Physics 9702 · AS & A Level · Force on a current-carrying conductor
Force on a current-carrying conductor — practice question
As shown in Fig. 6.1, a rectangular wire coil PQRS can turn freely about axis XY. Side QR is 5.4\,\text{cm} long, side PQ is 2.5\,\text{cm} wide, and the coil has 190 turns of wire. The coil’s plane makes an angle \theta with a uniform magnetic field of flux density 5.2 \times 10^{-3}\,\text{T}. Axis XY is perpendicular to the field. The current in the coil is 1.2\,\text{A}.
(a(i))[3]
Calculate the magnitude of the force on side $QR$ of the coil.
(a(ii))[2]
Use your answer in (a)(i) to show that the torque $\tau$ on the coil is $\tau = 1.6 \times 10^{-3}\cos\theta\,\text{N m}$.
(a(iii))[3]
Using the expression in (a)(ii), sketch, on the axes of Fig. 6.2, a graph showing how the torque $\tau$ varies with angle $\theta$ for values of $\theta$ from $0$ to $360^{\circ}$. Label the $\tau$ axis with an appropriate scale.
(b)[2]
The coil is now replaced by an identical coil wound on a ferrous core. Suggest, with a reason, how the torque on this coil compares with the torque on the original coil.
Worked solution & mark scheme
This 10-mark question has a full step-by-step worked solution and mark scheme. One marking point: “$F = BIL$” …