

  • When $C_1$ moves towards $C_2$, the distance between them ($d_{12}$) decreases. This causes the magnetic field $B_1$ from $C_1$ at $C_2$ to increase. Thus, $\Delta B_1 > 0$.
  • When $C_3$ moves away from $C_2$, the distance between them ($d_{23}$) increases. This causes the magnetic field $B_3$ from $C_3$ at $C_2$ to decrease. Thus, $\Delta B_3 < 0$.

The change in the net magnetic field is $\Delta B_{net} = \Delta B_1 - \Delta B_3$. Substituting the signs of the changes:

$\Delta B_{net} = (+ \text{ve}) - (- \text{ve}) = (+ \text{ve}) + (+ \text{ve})$

This means $\Delta B_{net}$ is definitely positive. Therefore, the net magnetic flux through $C_2$ is increasing in the positive direction (the direction of the field from $C_1$).

Applying Lenz's Law for Induced Current Direction

According to Lenz's Law, an induced current flows in a direction that opposes the change in magnetic flux that produced it.

Since the net magnetic flux through $C_2$ is increasing in the positive direction, the induced current in $C_2$ must create a magnetic field that opposes this increase. This opposing field must be in the negative direction (opposite to the field from $C_1$).

A clockwise current in coil $C_2$ generates a magnetic field pointing in the negative direction (towards the left, opposing the increasing flux from $C_1$). Therefore, the induced current in $C_2$ will be in the clockwise direction under these conditions.

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