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Physics > Mains electricity > Flashcards

Mains electricity Flashcards

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1
Q

Direct current (DC)

A

Electric current in a circuit that is in one direction only

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2
Q

Alternating current (AC)

A

Electric current in a circuit that repeatedly reverses its direction

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3
Q

Frequency

A

The number of wave crests passing a fixed point every second

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4
Q

Live wire

A

The wire of a mains circuit that has a potential that alternates from positive to negative and back each cycle

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5
Q

Neutral wire

A

The wire of a mains circuit that is earthed at the local sub-station so its potential difference is close to zero

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6
Q

Oscilloscope

A

A device used to display the shape of an electrical wave

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7
Q

f=1/t

A

frequency of an arc (Hz) = 1 ÷ time for one cycle (s)

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8
Q

Socket

A

A mains socket is used to connect the mains plug of a mains appliance to a mains circuit

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9
Q

Cable

A

Two or three insulated wires surrounded by an outer layer of rubber or flexible plastic

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10
Q

Three-pin plug

A

A three pin plug has a live pin, a neutral pin and an earth pin - the earth pin is used to earth the metal case of an appliance so the case cannot become live

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11
Q

Fuse

A

A fuse contains a thin wire that melts and cuts off the current if too much current passes through it

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12
Q

Circuit breaker

A

An electromagnetic switch that opens and cuts the current off if too much current passes through it

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13
Q

Residual current circuit breaker (RCCB)

A

An RCCB cuts off the current in a live wire when it is different from the current in a neutral wire

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14
Q

P=IV

A

power (W) = current (A) x potential difference (V)

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15
Q

Q=It

A

charge (C) = current (A) x time (s)

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16
Q

E=VQ

A

energy (J) = potential difference (V) x charge (Q)

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17
Q

What is mean by direct current and alternating current?

A

Direct current is in one direction only, whereas alternating current repeatedly reverses its direction

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18
Q

What do we mean by the peak voltage of an alternating potential difference?

A

The peak voltage of an alternating potential difference is the maximum voltage measured from zero volts

19
Q

What is the frequency of the UK mains system?

A

50 Hz

20
Q

What is the peak voltage of the UK mains system?

A

±325 V

21
Q

What is the direct voltage of the UK mains system?

A

230 V

22
Q

What do we mean by the live wire and the neutral wire of a mains circuit?

A

A mains circuit has a live wire that is alternately positive and negative every cycle and a neutral wire at zero volts

23
Q

How do we use an oscilloscope to measure the frequency of an alternating current?

A

Measure the time period taken by the waves then use the formula f=1/t

24
Q

What is the casing of a mains plug or socket made from and why?

A

Sockets and plug cases are made of stiff plastic materials that enclose the electrical connections. Plastic is used because it is a good electrical conductor

25
Q

What is a mains cable?

A

Mains cables are three core cables consisting of two or three insulated copper wires surrounded by an outer layer of flexible plastic material

26
Q

What colour is the live wire?

A

Brown

27
Q

What colour is the neutral wire?

A

Blue

28
Q

What colour is the earth wire?

A

Green and yellow striped

29
Q

Why does a 3-pin plug include an earth pin?

A

The earth wire is connected to the longest pin and is used to earth the metal case of a mains appliance

30
Q

Why is brass better than copper for the parts of three-pin plug?

A

While copper is more conductive, brass is harder, and so more durable

31
Q

What do we use a fuse for?

A

A fuse contains a thin wire that heats up, melts and cuts off the current if the current is too large

32
Q

Why is a fuse always on the “live” side of an appliance?

A

A fuse is always fitted in series with the live wire, thus cutting the appliance off from the live wire if the fuse blows

33
Q

What is a circuit breaker

A

A circuit breaker is an electromagnetic switch that opens (“trips”) and cuts off the current if too much current passes through the circuit breaker

34
Q

Why are appliances with plastic cases not earthed?

A

A mains appliance with a plastic case does not need to be earthed because plastic is an insulator and cannot become live

35
Q

What is the relationship between power and energy?

A

The power supplied to a device is the energy transferred to it each second

36
Q

How can we calculate the correct current for a fuse?

A

The correct rating for a fuse, in amperes, equals the electrical power divided by the potential difference

37
Q

What is an electric current?

A

An electric current is the rate of flow of charge

38
Q

What energy transfers take place when charge flows through a resistor?

A

When an electrical charge flows through a resistor, energy transferred to the resistor makes it hot

39
Q

What can we say about the electrical energy supplied by the battery in a circuit and the electrical energy transferred to the components?

A

When charge flows round a circuit for a certain time, the electrical energy supplied by the battery is equal to the electrical energy transferred to all the components in the circuit

40
Q

Why are electrical faults dangerous?

A

Electrical faults are dangerous because they can cause electric shocks and fires

41
Q

How can we prevent electrical faults?

A

Never touch a mains appliance with wet hands, and never touch a bare wire or a terminal at a potential of more than 30 V. Check cables, plugs and sockets for damage regularly, and check smoke alarms and infrared sensors regularly

42
Q

When choosing an electrical appliance, what factors should in addition to cost should we consider?

A

When choosing an electrical appliance, the power and efficiency rating of the appliance need to be considered

43
Q

How do different forms of lighting compare in terms of cost and energy efficiency?

A

Filament bulbs and halogen bulbs are much less efficient than low energy bulbs

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