Unit 2: 5 - Mains Electricity Flashcards

0
Q

What is alternating current (ac)?

A

When the mains supply passes the current in one direction, then reverses it in the other direction and repeats.

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1
Q

What is direct current (dc)?

A

When cells/batteries pass the current round the circuit in one direction.

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2
Q

What is the frequency of UK mains electricity?

A

50Hz

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3
Q

What does the frequency mean in ac?

A

How many times the flow changes direction. E.g. 20Hz means it changes direction 20 times a second.

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4
Q

What does the live wire in a plug do?

A

Gives the mains supply alternating between a positive and a negative potential, with respect to the neutral wire.

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5
Q

What ‘voltage’ is the mains supply in the UK?

A

230V

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6
Q

What ‘voltage’ is the neutral wire at?

A

0V

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7
Q

What peak voltages does the live wire alternate between?

A

+325 and -325 volts

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8
Q

How can the frequency of ac be determined from an oscilloscope?

A

f=1/T

Frequency = 1 / Time

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9
Q

What colour is the live wire?

A

Brown

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10
Q

What colour is the neutral wire?

A

Blue

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11
Q

What colour is the earth wire?

A

Green and yellow

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12
Q

The more current carried, the _____ the cable needs to be.

A

Thicker

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13
Q

Why are the pins in a plug made of brass?

A

Brass is a good electrical conductor, yet also hard and it will not rust or oxidise.

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14
Q

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

A

A 2-core does not have an earth wire, whereas a 3-core does.

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15
Q

What sorts of appliances must be earthed? And why?

A

Appliances with metal casings. If there is a fault, the live wire could touch the metal case and make it live, causing a shock to anyone who touches it.

16
Q

A fuse is fitted in ____ with the live wire.

A

Series

17
Q

How does a fuse work?

A

If a fault develops in an earthed appliance, a large current flows to earth and melts the fuse, cutting off the supply.

18
Q

How must the rating of a fuse compare to the normal working current of the appliance?

A

It should be slightly higher (else it would straight when the appliance is switched on).

19
Q

What is a circuit breaker?

A

A device that can be used instead of a fuse. It is an electromagnetic switch that cuts off if the current is bigger than a certain value.

20
Q

What is a residual current circuit breaker (RCCB)?

A

A device that cuts off the current in the live wire if it is different to the current in the neutral wire.

21
Q

What are the benefits of an RCCB?

A

It works faster than a fuse or an ordinary circuit breaker.

22
Q

Why don’t appliances with a plastic casing need to be earthed?

A

Because plastic is an insulator so will not deliver a shock.

23
Q

How do we work out the charge in Coulombs (using current)?

A

Q=I*t

Charge in coulombs = Current in amperes * time in seconds

24
Q

What is coulombs a unit for?

A

Charge

25
Q

Give 5 ways in which to avoid fire or shock with electrical appliances.

A
  • Don’t overload sockets
  • Check for damaged and worn cables regularly
  • Don’t use electrical appliances in a bathroom or with wet hands
  • Don’t touch a bare wire or terminal of more than 30V
  • Call in an electrician for mains repair