State what is meant by a magnetic field.
The apparatus in Fig. 8.1 is used to determine the magnetic flux density $B$ between the poles of a horseshoe magnet. Take the field between the poles to be uniform, and assume it is zero everywhere else. A rigid metal rod, $300\,\text{mm}$ long, is clamped so that it is at right angles to the direction of the magnetic field. Each pole of the magnet is $45\,\text{mm}$ long. A current in the rod produces a force on it. The balance is used to measure the size of this force. Figure 8.2 shows how the force $F$ on the rod changes with current $I$. Calculate the magnetic flux density $B$.
Calculate $B$, the magnetic flux density.
Explain why the electrons move in a circular path when they are in the magnetic field.
State the measurements needed to determine the charge to mass ratio, $e/m_e$, for an electron.