8 (a) Explain what is meant by a magnetic field. (b) The apparatus shown in Fig. 8.1 is used in an experiment to find the magnetic flux density B between the poles of a horseshoe magnet. Assume the magnetic field is uniform between the poles of the magnet and zero elsewhere. [Figure 8.1] The rigid metal rod of length 300mm is fixed in position perpendicular to the direction of the magnetic field. The poles of the magnet are both 45mm long. There is a current in the rod that causes a force on the rod. The balance is used to determine the magnitude of the force. The variation with current I of the force F on the rod is shown in Fig. 8.2. [Figure 8.2]

1. (a)Explain what is meant by a magnetic field.[1]
2. (b)Calculate the magnetic flux density B. B = ................................................................ T [2][2]
3. (c)(i)Explain why the electrons follow a circular path when inside the region of the magnetic field.[3]
4. (c)(ii)State the measurements needed in order to determine the charge to mass ratio, e/m_e, of an electron.[2]

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