P3 - Using Magnetic Fields

Question 1

How is an electromagnet made?

Answer 1

• Wrapping an insulated wire around an iron core.

- Passing a current through the wire magnetises the iron core.

Question 2

Why is steel unsuitable for use in an electromagnet?

Answer 2

Steel keeps its magnetism when the current is removed.

Question 3

Give four examples of everyday uses of electromagnets.

Answer 3

• Scrapyard crane
• Circuit braker
• Electric bell
• Relay

Question 4

How does a circuit breaker work?

Answer 4

• There is a switch in series with the electromagnet.
• The switch is held in place with an iron catch.
• When too much current passes through the electromagnet, the switch is pulled open and has to be reset manually.

Question 5

How does an electric bell work?

Answer 5

• When connected to the battery, the iron armature is pulled onto the electromagnet.
• The circuit is opened so the electromagnet is switched off and the armature springs back onto the bell.
• The armature then closes the switch making the process repeat.

Question 6

How does a relay work?

Answer 6

• When current passes through the electromagnet, the armature is pulled onto the electromagnet.
• The armature turns about the pivot and closes the switch.
• The small current through the electromagnet is used to turn on a larger current.

Question 7

How can the size of the force of the motor effect be increased?

Answer 7

• Increasing the current.

- Using a stronger magnet.

Question 8

How does the size of the force of the motor effect depend on the angle between the wire and magnetic field lines?

Answer 8

• Greatest when the wire is perpendicular to the magnetic field.
• Zero when the wire is parallel to the magnetic field lines.

Question 9

What happens to the forces in the motor effect when the current or the magnetic field is reversed?

Answer 9

The direction of the force is reversed.

Question 10

How can the speed of an electric motor be changed?

Answer 10

Changing the current.

Question 11

How can the direction of an electric motor be changed?

Answer 11

Reversing the current.

Question 12

What is a simple electric motor made of?

Answer 12

• Rectangular coil of insulated wire.
• Two graphite brushes.
• Split ring commutator.

Question 13

Why does the coil in a motor rotate?

Answer 13

• A force acts on each side of the coil.

- The force on each side is opposite.

Question 14

What is the purpose of the split ring commutator?

Answer 14

• It reverses the current round the coil every half turn.

- This means the coil is pushed in the same direction.

Question 15

Why does the diaphragm of a speaker vibrate?

Answer 15

• When a current passes through the coil, the force makes the coil move.
• When the current changes direction, the force reverses its direction making the coil move forwards and backwards.
• The vibrations produce sound waves.

Question 16

What is electromagnetic induction?

Answer 16

The process of creating a potential difference using a magnetic field.

Question 17

How can a magnet induce a potential difference?

Answer 17

When a conductor cuts the lines of a magnetic field, a potential difference is induced across the ends of the conductor.

Question 18

How can an electromagnet induce a potential difference?

Answer 18

When an electromagnet is switched on, the magnetic field is broken by the conductor.

Question 19

Who discovered electromagnetic induction?

Answer 19

Michael Faraday.

Question 20

Why do transformers only work with AC?

Answer 20

A changing magnetic field is necessary to induce AC in the secondary coil.

Question 21

What is a transformer for?

Answer 21

A device used to change the size of the alternating pd.

Question 22

What is the core of a transformer made from?

Answer 22

An iron core unless it is a switch mode transformer which has a ferrite core.

Question 23

Give two ways in which a switch mode transformer is different from an ordinary transformer.

Answer 23

• A switch mode transformer is lighter and smaller.

- It operates at high frequency.

Question 24

Why is an alternating pd induced in the secondary coil of a transformer when alternating current passes through the primary coil?

Answer 24

• Alternating current passing through the primary coil produces an alternating magnetic field.
• The lines of the alternating magnetic field pass through the secondary coil.
• The magnetic field is changing.

Question 25

What is a step-up transformer?

Answer 25

A transformer that makes the pd across the secondary coil greater.

Question 26

Which coil has more turns in a step-up transformer?

Answer 26

The secondary coil.

Question 27

What is a step-down transformer?

Answer 27

A transformer that makes the pd across the secondary coil smaller.

Question 28

Which coil has more turns in a step-down transformer?

Answer 28

The primary coil.

Question 29

Why wouldn't a DC transformer work?

Answer 29

There would be no moving magnetic field.

Question 30

Why is the iron core of a transformer laminated?

Answer 30

To cut out induced currents in the iron which would reduce efficiency.

Question 31

What range of frequencies do switch mode transformers work in?

Answer 31

50,000-200,000Hz

Question 32

Give two main differences between a switch mode transformer and regular transformers.

Answer 32

- It is lighter and smaller. | - It uses very little power when no device is attached.

Question 33

What are the three main circuits in a phone charger?

Answer 33

- Frequency converter. - Switch mode transformer. - Low voltage ac to dc converter.

Question 34

What is the equation for power in a transformer that is 100% efficient?

Answer 34

Vp x Ip = Vs x Is

Question 35

What is the transformer equation?

Answer 35

pd across primary / pd across secondary = turns on primary / turns secondary

Question 36

How is an alternating magnetic field set up in a transformer?

Answer 36

There is an alternating current.

Question 37

Give three ways of transporting 500MW.

Answer 37

- 500,000,000V at 1A - 1V at 500,000,000A - 500,000V at 1000A

Question 38

What is the problem with transporting at a high pd?

Answer 38

There would have to be a lot of insulation or the current would jump through the air.

Question 39

What is the problem with transporting at high current?

Answer 39

Thick wires would be needed to prevent melting.

Question 40

What is the relationship between current and energy loss?

Answer 40

- When the current is doubled, the heat energy produced is quadrupled. - Heat energy produced is directly proportional to current flowing squared.

Question 41

Why does the pd have to be kept high in the national grid?

Answer 41

To keep current as low as possible to prevent energy loss.

Question 42

What is Fleming's left hand rule?

Answer 42

- First finger field. - Second finger current - Thumb movement

Question 43

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Answer 43

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