Physics 9702 · AS & A Level · Force on a current-carrying conductor
Force on a current-carrying conductor — practice question
A stiff copper wire is positioned horizontally between the pole pieces of two magnets, as illustrated in Fig. 9.1. Each pole piece has a width of $8.5\text{ cm}$. In the space between the magnet poles, the uniform magnetic flux density $B$ is $3.7\text{ mT}$, while it is zero elsewhere. The angle between the wire and the direction of the magnetic field is $\theta$. The current in the wire flows in the direction indicated in Fig. 9.1.
(a)[2]
Using the side view of Fig. 9.1, state and explain the direction in which the force acts on the magnets.
(b(i))[2]
The steady current in the wire is $5.1\text{ A}$. With $\theta$ set to $90^\circ$, calculate the force acting on the wire.
(b(ii))[1]
The angle $\theta$ is now $60^\circ$. The section of wire in the magnetic field has length $\left(\dfrac{8.5}{\sin 60^\circ}\right)\,\text{cm}$. Calculate the force on the wire.
(c)[3]
The steady current in the wire is 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 direction of the magnetic field 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: “Application of **Fleming’s left-hand rule** gives an upward force on the wire” …