Question

Three identical coils $A,B,C$ carrying currents are placed coaxially with their planes parallel to one another. $A$ and $C$ carry currents as shown in figure. $B$ is kept fixed, while $A$ and $C$ both are moved toward $B$ with the same speed. Initially, $B$ is equally separated from $A$ and $C$. The direction of the induced current in the coil $B$ is :

• A. same as that in coil $A$
• B. same as that in coil $B$
• C. zero
• D. none of these

Answer 1

Answer: (C)

zero

The rings can be treated as two small bar magnets with polarities as shown.
Both A and C are equidistant from initially and are moving towards B with same speed. Therefore they will always be equidistant from B.
At anytime the flux in the coil B due to coil A will be equal and opposite to the flux in the coil B due to coil C.
Thus, at any time, the net flux through coil B is zero and so is the change in flux.
So no current will be induced in coil B