magnetism and electromagnetism

Question 1

Where are the magnetic forces the strongest?

Answer 1

The poles of a magnet are where the magnetic forces are the strongest.

Question 2

What happens when two magnets are brought close to each other?

Answer 2

When two magnets are brought close to each other, they exert a force on each other.

• two like poles repel
• two different poles attract

Question 3

What is a permanent magnet?

Answer 3

A permanent magnet produces its own magnetic field. For example, a bar magnet.

Question 4

What is an induced magnet?

Answer 4

An induced magnet will become a magnet when placed in a magnetic field. This always causes a force of attraction. It loses most or all of its magnetism quickly when it is taken away.

Question 5

What is a magnetic field?

Answer 5

A magnetic field is the region around a magnet where a force acts on another magnetic material or magnet.

Question 6

Name four magnetic materials.

Answer 6

magnetic materials always feel a force of attraction:

• iron
• steel (alloy of iron)
• cobalt
• nickel

Question 7

What direction does a magnetic field run from?

Answer 7

The magnetic field lines always run from the north to the south.

Question 8

How does a magnetic compass work?

Answer 8

A magnetic compass contains a small bar magnet. Naturally, the compass points in the direction of the earth’s magnetic field, which is due to its core.

Question 9

How is an electromagnetic field produced?

Answer 9

When a current flows through a conducting wire, a magnetic field is produced around the wire. There is proof for this due to the influence it has on compasses.

Question 10

What does the strength of a magnetic field depend on?

Answer 10

the strength of a magnetic field depends on:

• the magnitude of the current
• the distance from the wire/magnet

Question 11

Describe the right hand grip rule.

Answer 11

right hand grip rule:

• MUST use right hand
• put thumb in the direction of the current
• the direction in which the hand curls around the wire is the direction of the magnetic field around the wire

Question 12

What is a solenoid? Describe its magnetic field and how to increase its magnetic field.

Answer 12

A solenoid is formed is when you coil the wire that forms a magnetic field. It increases the strength of the field; it is strong and uniform inside the solenoid. Its magnetic field is similar in shape to a bar magnet.

• increasing the number of coils increases the strength of the magnetic field
• placing iron inside the solenoid increases the strength of the magnetic field

Question 13

What is an electromagnet? Why is this useful?

Answer 13

An electromagnet is a solenoid with an iron core. It is useful as the magnetic field is much stronger and you can turn it on or off and change the strength with the current in the wire.

Question 14

Give four uses of electromagnets.

Answer 14

uses of electromagnets:

• electric bell
• electrical relay
• circuit breaker
• electrical doorlock

Question 15

Describe how an electric bell works.

Answer 15

electric bell:

1) the switch closes
2) current flows, turning on the electromagnet
3) the soft iron armature is pulled towards the electromagnet and the hammer hits the gong
4) this causes the contact switch to open, cutting off the circuit
5) the spring pulls the armature back again, closing the contact switch
6) the current flows again and the hammer hits the gong again

Question 16

Describe how an electrical relay works.

Answer 16

electrical relay:

• using a low voltage circuit to set off a high voltage one
  1) the switch closes
  2) the small current provided by the cell causes the electromagnet to become magnetised
  3) an iron rocker arm is attracted to the electromagnet
  4) this causes contacts to close, setting off the high voltage circuit

Question 17

Describe how a circuit breaker works.

Answer 17

circuit breaker:

1) the current creates a magnetic field
2) when the current increases, so does the magnetic field
3) the magnetic field opens the switch
4) the current ‘trips’ and switches off the current

Question 18

Describe how an electrical doorlock works.

Answer 18

electrical doorlock:

1) switch closes
2) this sends a current to an electromagnet
3) this connects to an iron bolt and locks the door

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 magnetic field and the conductor exert a force onto each other.

Question 20

What is the unit for the strength of a magnetic field?

Answer 20

The measure for the strength of a magnetic field is magnetic flux density (in Tesla, T).

Question 21

What is Fleming’s left hand rule?

Answer 21

Fleming’s left hand rule:

• on left hand, hold thumb, first and second finger at right angles to each other
• first finger is the direction of the magnetic field, from the north to the south pole
• second finger is the direction of the current, from the positive to the negative end
• the thumb is the direction of force, or motion

Question 22

What force will be exerted on the conductor if the magnetic field produced by a magnet and a conductor are parallel to each other?

Answer 22

If a conductor is parallel to a magnetic field produced by a magnet, the conductor will not experience a force.

Question 23

How does an electric motor work?

Answer 23

An electric motor causes rotation due to Fleming’s left hand rule.

• two magnets opposite sides of each other, north facing south
• a circuit connected to a split ring commutator, which changes the +/- sides of the current so that the wire keeps on turning
• split ring commutator is connected to two conducting brushes (which are connected to a circuit with a cell) as it allows the +/- of current to be switched

Question 24

Describe how loudspeakers and headphones work.

