Three identical coils C1, C2 and C3 are closely placed such that they share a common axis. C2 is exactly midway. C1 carries current I in anti-clockwise direction while C3 carries current I in clockwise direction. An induced current flows through C2 will be in clockwise direction when
- C1 and C3 move with equal speeds away from C2
- C1 moves towards C2 and C3 moves away from C2
- C1 and C3 move with equal speeds towards C2
- C1 moves away from C2 and C3 moves towards C2
Answer
(2) C1 moves towards C2 and C3 moves away from C2
Solution
View all three coils from the same side along the common axis, and take the direction towards the observer as positive.
Anticlockwise current in C1 its field along the axis is towards the observer (positive flux through C2).
Clockwise current in C3 its field is away from the observer (negative flux through C2).
C2 is midway and the currents are equal, the two fluxes cancel initially: net = 0
A clockwise induced current in C2 produces a field away from the observer (negative direction).
By Lenz's law, it must oppose an increase of flux in the positive direction.
The positive flux (from C1) must increase and/or the negative flux (from C3) must decrease.
C1 moves towards C2 and C3 moves away from C2.
If C1 and C3 move with equal speeds in the same sense (both away or both towards), the two changes cancel and no current is induced; option (4) gives an anticlockwise current.
Answer: (2)