P7 Electromagnetism

Question 1

What is a magnetic field

Answer 1

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

Question 2

What is an induced magnet?

Answer 2

A material that becomes magnetic when it is placed in a magnetic field. When removed it is no longer magnetic

Question 3

What are permanent magnets?

Answer 3

Made from nickel, cobalt, some steels

Question 4

The earths magnetic field

Answer 4

The geographical North Pole is actually the magnetic South Pole

Question 5

The magnetic effect of a current

Answer 5

When a current flows through a wire it creates a magnetic field around the wire.

Question 6

Flemings right hand grip rule

Answer 6

The thumb is the current direction

The fingers show the field direction

Question 7

Magnetic field in a loop of wire

Answer 7

The magnetic field is strongest in the middle

Question 8

Magnetic field in a solenoid

Answer 8

Similar to a bar magnet.

Strongest in the middle and at poles than than the outside.

Question 9

The motor effect

Answer 9

When a wire with a current passing through it is placed between magnets the resultant force is stronger in one area and weaker in another. A force is produced on the wire causing it to move

Question 10

Flemings left hand rule

Answer 10

thuMb- Motion
First finger- Field
seCond finger- Current

Question 11

How can u increase the force on the wire

Answer 11

Increase:
Size of the magnetic field
Current passing through the wire
The length of the wire that is is the magnetic field

Question 12

What us an electromagnet?

Answer 12

A copper solenoid around an iron core. This magnetises the core creating an electromagnet which is 1000x stronger than the coil by its self. It can be switched on and off as it is a temporary magnet

Question 13

The electric bell

Answer 13

It’s circuit contains an electromagnet which attracts the hammer and iron contact this actions makes the bell ring but also breaks the circuit. The magnet is switched off as the circuit is broken. Once the hammer reaches the original position the circuit works again and the cycle continues.

Question 14

How do relays work?

Answer 14

A relay is a magnetic switch. It uses a small current to switch on a much larger current.

Question 15

Magnetic flux density (B)

Answer 15

The strength of a magnetic field measured in tesla (T)

Question 16

How do D.C. motors work?

Answer 16

When a current passes through a coil placed between the poles of a magnet there is a force acting on each side.left hand rule shows that each side of the coil will move.one side up other side down. It has a split ring commutator which allows the motor to spin.
When the coil is vertical the circuit is broken but the coil has gained enough momentum move to other side. The commutator is now in connection with the circuit and the current can flow.

Question 17

How do A.C. Motors work?

Answer 17

They work similar to a D.C. Motor accept they have slip rings. As the coil rotates, the direction of the current passing through the coil is reverses direction. When the coil has gone through half a turn, the direction of the current has reversed.

Question 18

Producing sounds

Answer 18

A series of oscillations or vibrations that travel in the same direction as the direction of the oscillations (longitudinal waves)
Vibrations travel through the air as pressure waves and transfers energy

Question 19

A moving coil loudspeaker

Answer 19

Has a permanent magnet and a coil of wire. The magnet is fixed but the coil can move forwards and backwards. Coil is attached to the cone. Outer edge of cone doesn’t move(fixed)

Question 20

How does a moving coil loudspeaker work?

Answer 20

Cylindrical magnet produces a strong radial magnetic field at normal to the coil. A signal (current) varies flow through coil. A change in current changes magnetic force on coil. Changing force makes coil move in and out. As current varies, coil moves. Coil attached to cone makes cone move setting up vibrations in the air.

Question 21

How do earphones work?

Answer 21

They work the same as loudspeakers except they are smaller and sound doesn’t have to travel as far

Question 22

What is the generator effect

Answer 22

When a wire is stationary between a magnet there is no current in the wire. If the wire moves it cuts the magnetic field lines. A potential difference is induced across the end of the wire and if the wire is part of a complete circuit an induced current passes along the wire.

Question 23

Inducing a potential difference

Answer 23

When a magnetic field changes near a conductor, even if the conductor does not move. Moving near a conductor changes the magnetic field around the conductor.

Question 24

The induced pd can be increased by:

Answer 24

Moving the wire or magnet faster
Using a stronger magnet
Replacing the wire with a coil

Question 25

What is the direction of the induced pd?

Answer 25

It is always such to oppose the motion or change producing it. Energy is transferred from the movement of the magnet into the coil to a flow of current.

Question 26

Moving coil microphone

Answer 26

Act like a loudspeaker in reverse. Energy is transferred from the moving air into the coil, causing a current to flow. Coil is connected to a diaphragm which is fixed. The coil surrounds a permanent magnet. Sound waves make to diaphragm vibrate causing the coil to move forward and backwards in the gap, inducing a pd and causing a varying current to flow

Question 27

Dynamo

Answer 27

Generates a direct current by moving the coil in a magnetic field. When it turns it cut the magnetic field lines and induces a potential difference between the end of the coil. Causing an induced current to pass through the coil and through the circuit the coil is part of.

Question 28

An induced current can be increased by:

Answer 28

Increasing number of turns on the coil Using more powerful magnets Turning the coil faster

Question 29

What does a transformer do?

Answer 29

Changes the size of an alternating pd.

Question 30

What is a transformer?

Answer 30

2 coils of wire wound separately on an iron core. An alternating pd is applied to primary coil producing an alternating current in the primary coil. Producing alternating magnetic field in the iron core. The strength of magnetic field increases as iron is easily magnetised. Alternating magnetic field in iron core induces pd in the secondary coil. This induces an alternating current in the circuit connected to the secondary coil.

Question 31

Step up transformer

Answer 31

Increases pd | More turns on secondary coil than on primary coil

Question 32

Step down transformers

Answer 32

Decreases pd | More turns on primary coil than on secondary coil