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Physics Michael > P2.4 Mains Electricity and Appliances > Flashcards

P2.4 Mains Electricity and Appliances Flashcards

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1
Q

What is electric current?

A

The movement of charge carriers

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2
Q

Cells and batteries supply direct current, what does this mean?

A

The current always keeps flowing in the same direction

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3
Q

The UK mains supply alternating current, what does this mean?

A

The current is constantly changing direction

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4
Q

What is the frequency and voltage of the UK mains supply?

A

The frequency is 50 cycles per second (50Hz) and the voltage is around 230V

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5
Q

What does a Cathode ray oscilloscope show?

A

How the voltage of an electricity supply changes over time

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6
Q

What trace is seen on a Cathode ray oscilloscope if you plug in a d.c. supply?

A

A straight line - the voltage is just the distance of the straight line trace to the centre line

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7
Q

What trace is seen on a Cathode ray oscilloscope if you plug in a a.c. supply?

A

The trace goes up and down in a regular pattern - some of the time it’s positive and some of the time it’s negative

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8
Q

On a CRO how do you find the voltage of an a.c. supply?

A

The vertical height of the peak of trace is the a.c. supplies potential difference

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9
Q

What is a time period on an OCR?

A

The time taken to complete one cycle e.g. from crest to crest

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10
Q

How do you work out the frequency on an OCR?

A

Frequency (Hz) = 1 ÷ time period (s)

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11
Q

What would the time period be between two peaks?

A

The horizontal difference between the two

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12
Q

What could ‘long cables’ result in? (2)

A

Someone could trip over them, or they could overheat and cause a fire if they’re bundled up

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13
Q

What could ‘frayed cables’ result in? (1)

A

Exposed wires could give an electrical shock

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14
Q

What could ‘too many plugs in one socket’ result in? (1)

A

The socket could overheat and cause a fire

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15
Q

What could ‘cables in contact with something hot’ result in? (1)

A

The could overheat and cause a fire

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16
Q

What could ‘water near cables or sockets’ result in? (2)

A

Water conducts electricity so you could get an electric shock or even cause electrocution

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17
Q

What could ‘shoving things into sockets’ result in? (2)

A

Electric shock or even cause electrocution

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18
Q

What could ‘damaged plugs’ result in? (1)

A

Exposed live parts could give an electric shock

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19
Q

What could ‘empty light bulb sockets’ result in? (1)

A

Touching the socket could give you an electric shock

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20
Q

What could ‘appliances without their covers on’ result in? (1)

A

Exposed live parts could give you an electric shock

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21
Q

What do three-core cables consist of?

A

A live wire, a neutral wire and an earth wire

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22
Q

What does the brown live wire do in a plug?

A

Alternates between a high positive and negative voltage

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23
Q

What does the blue neutral wire do in a plug?

A

It’s always at 0V - electricity normally only flows in and out through the live and neutral wires

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24
Q

What does the green and yellow earth wire do in a plug?

A

It protects the wiring - it works with the fuse to prevent fire and shocks

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25
Q

How does the earth wire prevent fire and shocks?

A

It is connected to the metal casing of the appliance and carries electrical charge away from you and towards earth in case something goes wrong

26
Q

What is the difference between a two-core cable and three-core cables?

A

Two-core cables don’t have an earth wire, instead the appliance they connected to, has a casing which won’t conduct electricity

27
Q

How are most appliances connected to the UK mains supply?

A

Using three-pin plugs and a three-core cable

28
Q

When assessing the safety of the wiring of a plug, what should you look out for? (3)

A

That the right coloured wire is connected to each pin and is firmly screwed in, no bare wires are showing inside the plug and the cable grip is tightly fastened over the cable outer layer

29
Q

When assessing the efficiency of a plug, what should you look out for? (3)

A

That the metal parts of a plug are made of copper or brass (as they’re good conductors) and that the case, cable grip and cable insulation is made of rubber or plastic as they’re insulators and flexible

30
Q

How do fuses and circuit breakers help in a plug?

A

They prevent damage during a current surge or if someone touches part of an appliance that’s live

31
Q

How do earth wires help to keep people safe?

A

It works with fuses or circuit breakers if there’s an electrical fault in an appliance

32
Q

What is a fuse made of? Where is it found?

A

A tube containing a short length of wire found in a three-pin plug

33
Q

What happens to the fuse when the current is too high in an appliance?

