7.2

Question 1

What is motor effect?

Answer 1

When a current-carrying conductor is placed in a magnetic field, the conductor and magnet exert a force on each other.

Question 2

What is the formula to calculate the size of the force generated by the motor effect.

Answer 2

Force (N) = magnetic flux density (T) × curent (A) × length in magnetic field (m)

Question 3

What is the technique used to determine the direction of the force.

Answer 3

Flemings left hand rule.
- ThuMb: Movement (direction of force)
- First finger: Field (kNuckle = North)
- SeCond finger: Current (+ to -)

Question 4

When is the motor effect applicable?

Answer 4

When the conductor is parallel to the magnetic field.

Question 5

Describe how an electric motor works.

Answer 5

1. Magnetic field exerts force on wire loop.
2. Wire rotates due to current travelling in opposite directions causing forces to act in opposite directions.
3. Split-ring commutator rotates with coil.
4. Contacts with negative and positive voltage swap, reversing current.
5. Forces on wire swap causing constant rotation of loop of wire.

Question 6

Describe how a moving-coil loudspeaker and headphones works.

Answer 6

1. As current passes through coil, magnetic field generated.
2. Magnetic fields of coil and magnet interact.
3. This creates resultant force, causing cone to move.
4. When a.c direction reverses, direction of force on cone reverses.
5. This causes cone to move in and out, generating sound waves.

Question 7

How can the sound waves generated by headphones or loudspeakers be modified?

Answer 7

• Change in frequency of a.c supply causes change in frequency of vibrations - changes pitch.
• Change in size of current, changes amplitude of vibration - changes volume.