(a)[3]
State what is meant by magnetic flux density.
(b)
A rigid copper wire is supported horizontally on a pivot, as shown in Fig. 8.1. The sections PQ, QR and RS of the wire lie in a uniform magnetic field of flux density $B$ between the poles of a permanent magnet. The perpendicular distance of PQRS from the pivot is $7.5\,\text{cm}$. When a current of $2.7\,\text{A}$ is sent through the wire, a small mass of $45\,\text{mg}$ is positioned $8.8\,\text{cm}$ from the pivot to bring the wire back into balance, as shown in Fig. 8.2.
(b(i))[2]
Explain why the balance of the wire is not altered by the current in sections PQ and RS when the current is switched on.
(b(ii))[3]
Section QR of the wire has length $1.2\,\text{cm}$. Calculate the magnetic flux density $B$.