P5 Household Electricity

Question 1

what do appliances do (in terms of energy)?

Answer 1

appliances transfer electrical energy into other forms of energy

Question 2

what type of energy do motors transfer?

Answer 2

kinetic energy

Question 3

what is power?

Answer 3

the rate at which energy is transferred

Question 4

what does 1 watt mean?

Answer 4

an energy transfer of 1 J per second

Question 5

what is usual with the power rating of appliances designed for producing thermal energy?

Answer 5

they have much higher power ratings than ones designed to transfer kinetic energy

Question 6

what is the equation which links E, P and t?

Answer 6

E = P x t

Question 7

how do you convert to kilojoules?

Answer 7

divide by 1000

Question 8

what is the equation that links P, V and I?

Answer 8

P = V x I

Question 9

what is the equation which links P, I and R?

Answer 9

P = I^2 x R

Question 10

what is direct current?

Answer 10

the electrons in the circuit only ever travel in one direction

Question 11

what type of current is the current from a cell?

Answer 11

direct current

Question 12

what is alternating current?

Answer 12

a current where the electrons are constantly changing direction

Question 13

what is the advantage of using alternating current?

Answer 13

it is very easy to use a tranformer to increase or decrease the the potential difference

Question 14

what is the frequency of the mains current?

Answer 14

50 Hertz

Question 15

what is the potential difference of the mains current?

Answer 15

230 Volts

Question 16

how do you calculate the frequency of an alternating current from an osciloscope trace?

Answer 16

measure the amount of time from one peak to the next
use the formula:
frequency = 1 / period

Question 17

why are wires made from copper?

Answer 17

it is a good conductor of electricity

Question 18

what is the brown wire and what does it do?

Answer 18

It is the LIVE wire

it carries the alternating potential difference from the supply

Question 19

what is the live wire connected to in the plug?

Answer 19

a fuse

Question 20

what is the blue wire and what does it do?

Answer 20

the NEUTRAL wire
it completes the circuit with the live wire
it has a potential difference of 0V compared to the live wire

Question 21

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

Answer 21

the EARTH wire

it is a safety wire to stop the appliance becoming live

Question 22

what would happen if a person touched the live wire?

Answer 22

a current would flow through the person into the earth and they would be electrocuted
this is because the potential difference of the earth is 0V

Question 23

why are appliances with metal cases dangerous?

Answer 23

if the live wire becomes loose and touches the case, the case can become live
this could gives someone a fatal electric shock

Question 24

what happens if a metal case of an appliance becomes live and there IS an earth wire?

Answer 24

the earth wire is connected to the ground by a metal rod and connected to the metal case of the appliance.
if the case becomes live, a huge current flows to the earth. this causes the fuse to melt and shut off the current

Question 25

what is the national grid?

Answer 25

a system of transformers and high voltage cables

Question 26

what does the national grid connect?

Answer 26

the power stations and the homes

Question 27

what is the issue with transporting the energy to homes?

Answer 27

energy is always lost in the power cables due to the resistance in the wires

Question 28

how does distance effect the energy loss in power cables?

Answer 28

it increases

Question 29

why are step up transformers used?

Answer 29

they increase the potential difference as less energy is lost in the cables when the potential difference is high

Question 30

what do step down transformers do?

Answer 30

reduce the potential difference before the electricity passes to homes