Physics 9702 · AS & A Level · Force on a current-carrying conductor
Force on a current-carrying conductor — practice question
As illustrated in Fig. 9.1, a rigid copper wire is positioned horizontally between the pole pieces of two magnets. Each pole piece has width $8.5\,\text{cm}$. The magnetic flux density $B$ is uniform in the space between the magnet poles, with value $3.7\,\text{mT}$, and it is zero beyond this area. The angle between the wire and the magnetic field direction is $\theta$. The current in the wire follows the direction shown in Fig. 9.1.
(a)[2]
Using the side view in Fig. 9.1, state and explain the direction in which the force on the magnets acts.
(b(i))[2]
The current in the wire is steady at $5.1\,\text{A}$. When $\theta = 90^{\circ}$, calculate the force on the wire.
(b(ii))[1]
The angle $\theta$ is altered to $60^\circ$. The length of wire within the magnetic field is $\left(\frac{8.5}{\sin 60^\circ}\right)\,\text{cm}$. Calculate the force on the wire.
(c)[3]
The steady current in the wire is now replaced by an alternating current with frequency $20\,\text{Hz}$ and root-mean-square (r.m.s.) value $5.1\,\text{A}$. The angle between the wire and the magnetic-field direction is $90^\circ$. On Fig. 9.2, sketch a graph to show how the force $F$ on the wire varies with time $t$ for two cycles of the alternating current.
Worked solution & mark scheme
This 8-mark question has a full step-by-step worked solution and mark scheme. One marking point: “Fleming’s left-hand rule gives an upward force on the wire” …