State the meaning of the tesla.
A magnet creates a uniform magnetic field of flux density $B$ in the region between its poles. A rigid copper wire carrying a current is supported on a pivot. The part $PQLM$ of the wire lies between the poles of the magnet, as shown in Fig. 8.1. The wire is kept horizontal by a small weight $W$.
Explain why section $QL$ of the wire produces a moment about the pivot.
Explain why sections $PQ$ and $LM$ of the wire have no effect on the equilibrium of the wire.
Section $QL$ of the wire has length $0.85\,\text{cm}$. The perpendicular distance of $QL$ from the pivot is $5.6\,\text{cm}$. When the current in the wire is altered by $1.2\,\text{A}$, $W$ is shifted $2.6\,\text{cm}$ along the wire to restore equilibrium. The mass of $W$ is $1.3 \times 10^{-4}\,\text{kg}$.
Show that the change in moment of $W$ about the pivot is $3.3 \times 10^{-5}\,\text{Nm}$.
Use the information in (i) to find the magnetic flux density $B$ between the poles of the magnet.