7.2 Magnetism and Electromagnetism (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 passes 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

when a force is exerted between a magnetic field and a current-carrying conductor placed in that field

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

the forefinger points in the direction of the magnetic field

Question 9

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

Answer 9

the second finger points in the direction of current flow in the conductor

Question 10

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

Answer 10

• the magnitude of the current flowing through the conductor
• the strength of the magnetic field that the conductor is placed in

Question 11

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

Answer 11

the direction of the force is reversed

Question 12

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

Answer 12

the strength of the force is increased

Question 13

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

Answer 13

the conductor must be at right-angles to the magnetic field it is placed in

Question 14

What is the unit used for magnetic flux density?

Answer 14

Tesla, T

Question 15

How does an electric motor work?

Answer 15

• A coil of wire, carrying a current, is placed in a magnetic field
• The forces on the two sides perpendicular to the field experience forces in opposite directions
• this causes a rotational effect

Question 16

How do loudspeakers make use of the motor effect?

Answer 16

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

Question 17

Explain how a loudspeaker works?

Answer 17

• 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

Question 18

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

Answer 18

• the frequency of the a.c current is altered
• this creates a different frequency of vibration in the cone