State two cases in which a charged particle in a magnetic field does not feel a force.
A lightly coiled metal spring hangs from a fixed point, as shown in Fig. 9.1. The ends of the spring are connected electrically by a flexible lead. The spring length is measured before the switch is closed and then again after the switch is closed. When the switch is closed, a magnetic field is produced around each coil of the spring. Using these magnetic fields, explain why the spring changes in length. State whether the spring extends or contracts.
With the switch in (b) closed, the small mass at the free end of the spring is now made to oscillate vertically. Use the principles of electromagnetic induction to explain why small fluctuations in the current in the spring are observed.