# P3 - Using Magnetic Fields Flashcards

1
Q

How is an electromagnet made?

A
• Wrapping an insulated wire around an iron core.

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

2
Q

Why is steel unsuitable for use in an electromagnet?

A

Steel keeps its magnetism when the current is removed.

3
Q

Give four examples of everyday uses of electromagnets.

A
• Scrapyard crane
• Circuit braker
• Electric bell
• Relay
4
Q

How does a circuit breaker work?

A
• 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.
5
Q

How does an electric bell work?

A
• 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.
6
Q

How does a relay work?

A
• 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.
7
Q

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

A
• Increasing the current.

- Using a stronger magnet.

8
Q

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

A
• Greatest when the wire is perpendicular to the magnetic field.
• Zero when the wire is parallel to the magnetic field lines.
9
Q

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

A

The direction of the force is reversed.

10
Q

How can the speed of an electric motor be changed?

A

Changing the current.

11
Q

How can the direction of an electric motor be changed?

A

Reversing the current.

12
Q

What is a simple electric motor made of?

A
• Rectangular coil of insulated wire.
• Two graphite brushes.
• Split ring commutator.
13
Q

Why does the coil in a motor rotate?

A
• A force acts on each side of the coil.

- The force on each side is opposite.

14
Q

What is the purpose of the split ring commutator?

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

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

15
Q

Why does the diaphragm of a speaker vibrate?

A
• 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.
16
Q

What is electromagnetic induction?

A

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

17
Q

How can a magnet induce a potential difference?

A

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

18
Q

How can an electromagnet induce a potential difference?

A

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

19
Q

Who discovered electromagnetic induction?

A

20
Q

Why do transformers only work with AC?

A

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

21
Q

What is a transformer for?

A

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

22
Q

What is the core of a transformer made from?

A

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

23
Q

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

A
• A switch mode transformer is lighter and smaller.

- It operates at high frequency.

24
Q

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

A
• 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.
25
Q

What is a step-up transformer?

A

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

26
Q

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

A

The secondary coil.

27
Q

What is a step-down transformer?

A

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

28
Q

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

A

The primary coil.

29
Q

Why wouldn’t a DC transformer work?

A

There would be no moving magnetic field.

30
Q

Why is the iron core of a transformer laminated?

A

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

31
Q

What range of frequencies do switch mode transformers work in?

A

50,000-200,000Hz

32
Q

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

A
• It is lighter and smaller.

- It uses very little power when no device is attached.

33
Q

What are the three main circuits in a phone charger?

A
• Frequency converter.
• Switch mode transformer.
• Low voltage ac to dc converter.
34
Q

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

A

Vp x Ip = Vs x Is

35
Q

What is the transformer equation?

A

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

36
Q

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

A

There is an alternating current.

37
Q

Give three ways of transporting 500MW.

A
• 500,000,000V at 1A
• 1V at 500,000,000A
• 500,000V at 1000A
38
Q

What is the problem with transporting at a high pd?

A

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

39
Q

What is the problem with transporting at high current?

A

Thick wires would be needed to prevent melting.

40
Q

What is the relationship between current and energy loss?

A
• When the current is doubled, the heat energy produced is quadrupled.
• Heat energy produced is directly proportional to current flowing squared.
41
Q

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

A

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

42
Q

What is Fleming’s left hand rule?

A
• First finger field.
• Second finger current
• Thumb movement
43
Q

Since you’ve been using these flashcards for free, please consider making a small donation for the hundreds of hours it took to make them.

A

http://bit.ly/21T6H3W

Thank you and good luck!