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Physics > Mains Electricity > Flashcards

Mains Electricity Flashcards

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1
Q

What is direct current?

A

Electric current in a circuit that is in one direction only.

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2
Q

What is an alternating current?

A

Electric current in a circuit that repeatedly reverses its direction.

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3
Q

What is the frequency?

A

The number of wave crests passing a fixed point every second.

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4
Q

What is the unit for frequency?

A

Hz

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5
Q

What is the peak voltage of an alternating potential difference?

A

The maximum voltage measures from zero volts

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6
Q

What is the mains frequency in the UK?

A

50Hz

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7
Q

What is the live wire?

A

It is alternatively positive and negative every cycle.

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8
Q

What is the neutral wire?

A
  • it’s earthed at the local substation.

* it is at zero volts.

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9
Q

What is an oscilloscope?

A

A device used to display the shape of an electrical wave.

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10
Q

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

A

Measure the time period of the waves then use the formula:

Frequency = 1 / time for one cycle

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11
Q

What does the live wire alternate between?

A

+325V and -325V

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12
Q

Where is the earth wire connected to?

A

The ground at your home.

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13
Q

Why is there a longer pin of a three-pin plug?

A

It makes contacts with the earth wire of a wall socket circuit. When you plug an appliance with a metal case to a wall socket, the case is automatically earthed.

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14
Q

Why are sockets and plugs made of stuff plastic materials?

A

Plastic is a good electrical insulator.

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15
Q

Why are the pins made out of brass?

A
  • it is a good electrical conductor.

* it does not rust/oxidise.

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16
Q

Why isn’t copper used instead of brass?

A

It isn’t as hard as brass although it conducts better.

17
Q

Why is there a fuse in the plug?

A

If too much current passes through the wire in a the fuse, it melts and cuts the live wire off.

18
Q

What colour is the live wire?

A

Brown.

19
Q

What colour is the neutral wire?

A

Blue.

20
Q

What colour is the earth wire?

A

Yellow and green.

21
Q

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

A

A two-core cable does not have an earth wire.

22
Q

Why is copper used for the wires?

A
  • good electrical conductor.

* bends easily.

23
Q

Why is plastic used to surround the copper wires?

A

It is a good electrical insulator so it prevents anyone touching the cable from receiving an electric shock.

24
Q

What are two-core cables used for?

A

Appliances with plastic cases.

25
Q

What is a fuse?

A

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

26
Q

Why is a fuse always fitted in series with the live wire?

A

It cuts the appliance off from the live wire if the fuse blows.

27
Q

What is the rating of a fuse?

A

The maximum current that can pass through it without melting the fuse wire.

28
Q

What is a circuit breaker?

A

An electromagnetic switch that opens and cuts off the current if too much current passes through the circuit breaker.

29
Q

How does the circuit breaker cut off the current?

A

If the current in the live wire is too large, the magnetic field of the electromagnetic is strong enough to pull the switch contacts apart.

30
Q

Why are circuit breakers used instead of fuses?

A
  • they work faster than fuses.

* they can be reset more quickly.

31
Q

What is a residual current circuit breaker (RCCB)?

A

It cuts off the current in the live wire when it is different from the current in the neutral wire.

32
Q

When can the RCCB be used?

A

When there is no earth connection.

33
Q

Why is the RCCB better than a fuse or circuit breaker?

A

It is more sensitive.

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