P7

Question 1

Where are the magnetic forces strongest on a magnet?

Answer 1

At the poles

Question 2

Attraction and repulsion between two magnetic poles are examples of what kind of force?

Answer 2

Non-contact

Question 3

What is a permanent magnet?

Answer 3

A magnet which produces its own magnetic field

Question 4

What is an induced magnet?

Answer 4

A material that becomes a magnet when it is placed in a magnetic field

Question 5

How can you demagnetise a permanent magnet?

Answer 5

By applying force or heating the magnet as this disorganises the particles

Question 6

What happens when an induced magnet is removed from the magnetic field?

Answer 6

It loses all/most of its magnetism quickly

Question 7

What is the magnetic field?

Answer 7

The region around a magnet where a force acts on another magnet or on a magnetic material

Question 8

What is the force between a magnet and magnetic material always?

Answer 8

one of attraction

Question 9

What does the strength of the magnet depend on?

Answer 9

The distance from the magnet

(the field is strongest at the poles)

Question 10

In which direction does the magnetic field always go?

Answer 10

From North to South

Question 11

How did explorers navigate the Earth using magnetic fields?

Answer 11

• In magnetic compasses there are small bar magnets
• The Earth has a magnetic field and the compass needle points in the direction of the Earth’s magnetic field
• The north of the compass points to the North Pole (Earth’s magnetic South)
• The south of the compass points to the South Pole (Earth’s magnetic North)

Question 12

How can you check the direction of the magnetic field?

Answer 12

• right hand
• thumbs up pointing in the same direction as the current
• the way your other fingers wrap around (knuckle to tip) is the direction of the magnetic field.

Question 13

What does the strength of this magnetic field rely on?

Answer 13

• the distance from the wire
• the current through the wire

Question 14

What is a solenoid?

Answer 14

A coil of current carrying wire

Question 15

What happens when a current flows through a wire?

Answer 15

A magnetic field is produced perpendicular to the wire in concentric circles around it

Question 16

How does shaping a wire to form a solenoid increase the strength of the magnetic field?

Answer 16

The field lines around each loop of wire line up with each other and form a stronger magnetic field. The magnetic field is strongest and uniform through the middle of the solenoid

Question 17

How can you further increase the strength of the magnetic field of a solenoid?

Answer 17

By adding an iron core

Question 18

What is an electromagnet?

Answer 18

A solenoid with an iron core

Question 19

What is the motor effect?

Answer 19

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

Question 20

How must the wire be positioned to experience the full force?

Answer 20

At 90 degrees to the field (perpendicular)

Question 21

What is Fleming’s left hand rule used for?

Answer 21

To determine the direction of the force

• First Finger= magnetic Field
• second finger= Current
• thumb = motion (direction of the Force)

Question 22

What does the force on a conductor in a magnetic field depend on?

Answer 22

• The magnetic flux density measured in tesla (T)
• The current (amps)
• The length of the wire in metres

Question 23

What is the equation used to calculate force?

Answer 23

• F= B x I x lF= force (N)B= magnetic flux density (T)I= current (A)l= length (m)

Question 24

What is the basis of an electric motor?

Answer 24

A coil of wire carrying a current in a magnetic field tends to rotate.

Question 25

How does a motor work?

Answer 25

- The power supply applies a potential difference across the coil - A current flows through the coil - A magnetic field is created around the coil - The magnetic field interacts with the magnetic field of the permanent magnet - This creates a force that makes the coil spin - Every half-turn, the direction is reversed by a split-ring commutator

Question 26

What is the function of a split-ring commutator?

Answer 26

A split-ring commutator reverses the direction of the current in the wire every half-turn - it connects to the circuit with electrical contacts called brushes

Question 27

How can you increase the speed of a motor?

Answer 27

- increase the current - increase the strength of the magnetic field - increase the number of coils

Question 28

Why do loudspeakers/headphones vibrate when AC current flows through the coil?

Answer 28

- the current flowing through the coil creates a magnetic field - this interacts with the permanent magnetic field and produces a resultant force - When the direction of the current changes, the direction of the force changes to the opposite direction - This causes vibrations in the cone which produce pressure changes in the air to produce sound

Question 29

What is the generator effect?

Answer 29

If an electrical conductor moves relative to a magnetic field or if there is a change in the magnetic field around a conductor a potential difference is induced (and current if there's a complete circuit) across the ends of the conductor

Question 30

How does induced current work?

Answer 30

An induced current generates a magnetic field that opposes the original change (either the movement of the conductor or the change in magnetic field)

Question 31

Explain how alternators work

Answer 31

- generates AC (alternating current) - As the coil spins, a current is induced in the coil - every half-turn, the current changes direction

Question 32

Explain how dynamos work

Answer 32

- generates DC (direct current) - They use a split-ring commutator as their electrical contact - This swaps the connection every half-turn to keep current flowing in the same direction

Question 33

What do microphones use the generator effect to do?

Answer 33

To convert the pressure variations in sound waves into variation in current in electrical circuits (converts sound waves into electrical signals)

Question 34

How does a moving-coil microphone work?

Answer 34

- the sound waves cause the diaphragm to oscillate - This causes the cone to vibrate which interacts with the permanent magnetic field (it moves through) - As a result, a potential difference is induced

Question 35

What is a transformer?

Answer 35

A device that changes the potential difference of a device, using the generator effect

Question 36

How is the effect of an alternating current in one coil in inducing current in another used in transformers?

Answer 36

- The current flowing through a primary coil causes a magnetic field - this creates an induced magnetic field in the iron - an AC causes the induced magnetic field to also alternate - the constantly changing magnetic field cuts through the secondary coil - this induces an alternating potential difference in the secondary coil (due to the generator effect) - if the secondary coil is part of a complete circuit a current will flow with some frequency of AC in the primary coil

Question 37

Why are iron cores used?

Answer 37

Iron is easily magnetised

Question 38

What does the ratio of potential differences across the primary and secondary of coils of a transformer depend on?

Answer 38

The number of turns on each coil Vp/Vs = np/ns Vp/Vs= potential difference (primary and secondary) np/ns= number of turns on each coil (primary and secondary)

Question 39

How can you manipulate the amount of potential difference induced?

Answer 39

by varying the number of coils wrapped around

Question 40

How does potential difference and the number coils differ in step up and step down transformers?

Answer 40

step-up: higher potential difference in the secondary coil, more turns on the secondary coil - increases V step-down: higher potential difference in the primary coil, more turns on the primary coil - decreases V

Question 41

What would happen if transformers were 100% efficient?

Answer 41

the electrical power input and output would be equal