A

The fuse wire will melt and break the circuit

34
Q

How do circuit breakers protect devices from current surge?

A

They are similar to fuses, when they detect a surge they break the circuit by opening a switch in the live wire

35
Q

What benefits are there in using a circuit breaker over a fuse? (2)

A

They operate much faster than fuses - they break as soon as there is a surge which makes them safer and they can easily be reset whereas fuses melt and need to be replaced

36
Q

What drawbacks are there in using circuit breakers?

A

They’re more expensive than fuses

37
Q

How does the earth wire work with a fuse or a circuit breaker to prevent damage? (3 steps)

A
  1. If the live wire touches the case of the appliance, the case is now earthed and too great a current flows in through the live wire, the case and the live wire
  2. This surge in current melts the fuse (or trips the circuit breaker in the live wire) when the amount of current is greater than the fuse rating . This breaks the circuit
  3. This stops electrical current from flowing so it is impossible to get an electric shock and prevents the risk of fire
38
Q

How are fuses rated?

A

As near as possible but just higher than the normal operating current of the appliance they’re in

39
Q

Why do appliance with large amounts of current require thicker cables?

A

More powerful appliances draw more current from the mains supply so it is for safety purposes

40
Q

How do Residual Current Circuit Breakers work?

A

Normally the same amount of current passes through the live and neutral wires, if someone touches the live wire there will be more current in the live wire than the neutral wire - this difference in current causes the RCCB to quickly cut off the power by opening a switch

41
Q

What is a double insulated appliance?

A

An appliance that has a plastic casing and no metal parts - they don’t require earth wires as the case can’t conduct electricity

42
Q

What is a cable called containing just a live and neutral wire?

A

A two-core cable

43
Q

What is electrical energy converted into when current flows through something with electrical resistance?

A

Heat energy

44
Q

How does the heat produced change as you increase the current flowing through a circuit with an electrical resistor?

A

The more current flows, the more heat produced

45
Q

How does the heat produced change as you increase the voltage of a circuit with an electrical resistor?

A

A bigger voltage means more heating because it pushes more current through

46
Q

What issues are there with converting electrical energy into heat energy due electrical resistors?

A

It wasting energy which can be used as useful energy (e.g. the job the appliance is supposed to carry out)

47
Q

What are energy efficient appliances?

A

Appliances that transfer more of their total electrical energy input to useful energy

48
Q

What determines the total energy transferred by an appliance?

A

How long the appliance is on and the power at which it’s operating

49
Q

What is the equation for finding the power of an appliance?

A

P = E/t ==> Power (W) = energy transferred (J) / Time (s)

50
Q

What does a more powerful appliance do?

A

Transfer more energy in the same amount of time as a less powerful appliance

51
Q

What formula can you use to work out the power of an appliance when you know the p.d. across the appliance?

A

P = I x V ==> Power (W) = current (A) x p.d. (V)

52
Q

How do you work out the fuse needed for an appliance?

A

You need to work out the current that the appliance will use if it’s operating at its power and voltage ratings

53
Q

What does the power rating give you?

A

The maximum power at which you can safely use the appliance at the voltage given

54
Q

What are the power and voltage ratings for most UK appliances?

A

The power and voltage at which they operate when they’re plugged into a 230 V mains supply

55
Q

What formula can you use to work out the fuse needed for an appliance?

A

I = P/V ==> Current = power / p.d.

56
Q

What fuse do you choose for an appliance after working out its normal current?

A

The fuse you choose should be rated just a little higher than the normal current

57
Q

What happens when an electrical charge goes through a change in p.d.?

A

Energy is transferred

58
Q

How do you raise the potential of a power source?

A

By supplying energy to the power source of a circuit

59
Q

When does the charge of a circuit give up the energy it gains from an increase in potential?

A

When it ‘falls’ through any potential drop in components elsewhere in the circuit

60
Q

What is the p.d. between two points?

A

The energy transferred per unit charge passing between the two points

61
Q

How do you calculate the p.d. between two points?

A

E = V x Q ==> Energy transferred (J) = p.d. (V) x charge (C)

62
Q

How does a battery with a bigger voltage affect a circuit?

A

It will supply more energy to the circuit for every coulomb of charge which flows round it, because the charge is raised up ‘higher’ at the start and more energy will be transferred in the circuit too

Physics Michael (14 decks)

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