P2.4 Using mains electricity safely

Question 1

What is a direct current?

Answer 1

a current that always flows in the same direction

it is always at the same voltage

Question 2

What type of current does mains electricity supply?

Answer 2

an alternating current

Question 3

What is an alternating current (a.c) ?

Answer 3

A current that is constantly changing direction

Question 4

What voltage is the mains electricity in the UK?

Answer 4

about 230 v

Question 5

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

Answer 5

50 cycles per second (50 hertz)

Question 6

Describe the oscilloscope trace of an alternating current

Answer 6

a regular repeating wave.

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

Question 7

Describe the oscilloscope trace of a direct current

Answer 7

a straight line

Question 8

What is the time period of a wave?

Answer 8

The time taken to complete 1 cycle

Question 9

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

Answer 9

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

Question 10

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

Answer 10

frequency = 1 (divided by) time period

Question 11

On an oscilloscope, what does the timebase dial control?

Answer 11

how many milliseconds each devision represents on the horizontal axis

Question 12

On an oscilloscope, what does the gain dial control?

Answer 12

how many volts each cm division represents on the horizontal axis

Question 13

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

Answer 13

the input voltage at that point

Question 14

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

Answer 14

By measuring the hight of the trace

Question 15

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

Answer 15

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

Question 16

What are most electrical appliances connected to the mains using?

Answer 16

a cable and three-pin-plug

Question 17

What type of current do cells and batteries supply?

Answer 17

a direct current (d.c)

Question 18

Describe the structure of an electric cable

Answer 18

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

Question 19

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

Answer 19

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

Question 20

What materials are used in a three-pin plug?

Answer 20

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

Question 21

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

Answer 21

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

Question 22

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

Answer 22

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

Question 23

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

Answer 23

The fuse will melt, breaking the circuit

Question 24

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

Answer 24

Thicker cables have less resistance, so carry more current

Question 25

What does it mean if an appliance is double insulated?

Answer 25

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

Question 26

What are two-core-cables?

Answer 26

cables that only carry the live and neutral wire They do not need an earth wire as the appliance is double insulated

Question 27

What is 'earthing' an appliance?

Answer 27

appliances with metal cases are usually earthed to reduce the danger of electric shock means the case must be attached to an earth wire.

Question 28

What does RCCBs stand for?

Answer 28

Residual Current Circuit Breakers

Question 29

What do RCCBs do and how do they work?

Answer 29

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

Question 30

What are some advantages of using RCCBs over fuses?

Answer 30

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

Question 31

What happens when an electrical charge flows through a resistor?

Answer 31

the resistor gets hot

Question 32

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

Answer 32

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

Question 33

What is an example of a power saving lamp?

Answer 33

Compact Fluorescent Lamps (CFLs)

Question 34

In the equation P= E (divided by) t what do the letters stand for and what are the units?

Answer 34

P- power (W) E- energy (J) t- time (s)

Question 35

In the equation P= I x V What do the letters stand for and what are the units?

Answer 35

P- power (W) I-current (A) V- potential difference (V)

Question 36

In the equation E= V x Q what do the letters stand for and what are the units?

Answer 36

E- energy (J) V- potential difference (V) Q- charge in coulombs (C)

Question 37

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

Answer 37

As the motor will not be 100% efficient. some of the energy will be transformed into heat and sound

Question 38

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

Answer 38

The case may become live if there is an electrical fault this causes a danger of electric shock

Question 39

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

Answer 39

contains a live, neutral and earth wire theses wires should be insulated with plastic

Question 40

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

Answer 40

a. c is constantly changing direction | d. c flows in one direction only