Topic 7- Magnetism and electromagnetism

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 fore finger represent?

Answer 8

The forefinger points in the direction of the magnetic field

Question 9

When using Fleming’s left rule what does the middle finger represent?

Answer 9

The middle 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 , R

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

Question 19

What happens when an electrical conductor moves relative to a magnetic field?

Answer 19

A potential difference is induced across the ends of the conductor

Question 20

What happens to an electrical conductor when there is a change to the magnetic field that it is placed in?

Answer 20

A potential difference is induced across the ends of the conductor

Question 21

What is the requirement for an induced potential difference to cause a current flow

Answer 21

The conductor must form a closed loop or be part of a complete circuit

Question 22

What can be said about the direction of the magnetic field produced by an induced current?

Answer 22

The direction of the field is such that it opposes the change that induced the current

This change is either the movement of the conductor or a change to the field

Question 23

What is an advantage of elctromagnets?

Answer 23

You can turn them on and off

Question 24

How can you increase field strength in solenoids?

Answer 24

Increase current

Add more coils

Add iron core

Question 25

What are two ways that the generator effect is used to generate different types of current?

Answer 25

In an alternator to produce alternating current

In a dynamo to produce direct-current

Question 26

What electromagnetic effect does a microphone take advantage of and how?

Answer 26

The generator effect

It converts the pressure variations in sound waves into alternating current in a circuit

Question 27

Describe the makeup of a basic transformer

Answer 27

A primary coil and a secondary coil of wire wrapped around an iron core`

Question 28

Why is iron used as the core for a transformer?

Answer 28

It is easily magnetised

Question 29

Explain how a transformer works

Answer 29

An alternating current flows through the primary coil

This induces a changing magnetic field in the core

This changing magnetic induces a current to flow in the secondary coil

Question 30

Why must the current flowing through the primary coil of a transformer be alternating?

Answer 30

For current to be induced in the secondary coil, the magnetic field in the core must be continuously changing

For the magnetic field to be changing, the current in the primary coil must be alternating

Question 31

What can be said about the electrical power input and output of a 100% efficient transformer?

Answer 31

The electrical power input is equal to the electrical power output

Question 32

What do step-up transformers do?

Answer 32

Increase p.d

Decrease current

Question 33

What do step-down transformers do?

Answer 33

Decrease p.d

Increase current

Question 34

What is the generator effect?

Answer 34

Using movement and a permanent field to induce a current

Question 35

How can you increase the efficiency of a transformer?

Answer 35

Insulating layers in core (laminations)

Used in national grid to transfer electricity at low currents so less heat energy is wasted