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Physics- P2 > P2.4 Using mains electricity safely > Flashcards

P2.4 Using mains electricity safely Flashcards

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1
Q

What is a direct current?

A

a current that always flows in the same direction

it is always at the same voltage

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2
Q

What type of current does mains electricity supply?

A

an alternating current

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3
Q

What is an alternating current (a.c) ?

A

A current that is constantly changing direction

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4
Q

What voltage is the mains electricity in the UK?

A

about 230 v

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5
Q

What is the frequency of the mains electricity in the UK?

A

50 cycles per second (50 hertz)

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6
Q

Describe the oscilloscope trace of an alternating current

A

a regular repeating wave.

some of the time it is positive and some of the time it is negative

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7
Q

Describe the oscilloscope trace of a direct current

A

a straight line

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8
Q

What is the time period of a wave?

A

The time taken to complete 1 cycle

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9
Q

On an oscilloscope trace, how do you find the time period of the wave?

A

measure the horizontal difference between 2 peaks then multiply by the time base which will be given.

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10
Q

On an oscilloscope trace, how do you find out the frequency of the wave?

A

frequency = 1 (divided by) time period

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11
Q

On an oscilloscope, what does the timebase dial control?

A

how many milliseconds each devision represents on the horizontal axis

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12
Q

On an oscilloscope, what does the gain dial control?

A

how many volts each cm division represents on the horizontal axis

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13
Q

On an oscilloscope screen, what does the vertical hight of the a.c show?

A

the input voltage at that point

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14
Q

How can you measure the potential difference of an a.c supply on an oscilloscope?

A

By measuring the hight of the trace

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15
Q

How can you measure the potential difference of a d.c on an oscilloscope screen?

A

measure the distance from the straight line trace to the centre line

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16
Q

What are most electrical appliances connected to the mains using?

A

a cable and three-pin-plug

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17
Q

What type of current do cells and batteries supply?

A

a direct current (d.c)

18
Q

Describe the structure of an electric cable

A

3 wires inside, each with a core of copper and a coloured plastic coating

Live wire- brown, alternates between a positive and negative voltage

Neutral wire- blue, always at 0V

Earth wire- green and yellow- protects the wiring and for safety. works together with the fuse to prevent fire and shocks. It is attached to the metal casing of the appliance and carries electricity to the earth if something went wrong or if the live or neutral wires touched the metal case

19
Q

Describe the function of the earth wire in an electrical cable and three-pin-plug

A

Earth wire- green and yellow

protects the wiring and for safety.

works together with the fuse to prevent fire and shocks.

It is attached to the metal casing of the appliance and carries electricity to the earth if something went wrong or if the live or neutral wires touched the metal case

20
Q

What materials are used in a three-pin plug?

A

main parts are made of copper or brass- as good conductors

case, cable grip, cable insulation - rubber or plastic as they are good insulators and flexible

21
Q

What are some safety features of a three-pin-plug?

A

right coloured wire is connected to each pin, and firmly screwed in

no bare wires showing in the plug

cable grip is tightly fastened over the cable outer layer

22
Q

What happens if an electrical fault causes too great a current?

A

the circuit is disconnected by a fuse or circuit breaker in the LIVE wire

23
Q

What happens when the current through a fuse exceeds the rating of the fuse?

A

The fuse will melt, breaking the circuit

24
Q

Why does the fuse rating needed for cables increase with cable thickness?

A

Thicker cables have less resistance, so carry more current

25
Q

What does it mean if an appliance is double insulated?

A

if the appliance has a plastic casing and no metal parts are showing. It does not need an earth wire.

26
Q

What are two-core-cables?

A

cables that only carry the live and neutral wire

They do not need an earth wire as the appliance is double insulated

27
Q

What is ‘earthing’ an appliance?

A

appliances with metal cases are usually earthed to reduce the danger of electric shock

means the case must be attached to an earth wire.

28
Q

What does RCCBs stand for?

A

Residual Current Circuit Breakers

29
Q

What do RCCBs do and how do they work?

A

detect a difference is the current between the live and neutral wires and break the circuit by opening a switch

30
Q

What are some advantages of using RCCBs over fuses?

A

Safer- operate much faster as no time is wasted waiting for the current to melt the fuse

More effective- work for small currents that may not be enough to melt a fuse

more convenient- can easily be reset by flicking a switch whereas fuses need to be replaced

31
Q

What happens when an electrical charge flows through a resistor?

A

the resistor gets hot

32
Q

How do filament lamps work and why is a lot of energy wasted?

A

work by passing a current through a very thin wire. - means there is lots of resistance- heated up so much that it glows.

therefore, lots of energy is wasted as heat

33
Q

What is an example of a power saving lamp?

A

Compact Fluorescent Lamps (CFLs)

34
Q

In the equation

P= E (divided by) t

what do the letters stand for and what are the units?

A

P- power (W)

E- energy (J)

t- time (s)

35
Q

In the equation

P= I x V

What do the letters stand for and what are the units?

A

P- power (W)

I-current (A)

V- potential difference (V)

36
Q

In the equation

E= V x Q

what do the letters stand for and what are the units?

A

E- energy (J)

V- potential difference (V)

Q- charge in coulombs (C)

37
Q

Explain why the kinetic output of a motor will always be less than predicted?

A

As the motor will not be 100% efficient.

some of the energy will be transformed into heat and sound

38
Q

Why may an appliance not be safe if there is no earth wire?

A

The case may become live if there is an electrical fault

this causes a danger of electric shock

39
Q

Describe the structure of a cable that connects a cooker to the mains electricity supply

A

contains a live, neutral and earth wire

theses wires should be insulated with plastic

40
Q

What is the difference between an alternating current and a direct current?

A

a. c is constantly changing direction

d. c flows in one direction only

Physics- P2 (6 decks)

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