The motor effect

Question 1

What is produced when current flows through a conducting wire?

Answer 1

A magnetic field is produced around the wire.

Question 2

What determines the strength of the magnetic field around a current-carrying wire?

Answer 2

• The magnitude of the current flowing through the wire.

- The distance from the wire.

Question 3

What is a solenoid?

Answer 3

A coil of wire which when current pass through creates a strong magnetic field.

Question 4

Describe the magnetic field found inside a solenoid

Answer 4

Strong and uniform.

Question 5

What is an electromagnet?

Answer 5

• A solenoid with an added iron core.

- Adding the iron core increases the strength of the magnetic field.

Question 6

What is the motor effect?

Answer 6

A wire carrying a current creates a magnetic field. This can interact with another magnetic field, causing a force that pushes the wire at right angles.

Question 7

What rule is used to determine the force experienced due to the motor effect?

Answer 7

Fleming’s left-hand rule.

Question 8

When using Fleming’s left hand rule, what does the forefinger represent?

Answer 8

Direction of magnetic field (north to south).

Question 9

When using Fleming’s left hand rule, what does the second finger represent?

Answer 9

Direction of current flow (conventional current + to -).

Question 10

When using Fleming’s left hand rule, what does the thumb represent?

Answer 10

Direction of force.

Question 11

What factors affect the size of the force on a current-carrying wire in a magnetic field?

Answer 11

• The magnitude of the current flowing through the wire/conductor.
• The strength of the magnetic field that the wire/conductor is placed in.

Question 12

What factors affect the size of the force on a current-carrying wire in a magnetic field?

Answer 12

• The magnitude of the current flowing through the wire/conductor.
• The strength of the magnetic field that the wire/conductor is placed in.

Question 13

If the direction of current in a current-carrying wire placed in a uniform magnetic field is reversed, what happens to the force?

Answer 13

The direction of force is reversed.

Question 14

State the equation linking force, magnetic flux density, current and length. Give appropriate units

Answer 14

Force = magnetic flux density x current x length
F = B I L
Force (newtons), magnetic flux density (tesla/T), current (amps), length (meters).

Question 15

What criteria must be met for the equation linking force, magnetic flux density, current and length to hold?

Answer 15

The wire/conductor must be at right angles to the magnetic field it is placed in.

Question 16

How does an electric motor work?

Answer 16

• A coil of wire, carrying a current, is placed in a magnetic field.
• The forces on the 2 sides perpendicular to the magnetic field experiences forces in opposite directions.
• This causes a rotational effect.
• A split ring commutator changes the direction of current every half turn to allow the rotational effect in the same direction.

Question 17

How do loudspeakers make use of the motor effect?

Answer 17

The motor effect is used to convert variations in the current of an electrical circuit into pressure variations which produce audible sound.

Question 18

Explain how a loudspeaker works

Answer 18

• A cone with a wire wrapped around it is connected to an a.c. power supply and is placed in a permanent magnetic field.
• When current flows through the wire, it creates a second magnetic field, which interacts with the permanent field.
• This produces a force which causes the cone to vibrate.
• The direction of current reverses causing the opposite and a vibration effect.
• This vibration in air produces pressure variation that cause sound waves.

Question 19

How is the pitch of the sound from a loudspeaker changed?

Answer 19

• The frequency of the a.c. current is altered.

- This creates a different frequency of vibration in the cone.