question_answer 27)

  A circular coil expands radially in a region of magnetic field and no electromotive force is produced in the coil. This can be 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) there is a constant magnetic field in the perpendicular (to the plane of the coil) direction

Answer:

                                    (b,c)When a circular coil expands radially in a region of magnetic field, induced e.m.f developed is Here, magnetic field B is in a plane perpendicular to the plane of circular coil. As e = 0, magnetic field must be in the plane of circular coil so that its component perpendicular to plane of coil is zero. Further, if the magnetic field has a component perpendicular to the plane of the coil, whose magnitude is decreasing suitably so that magnetic flux linked with the coil stays constant then  Choices (b) and (c) are correct.