Topic 7 - Magnetism and Electromagnetism

Question 1

What causes the Earth’s magnetic field?

Answer 1

The Earth’s core

Question 2

What materials can become magenetised?

Answer 2

1.Iron
2.Steel
3.Nickel
4.Cobalt

Question 3

What is a magnetic field?

Answer 3

The region around a magnet where other magnets or magnetic materials experience a force.

Question 4

Where is the magnetic field strongest?

Answer 4

The poles

Question 5

What can we use to see a magnetic field?

Answer 5

1.Iron filings
2.Plotting compasses

Question 6

Which direction do magnetic field lines travel?

Answer 6

North to south

Question 7

How do we show the magnetic field getting weaker with a diagram?

Answer 7

The magnetic field lines get further apart

Question 8

What is a uniform magnetic field?

Answer 8

A uniform magnetic field is one that has the same strength and direction at all points.

Question 9

What is an electromagnet?

Answer 9

1. When a conductor has a current flowing through it
2. It creates a magnetic field around the wire
3. Which can be turned on and off when the current is turned on and off.

Question 10

Which direction does a conventional current flow?

Answer 10

Bottom of the page to the top

Question 11

How can we find the direction of a magnetic field from an electromagnet?

Answer 11

The right hand rule

Question 12

What is the right hand rule?

Answer 12

1.Thumb goes in the direction of current
2. The fingers curl in the direction of the field

Question 13

What is a solenoid?

Answer 13

A solenoid is a long piece of wire that is curled to form a coil. When an electric current is passed through this coil, it creates a uniform magnetic field. A type of electromagnet

Question 14

What is the relationship between the number of coils and magnetic field strength?

Answer 14

The more coils the stronger the field

Question 15

What is created inside a solenoid?

Answer 15

A very strong uniform magnetic field

Question 16

What does the direction of the magnetic field in an electromagnet depend on?

Answer 16

The direction of current

Question 17

What is the motor effect?

Answer 17

Where current flowing in a magnetic field creates a force which causes motion

Question 18

What is the left hand rule?

Answer 18

1.The thumb = motion
2.The first finger = magnetic field
3.The second finger = conventional current

Milwall Football Club

Question 19

What does the size of the force from the motor effect depend on?

Answer 19

1.length of wire
2.Strength of magnets
3.Size of current

Bigger = more

Question 19

What is the equation linking force, magnetic field strength, current and length?

Answer 19

force = magnetic field strength x current x length

F = B I l

Question 19

What unit is magnetic field strength measured in ?

Answer 19

Tesla ( T )

Question 20

How does the motor effect work?

Answer 20

1.The magnetic field from the current interacts with the magnetic field from the permanent magnet
2.On one side the magnetic fields cancel eachother out, and on the other they add together.
3.This creates a force in the direction away from the stronger magnetic field

Question 21

How can we make an electric motor stronger/

Answer 21

1. Add more turns coils
2. Add a stronger magnet
3. Add a higher current

Question 22

What is a permanent magnet?

Answer 22

Objects which produce their own magnetic field

Question 23

What is an induced magnet?

Answer 23

Magnetic materials which become magnets when in a magnetic field

Question 24

What are 2 uses of electromagnets?

Answer 24

1.Used in cranes to pick things up
2.Used in the electric starters of motors

Question 25

What must be true to experience the strongest force from the motor effect

Answer 25

The current must flow perpendicular to the field

Question 26

How does dc motor work?

Answer 26

1.There is a coil of wire on a spindle, which loops around between 2 permanent magnets
2.The forces acting here are the same as the regular motor effect
3.As the current flows through the coil, one side of the loop experiences an upward force, whilst the other is forced downwards, causing it to spin
4.The split ring commutator swaps the contacts every half turn to keep the motor rotating in the right direction.

Question 27

How do loudspeakers work?

Answer 27

1.An ac is sent through a coil of wire attached to the base of a paper cone
2.The coil surrounds one pole of a permanent magnet and is surrounded by the other pole, so the current causes a force on the coil, which causes the cone to move
3.When the current reverses, the force reverses and the cone comes back down
4.Different frequencies of ac can make the cone vibrate at different frequencies, creating different sounds

Question 27

What is the generator effect?

Answer 27

The induction of a potential difference (and current if there is a permanent circuit) in a wire which is moving relative to a magnetic field, or experiencing a change in magnetic fields.

Question 28

What is the relationship between the induced current and the change that made it?

Answer 28

The induced current always opposes the change that made it

Question 29

How does the generator effect work?

Answer 29

1. A magnet in a coil of wire moves or vice versa
2. THis cuts the magnetic field lines
3. The shifting from side to side creates a pd and current if in a circuit
4. If you keep moving the magnet or coil backwards and forwards, an alternating pd is made

Question 30

How can you increase the size of an induced pd?

Answer 30

1.Increasing the speed of movement, which cuts more magnetic fields in a shorter period
2.Increasing the strength of the magnetic field, so there are more field lines to be cut

Question 31

How do alternators work?

Answer 31

1.A coil is rotatesd in a field between to permanent magnets
2.As the coil spins, a current is induced in the coil, which changes direction every half turn.
3.The generator has slip rings and brushes so the contacts don’t swap every half turn.
4.So they produce an alternating pd, and an ac

Question 32

How do dynamos work?

Answer 32

1.same as alternators
2.BUT - they have a split ring commutator instead of slip rings
3.This swaps the connection every half turn, so the current keeps flowing in the same direction, so u get dc

Question 33

What does the oscilloscope look like for alternators and dynamos?

Answer 33

1.Alternators - a wavy line through the x - axis
2.Dynamo - a line which bounces along the x - axis

Question 34

What does increasing frequency of revolutions do to a dynamo and alternator?

Answer 34

1.Increases the overall pd and creates more peaks

Question 35

How do microphones work?

Answer 35

1.Loudspeakers in reverse
2.Sound waves hit a flexible diaphragm that is attached to a coil of wire around a magnet
3.This causes the coil to move in the field, which generates current
4.The movement depends on the sound wave

Question 36

What do transformers do?

Answer 36

change the size of the pd of an ac

Question 37

How do transformers work?

Answer 37

1.There are 2 coils of wire, a primary and secondary, joined with an iron core
2.When an alternating pd and ac is applied across the primary coil, it creates an alternating magnetic field around it.
3.This field creates a magnetic field in the core which changes directions.
4.This changing field induces an alternating pd in the second coil

Question 38

What are the 2 equations to do with transformers?

Answer 38

1. Vp/Vs = np/ns
2. Vp x Ip = Vs x Is

1. pd1/pds = coils1/coils2
2. volt1 x current1 = volt2 x current 2