The diagram shows a wire frame used in a repair workshop to check the direction of the motor effect. The frame is placed between the pole pieces of a strong...

The diagram shows a wire frame used in a repair workshop to check the direction of the motor effect. The frame is placed between the pole pieces of a strong magnet. A battery sends current down the vertical wire section shown. The field is directed into the page, represented by crosses. The mechanic wants to predict the force before connecting the full supply, because a moving wire could touch the glass safety screen. The current, magnetic field and force are at right angles to one another.

(a) What do the crosses in Fig. 1 represent? [1]
(b) Which rule can be used to find the direction of the force on the wire? [1]
(c) Sketch an arrow on a copy of Fig. 1 to show the force direction. [2]

Fig. 1 shows a loudspeaker coil used in an emergency announcement system at a sports stadium. An alternating current from the amplifier passes through a coil fixed to a paper cone. The coil is in the radial magnetic field of a permanent magnet. The current repeatedly changes direction, making the cone move in and out. This movement produces pressure changes in the air. The diagram is not drawn to scale, but it shows the narrow air gap where the magnetic force is greatest.

(a) What causes a force on the loudspeaker coil? [1]
(b) When does the direction of this force reverse? [1]
(c) Which two changes would make the force on the coil larger? [2]