mains electricity - 2.2

Question 1

what is the relationship between current, power, voltage? (equation)

Answer 1

P = IV
power = current x voltage

Question 2

what is the equation linking energy transferred, current, voltage, time?

Answer 2

E-ItV
Energy tranferred = current x time x voltage

Question 3

what is the relationship between power, energy transftered, and time?

Answer 3

P=E/t
(power = energy transferred/time)

Question 4

what is the unit for energy transferred

Answer 4

joules

Question 5

what is the differenc between AC and DC?

Answer 5

Ac- alternating current, which is produced by alternating voltage
DC- movement of the charges is only in one direction and it is creased by direct voltage

Question 6

what type of supply is battery supply?

Answer 6

DC

Question 7

what type of supply is mains supply?

Answer 7

AC

Question 8

how can you turn AC into DC (not necessary to know in this topic)

Answer 8

diode

Question 9

how are most appliances connected to the mains supply?

Answer 9

by three-core cables, meaning they have 3 wires inside them made with a core of copper and a coloured plastic coating

Question 10

what are three core cables made up of

Answer 10

they have 3 wires inside them made with a core of copper and a coloured plastic coating

Question 11

what does colour on a three-core cable indicate?

Answer 11

its purpose

Question 12

what is the blue wire called? what does it do?

Answer 12

the blue wire is called the neutral wire. the neutral wire completes the circuit. when the appliance is operating normally , current flows through the live nd neutral wire. it is around 0 volts (there is no potential difference)

Question 13

what is the brown wire called? what does it do?

Answer 13

the brown wire is called the live wire. it carries the voltage (potential difference). it alternates between a high +ve and -ve voltage of about 230V

Question 14

what is the green and yellow wire called? what does it do?

Answer 14

this is called the earth wire. it is for safety and protecting the wiring. it carries the current away if something goes wrong and stops the appliance casing sbecoming live. It is also 0 volts.

Question 15

which two wires are used for an appliance to work properly?

Answer 15

live and neutral

Question 16

what is the P.D between the live wire and the neutral wire (this is the supply P.D)?

Answer 16

230V for the mains

Question 17

what is the P.D between the live wire and the earth wire?

Answer 17

230V for a mains-connected appliance

Question 18

what is the P.D between the neutral wire and the earth wire?

Answer 18

no p.d, they are both 0 volts

Question 19

is mains electricity AC or DC?

Answer 19

AC

Question 20

what do plug sockets have which are connected to the live wire? why?

Answer 20

switches- so that the circuit can be broken and the appliance becomes isolated and the risk of electric shock is reduced

Question 21

what is attatched to the live wire? why?

Answer 21

(switches), fuses, circuit breakers (in order to isolate the appliance if something goes wrong)

Question 22

what happens if you touch the live wire? why?

Answer 22

your body is at 0V. this means if you touch the live wire, a lage voltage is produced across youe body and a current flows through you. this causes a large electric shock which could injure or even kill you.
Even is the plug socket is turned off (so the switch is open) there is still a danger of an electric shock. A current isn’t flowing, but there is still a p.d in the live wire so if you made contact with it your body would provide a link between the supply and earth, so a current would flow through you. Any connection between the live and neutral wire can be dangerous. if the link creates a low resistance path to eart, a huge current wil flow, which could reuslt in a fire

Question 23

when is earthing/ fuse used in a wire. give an example

Answer 23

surges in current can occur because of changes in a current (like if it is suddenly switched off) or because of a fault in the electrical appliance. thesecan lead to wiring and circuits in your appliance to melt, causing a fire, and faulty appliances can cause deadly electric shocks.
The earth wire and fuse are used to prevent this from happening.

Question 24

give an example of why an earth wire might be used (toaster)

Answer 24

current flows through the live and neutral wire. it also flows through a heating coil. if the live wire touches a metal casing, the toaster becomes faulty. this causes a large current surge as current flows through the live wire casing and the earth wire.
the surge in the current melts the fuse when the amount of current is greater than the fuse rating. Fuses are connected to the live wire, so that breaking the fuse breaks the circuit and cuts off the live supply. This isolates the whole appliance, making it impossible to get an electric shock form the case. It also prevents the risk of fire caused by the heating effect of a large current

Question 25

what is a fuse rating?

Answer 25

the min current needed to break a fuse

Question 26

how should fuses be rated?

Answer 26

as near as possible but just higher than the normal operating current, the arger the current the thicker the cable you need to stop it from getting too hot and melting (which is why fuse rating for cables usually increases with cable thickness)

Question 27

are circuit breakers safer than fuses?

Answer 27

yes

Question 28

what do circuit breakers do? compare them to a fuse

Answer 28

instead of melting a fuse, a large current may 'trip' a circuit breaker. the circuit breakers turn off quicker than the time taken for a fuse to melt and they can also be reset rather than having to replace a fuse (however they are more expensive than fuses)

Question 29

a 1kW hair dryer is connected to a 230V supply. Suggest whether a 3A,5A, or 13A fuse is needed

Answer 29

1. converd kW into W 1kW=1000W 2. rearrange the equation of P=IV so I is the subject I=P/V 3. substitute what is given 1000/230 = 4.3... 4. choose a rating which is just higher that the calculated current 5A

Question 30

what is double insulation? why is it double?

Answer 30

when you insulate materials. it is double because appliances have metal casing which is earthed (attatched to earth wire), reducing the danger of electric shock, and it has a plastic casing (with no metal parts showing) double insulation doesn't require an earth wire, only live and neutral (2 core cable) note: this doesn't really make sense, the textbook is a bit confusing

Question 31

why does the fuse need to be rated as near as possible to the normal operating current?

Answer 31

because.. too low- the fuse will blow whilst the appliance is functioning safely too high- it won't melt properly if there is a huge surge in the current

Question 32

why does a current in a resistor create heat?

Answer 32

A current passing through a resistor (or wire) results in the electrical transfer of energy (current is the rate of flow of charge) The temperature of a resistor increases due to the collisions of the free electrons within the wire Some of the energy is dissipated into the surroundings by heating (V=IR)

Question 33

what is power?

Answer 33

The rate of energy transfer or the amount of energy transferred per second

Question 34

how can heat produced by current in a resistor be utilised?

Answer 34

This heating effect is utilised in many domestic contexts, including: Electric heaters Electric ovens Electric hob Toasters Kettles

Question 35

what are the pros of circuit breakers over fuses?

Answer 35

they operate much faster than fuses (they break the circuit as soon as there is a current surge, no time wasted waiting for current to melt a fuse). they also work for small current changes that might not be large enough to melt a fuse, making them more effective at protecting against electrocution

Question 36

what is a common type of circuit breaker?

Answer 36

RCCB - residual current circuit breaker