Physics - Motor Effect

Question 1

What happens to a charged particle when it is moving in a magnetic field?

Answer 1

There is a force on the charged particle moving through the magnetic field perpendicular to its motion as long as its motion is not parallel to the field

Question 2

Where can this effect be used?

Answer 2

Deflecting magnets in particle accelerators, the aurora, mass spectrometer, old fashioned TV, oscilloscope

Question 3

Write down the equation for the force on the particle moving at right angles to the magnetic field

Answer 3

Force = magnetic field strength x charge x velocity

F=Bqv

Question 4

Define a current

Answer 4

Rate of flow of charged particle

Question 5

Explain why a current carrying wire feels a force when placed perpendicularly into a magnetic field

Answer 5

The flow of charge in the wire is moving through the magnetic field and hence feels a force.

Question 6

Describe how Fleming’s left-hand-rule is used to predict the direction of the motor force when a wire carries a current perpendicular to a magnetic field

Answer 6

1st – magnetic field
2nd – conventional current
Thumb – motion/force

Question 7

How can the magnitude of the motor force be increased?

Answer 7

Increase magnetic field strength, length of wire in field, current through the wire

Question 8

Write down the equation for the force on a wire at right angles to a magnetic field

Answer 8

Force = magnetic field x current x length of wire

F=BIL

Question 9

Describe the basic construction of a simple DC motor

Answer 9

(See diagram) Must have labelled: South Pole, North Pole, armature, brush and commutator

Question 10

Describe how a DC motor uses the motor effect to achieve rotation

Answer 10

Wires to left and to the right both feel a clockwise force.

When turned 90 degrees there is no force, but momentum keeps coil turning.

When at 180 degrees the current flows the other way around the coil due to the brushes touching the other terminal. This means the force is still clockwise.

This then repeats

Question 11

What is the function of a commutator?

Answer 11

It and brushes allows the current to switch in the coil so the force is always in the same turning direction

Question 12

Give domestic examples of where electric motors are used

Answer 12

Microwave, washing machine, food processor

Question 13

Describe the basic construction of a simple loudspeaker

Answer 13

(See diagram) Must have: Soft iron core, speaker cone , field coil, voice coil

Question 14

Explain how the motor effect is used to achieve vibration of a loudspeaker cone

Answer 14

The speaker cone coil (voice coil) is placed over a magnetic soft iron core. This is made magnetic using a dc power supply.

When a current passes through the coil, the coil will feel a force due to the motor effect. The direction and size of the force depends on the direction of current and the magnitude of the current

Question 15

Explain how a bell works

Answer 15

When the switch is closed a current flows through the wire.
This causes the horse shoe soft iron bar to become magnetic and attracts the bell hammer.
As the hammer is attracted the connection is broken and the current stops flowing
This switches off the magnet and the hammer goes back to the original position.
The current is now made again and the magnet switches back on and the process repeats

Question 16

Explain how a relay switch works

Answer 16

When an input current passes through the wire the soft iron bar and coil become magnetic.

This closes the switch in the relay and allows current to flow in the secondary circuit.