Answer 24

loudspeakers and headphones:

• a permanent magnet in E shape, middle line is N pole, top and bottom lines are S poles, cone on N with wider part of cone facing away from it
• coil of wire on cone on the N pole connected to AC supply
  1) magnetic field is generated by the coil of wire
  2) this interacts with the magnetic field from the permanent magnet
  3) this causes the cone to move
  4) AC means it vibrates by moving inwards / outwards
  5) this generates sound waves
• changing the frequency of the current changes the speed of the vibrations
• increasing the size of the current increases the amplitude, increasing the volume

Question 25

How is a potential difference induced? How is this lost?

Answer 25

If we move a wire through a magnetic field, a potential difference is induced. - if the wire stops moving, this induced potential difference is lost - it is lost of the wire moves along the magnetic field

Question 26

What is the generator effect?

Answer 26

If a wire that has an induced potential difference is part of a complete circuit, a current is induced. This is the generator effect. - this is lost if the wire moves along the magnetic field - this induced current creates its own magnetic field, which opposes the movement of the magnet

Question 27

Give three ways in which an induced current or voltage can be made larger.

Answer 27

increasing induced I/V: - use a stronger magnetic field - move the wire more rapidly - shape the wire into a coil (the more turns, the higher the induced I/V)

Question 28

Give two ways to change the direction of an induced current or voltage.

Answer 28

changing direction of induced I/V: - movement of wire or magnets change direction - switching the poles of the magnet

Question 29

Describe the nature of the magnetic field produced by an induced current.

Answer 29

An induced current creates its own magnetic field, which opposes the movement of the magnet.

Question 30

Give two uses of the generator effect.

Answer 30

uses of the generator effect: - alternator - dynamo

Question 31

Describe what an alternator is and how it works.

Answer 31

alternator: - this is to generate an alternating current - this is a coil of wire rotating in a magnetic field - the coil is connected to two commutators, which allow the current to pass out of the coil - because it is two commutators, this allows for an alternating current - a potential difference is induced when it is passed through a magnetic field (just like sin(x) graph)

Question 32

What three things can be deduced from the graph of the induced potential difference of an alternator?

Answer 32

graph of induced pd of alternator: - maximum potential difference (crest of graph) is when the coil is horizontal as the wire is going directly through the magnetic field at the fastest possible rate - reaches 0V when the coil is vertical as the coil is moving parallel to the field, and so it not cutting through the magnetic field lines - minimum potential difference (trough of graph) is same as first point, but opposite as each side of the coil is on the opposite side as they were before

Question 33

How can you increase the size and frequency of the AC in an alternator?

Answer 33

increasing size of AC in alternator: - increasing the strength of the magnetic field - increasing the turns in the coil - increasing the area of the coil - increasing the rotation speed of the coil (also increases the frequency of the AC)

Question 34

Describe what a dynamo is and how it works.

Answer 34

dynamo: - this is to generate a direct current - it has a split ring commutator (two sides separated by a gap) - a potential difference and current are induced as they cut through the magnetic field lines - each side of the wire is always connected to the same side of the commutator, and this is why a direct current is produced; potential difference does not reverse

Question 35

What three things can be deduced from the graph of the induced potential difference of a dynamo?

Answer 35

graph of induced pd of dynamo: - reaches 0V as the coil is vertical, so it is moving parallel to the magnetic field, so nothing is produced - two peaks in one full rotation of the coil as it is a direct current - maximum potential difference (crest of graph) is when the coil is horizontal as the wire is going directly through the magnetic field at the fastest possible rate

Question 36

Describe how a microphone works.

Answer 36

microphone: - a permanent magnet in E shape, middle line is N pole, top and bottom lines are S poles, tube on N, connected to a diaphragm (thin sheet of plastic) - coil of wire on tube on the N pole connected to AC supply 1) sound waves hit the diaphragm, causing it to vibrate 2) the coil moves in and out of the magnetic field 3) this induced a potential difference across the ends of the wire 4) this is passed through an amplifier into a moving coil loudspeaker

Question 37

Why is an iron core used in a transformer?

Answer 37

An iron core is used in a transformer as it is easily magnetised, and it increases the strength of the magnetic field.

Question 38

Describe how a transformer works.

Answer 38

transformer: - the primary coil is connected to an AC, so it generates a changing magnetic field - this is transmitted along the iron core and through the secondary coil - this induces a potential difference

Question 39

Why do transformers only work with AC's?

Answer 39

Transformers only work with AC's as they induce a potential difference, whereas DC's don't.

Question 40

How can a transformer make the V equal on each side?

Answer 40

If the primary and secondary coil have the same number of turns, then the potential difference will be the same. This will only work if the transformer is 100% efficient.

Question 41

How do step-up transformers work?

Answer 41

Step-up transformers have more turns in the secondary coil as it increases the potential difference.

Question 42

How do step-down transformers work?

Answer 42

Step-down transformers have less turns in the secondary coil as it decreases the potential difference.