Question

A circular coil expands radially in a region of magnetic field and no electromotive force is produced in the coil. This is because

• A. The magnetic field is constant.
• B. The magnetic field is in the same plane as the circular coil and it may or may not vary.
• C. The magnetic field has a perpendicular (to the plane of the coil) component whose magnitude is decreasing suitably.
• D. Both (2) and (3).

Answer 1

Answer: (D)

Both (2) and (3).

Answer 2

When a circular coil expands radially in a region of magnetic field, induced emf developed is

$\varepsilon = Blv = B \times$rate of change area

Here, magnetic field B is in a plane perpendicular to the plane of the circular coil.

As $\varepsilon = 0$, magnetic field must be in the plane of circular coil so that its component perpendicular to the plane of the coil, whose magnitude is decreasing suitably so that magnetic flux linked with the coil stays constant then

$\varepsilon = \frac{d\varphi}{dt} = 0$

So both options (2) and (3) are correct.