D. C. Motor,D. C. Motor,moving moving --coil Loudspeakercoil Loudspeaker

Prepared in Dec 1998Second editing in March 2000

At the end of this unit you should be able to :1. describe an experiment to show the force on a

current-carrying conductor, and a beam of charged particles, in a magnetic field, including the effect of reversing (i) the current, (ii) the direction of the field.

2. show understanding of the relative directions of force, field and current (Fleming’s Left Hand Rule).

3. relate the turning effect to the action of an d.c. motor.

4. describe the action of a loudspeaker (details of the cone will not be required).

Learning objectivesLearning objectives

• Hand the wire between the poles of a powerful magnet. (see fig.)

• When you press the switch and allow the current flows through the wire.

• The wire will then moves horizontally outward.

• From this observation, what conclusion we can make ?

switchforce

Magnetic forceon a current-carrying conductor

(continue on next slide)

Magnetic forceon a current-carrying conductor

• The wire moves because the magnetic field of the permanent magnet reacts with the magnetic field of the current in the wire.

• Fig. shows the combined field of the magnet and wire.

Fleming’s Left Hand RuleFleming’s Left Hand Rule

This rule is useful to illustrate the relationship among the direction of magnetic field, the direction of conventional current flows and the direction of the motion.

It is mainly used to illustrate Electric Motors.

Fleming’s Left-Hand Rule

• Hold your left hand in a fist and then spread out the thumb, first finger and second finger so that they are at right angles to each other.

• Point your first finger in the direction of the magnetic field (from north pole to south pole). (continue on next slide)

Fleming’s Left-Hand Rule• Rotate your hand

about that finger until your second finger points in the direction of the current ( from + to -).

• Then your thumb points in the direction of the movement of the wire (or direction of the force).

d.c. Motorsd.c. Motors

Simple d.c. Motor• The current from the

battery travelling through a brush contact to one half of the commutator.

• After travelling round the coil, the current passes through the other half of the commutator and back to battery. (continue on next slide)

Simple d.c. Motor• By Fleming’ Left-

Hand rule, the current that near the N-pole of the magnet move upwards.

• So the coil turns in clockwise direction.

• When the coil turns through 900 (so that the coil is vertical) as shown in fig. (b), the current stop flowing because of the gap in the commutator break the circuit.

• However, the coil keeps turning because of its own momentum.

Simple d.c. Motor

(a) (b) (c)

Improvement of d.c. Motor• Practically, d.c. motor can be made more

powerful, if

– A large number of turns are wound on the coil.

– A soft iron core is used. (Why ?)

– Use stronger magnet�

LoudspeakersLoudspeakers

Moving-coil Loudspeaker

• It consists a movable coil attached to the large cone.

• The cone fits loosely over the centre of cylindrical permanent magnet so that the coil is in a strong magnetic field.

• The wire of the coil is at right angles to the field.

(continue on next slide)

Moving-coil Loudspeaker

• If the current flows in the direction shown, the coil will move to the right (use Fleming’s left-hand rule to check this).

• If the direction of the current reverses, the coil moves in the opposite way.

Moving Coil Loudspeaker