Using magnetic fields to keep things working

Question 1

What are the ends of a magnet called?

What is the magnetic field?

Answer 1

The ends of a magnet are called the magnetic poles, there is a north pole at one end and a south pole at the other.
The magnetic field is the region around a magnet in which a piece of iron or steel will be attracted to the magnet.

Question 2

What will happen to iron filings placed near the magnet?
What are these lines called?

In which direction will a plotting compass placed in the magnetic field point?

Answer 2

Iron filings placed near a magnet will form a pattern of lines that loop from one pole to the other. These are lines of force or magnetic field lines.
A plotting compass placed in the magnetic field will always point along a field line.

Question 3

If two magnets are brought together with like poles together, how will they behave?
If unlike poles are together how will the magnets behave?

Answer 3

They will repel each other.

They will attract each other.

Question 4

What is produced around a wire when current flows through it?

Answer 4

When current flows through a wire, a magnetic field is produced around the wire.

Question 5

How is an electromagnet made?
What happens when a current flows through the wire?
What happens when the current is switched off?

Answer 5

An electromagnet is made by wrapping insulated wire around a piece of iron, called the core.
When a current flows through the wire the iron becomes strongly magnetised, when the current is switched off, the iron loses its magnetism.

Question 6

What makes electromagnets very useful?

Name four devices that use electromagnets.

Answer 6

Electromagnets are very useful because of the temporary magnetism when current flows through the wire.

1) Scrapyard cranes
2) Circuit breakers
3) Electric bells
4) Relays

Question 7

What is the motor effect?

When is the force a maximum and when is it zero?

Answer 7

The motor effect is the force experienced by a wire carrying an electric current when it is placed in a magnetic field.
The force is a maximum if the wire is at an angle of 90º to the magnetic field and zero if the wire is parallel to the magnetic field.

Question 8

For what is Fleming’s left hand rule used?
How can it be represented on someones hand?
What does each of the fingers and the thumb represent?

Answer 8

Fleming’s left hand rule is used to determine the direction of the force, in the motor effect.
The thumb and first two fingers on the left hand are all held at right angles to each other:
1) The first finger represents the magnetic field (pointing north to south)
2) The second finger represents the current (pointing positive to negative)
3) The thumb represents the direction of the force.

Question 9

How can the size of the force in the motor effect be increased? (2)

Answer 9

1) Increasing the strength of the magnetic field.

2) Increasing the size of the current.

Question 10

The direction of the force on the wire in the motor effect is reversed by what two things?

Answer 10

1) If the direction of the current is reversed.

2) If the direction of the magnetic field is reversed.

Question 11

How can the speed of an electric motor be increased?

How can the direction of an electric motor be reversed?

Answer 11

The speed of the motor is increased by increasing the size of the current.
The direction of the motor can be reversed by reversing the direction of the current.

Question 12

Why does the coil in a motor spin when a current passes through the coil? (2)

Answer 12

1) A force acts on each side of the coil due to the motor effect.
2) The force on one side of the coil is in the opposite direction to the force on the other side.

Question 13

What does the split ring commutator do to the direction of the current?
Why is the coil always pushed in the same direction?

Answer 13

The split ring commutator reverses the direction of the current around the coil every half-turn.
Because both sides swap over each half-turn, the coil is always pushed in the same direction.

Question 14

What happens if an electrical conductor ‘cuts’ through magnetic field lines?

Answer 14

If an electrical conductor ‘cuts’ through magnetic field lines, a potential difference is induced across the ends of the conductor.

Question 15

What happens if a magnet is moved into a coil of wire?
What is this process called?
What happens if the wire or coil is part of a complete circuit?

Answer 15

If a magnet is moved into a coil of wire, a potential difference is induced across the ends of the coil. This process is called electromagnetic induction.
If the wire or coil is part of a complete circuit, a current passes through it.

Question 16

In electromagnetic induction what will happen if the direction of movement of the wire or coil is reversed, or the polarity of the magnet is reversed?
When is a pd induced?

Answer 16

If the direction of movement of the wire or coil is reversed, or the polarity of the magnet is reversed, the direction of the induced pd is also reversed.
A pd is only induced while there is movement.

Question 17

The size of the induced pd in electromagnetic induction is increased by increasing what three factors?

Answer 17

1) The speed of movement
2) The strength of the magnetic field
3) The number of turns of the coil

Question 18

What does a transformer consist of?
What happens when an alternating current is passed through the primary coil?
What happens to this?

Answer 18

A trans former consists of two coils of insulated, called the primary coil and the secondary coil. These coils are wound on to the same iron core.
When an alternating current passes through the primary coil, it produces an alternating magnetic field in the core. This field continually expands and collapses.

Question 19

In transformers:
What affect do the alternating magnetic field lines have on the secondary coil?
What if the the secondary coil is part of a circuit?

Answer 19

The alternating magnetic field lines pass through the secondary coil and induce an alternating potential difference across its ends.
If the secondary coil is part of a complete circuit and alternating current is produced.

Question 20

In tranformers:
Why are the coils of wire insulated?
Why is the core made of iron?

Answer 20

The coils of wire are insulated so that current does not short across either the iron core or adjacent turns of the wire, but flows around the whole coil.
The core is made of iron so it is easily magnetised.

Question 21

What does a step-up transformer do?

What does a step-down transformer do?

Answer 21

A step-up transformer makes the pd across the secondary coil greater than the pd across the primary coil. Its secondary coil has more turns than the primary coil.
A step-down transformer makes the pd across the secondary coil less than the pd across the primary coil. It secondary coil has fewer turns than its secondary coil.

Question 22

From what is the core made in a switch mode transformer?

What are the advantages of a switch mode transformer? (3)

Answer 22

A switch mode transformer has a ferrite core.

1) A switch mode operates at a much higher frequency.
2) It is lighter and smaller.
3) It uses very little power when there is no device connected its output terminals.

Question 23

For what does the national grid use transformers and why is this necessary?

What are step-down transformers used for?

Answer 23

The national grid uses transformers to step up the pd from power stations, this is because the higher the pd at which electrical energy is transmitted across the grid, the less energy is wasted in the cables.
Step-down transformers are used to reduce the pd so that it is safe to be used by consumers.

Question 24

What equation relates the pd across, and the number of turns on, the primary and secondary coils?

Answer 24

Vp/Vs = Np/Ns
Vp is the pd across the primary coil in volts, V
Vs is the pd across the secondary coil in volts, V
Np is the number of turns on the primary coil
Ns is the number of turns on the secondary coil

Question 25

How efficient are transformers?

How is the efficiency of a transformer represented?

Answer 25

100%.
Vp x Ip = Vs x Is
Ip is the current in the primary in amperes, A
Is is the current in the secondary in amperes, A

Question 26

What is an ECG used for?

Answer 26

An ECG or electrocardiogram is used to measure the potential differences generated by the heart.

Question 27

What are electronic devices used for in hospitals?

What are digital thermometers used for?

Answer 27

Electronic devices are used to measure the blood pressure.

Digital thermometers are used to measure temperature.

Question 28

What is an endoscope used for?

What are x-rays used for?

Answer 28

An endoscope contain bundles of fibre optics is used to look inside the body without making large incisions.
X-rays are used to take pictures of suspected broken bones.

Question 29

What are CT scanners used for?

What are MR scanners used for?

Answer 29

CT scanners are used to build up digital pictures of a cross-section through the body.
MR scanners use radio waves to produce detailed digital pictures of the body.