(a) A circular coil of 30 turns and radius `8.0cm`. Carrying a current of `6.0A` is suspended vertically in a uniform horizontal magnetic field of magnitude `1.0T`. The field lines make an angle of `60^@` with the normal to the coil. Calculate the magnitude of the counter torque that must be applied to prevent the coil from turning.
(b) Would your answer change if the circular coil in (a) were replaced by a planar coil of some irregular shape that encloses the same area? (All other particulars are also unaltered).

Explanation

(a) Number of turms on the circular coil, `n = 30`
Radius of the coil, `r = 8.0 cm = 0.08 m`
Area of the coil `= pir^(2) = pi (0.08)^(2) = 0.0201 m^(2)`
Current flowing in the coil, `I = 6.0 A`
Magnetic field strength, `B = 1I`
Angle between the field lines and normal with the coil surface,
`theta = 60^(@)`
The coil experiences a torque inthe magneitc field. Hence, it turns. the counter torque applied to prevent the coil form turning is given by the relation,
`T = nIBA sin theta` ...(1)
`=30 xx 6 xx 1 xx 0.0201 xx sin 60^(@)`
`=3.133Nm`
(b) It can be inferred from relation (i) that the magnitude of the applied torque is not dependent on the shape of the coil. It depends on the area of the coil. Hence, the answer would not change if the circular coil in the above case in replaced by a planar coil of some irrengular shape that enclose the same area.