Paper 1 Electricity in the home

Question 1

Direct current

Answer 1

Current travels in one direction

e.g. simple circuit

Question 2

Alternating current

Answer 2

Current repeatedly reverses its direction
Frequency = number of waves per second
Oscilliscope measures these waves

Question 3

What supplies a d.c

Answer 3

Cells and batteries supply current which always passes in the same direction

Question 4

What supplies an a.c

Answer 4

Mains electricity is an a.c supply.

The live wire alternates between -325V and +325V

Question 5

How does the current flow in a wire

Answer 5

Metal wire has millions of atoms, each with delocalised electrons. Wire made up of a lattice of positive ions surrounded by delocalised electrons

Question 6

What happens to the electrons when a battery is attached to the wire

Answer 6

the free electrons are repelled by negative terminal and attracted to +. they still have random movement but now move in the same direction through the wire at a ‘drift velocity’

Question 7

What happens to the electrons in an a.c

Answer 7

they are pushed backwards and forwards so change direction every 0.1 seconds. this means electrons don’t travel all the way from the battery to the appliance

Question 8

How does a main powered appliance get energy if electrons don’t travel all the way to the appliance from the power source

Answer 8

The power source cause all electrons in a circuit loop to vibrate at once. energy is transformed from electrons to appliance

Question 9

Green and yellow wire

Answer 9

Earth wire, prevents the appliance from becoming ‘live’

Question 10

Brown wire

Answer 10

Live wire, alternates between +325V and -325V

Question 11

Blue wire

Answer 11

Neutral wire, completes the circuit

Question 12

Why is the casing of a plug made of plastic

Answer 12

Because it doesn’t conduct electricity and is a good insulator

Question 13

Why do the cables contain copper wires

Answer 13

Because copper is a good conductor

Question 14

Why is the copper made of lots of thin stands twisted together

Answer 14

Makes cables more flexible instead of being one thick wire

Question 15

Why are the pins made from brass

Answer 15

Because it’s a good conductor and is stiffer and doesn’t oxidise (unlike copper)

Question 16

Where and why does the plug contain a fuse

Answer 16

Between live pin and live wire so if the current gets too high the fuse melts and breaks the circuit and cuts off the live wire

Question 17

Which pin is longest and why

Answer 17

Earth pin to make contact with the earth wire of the socket before contact with the live wire

Question 18

What is the purpose of different thickness of wires

Answer 18

Low current (e.g. lighting) will be thinner than high current (e.g. cooker). Thicker cables less likely to overheat.

Question 19

What is a short circuit

Answer 19

An electrical circuit which allows a current to travel along an unintended path with no or low resistance meaning a high current which overheats the wires. this can damage equipment and could cause a fire

Question 20

Why do bulbs ‘blow’ when they have just been switched on

Answer 20

When first switched in filament cold so low resistance and a very high current flows (sometimes high enough to melt the wire in the bulb)
As the lamps temp increases so does resistance forcing current down which won’t melt the filament

Question 21

How do electrical companies bill their customers

Answer 21

Joule is a very small amount of energy so companies use a unit called kilowatt hours. 1kWh is the energy delivered when a 1kW device is used for 1 hr and is = to 3.6 million J

Question 22

What does the cost of using a particular device depend on

Answer 22

How fast it uses up the energy and how long the device is used for

Question 23

Equation for working out the cost of electrical energy

Answer 23

No. of units used (kWh)= power x time

total cost= no. of units used x unit price

Question 24

What’s a fuse and how does it work

Answer 24

Fuse contains a thin wire that heats up and melts if too much current passes through
A fuse must always have correct rating for appliance

Question 25

What happens if the current rating of the fuse is too large

Answer 25

The fuse won’t blow when it should and the heating effect of the current could cause a fire in the appliance or its connecting cable

Question 26

Calculating which fuse rating to use

Answer 26

Domestic appliances usually 3A, 5A or 13A fuse.

To work out which one to use, need to know power rating and p.d of the device to calculate it’s working current

Question 27

Why is earthing important

Answer 27

Earth wire is a low resistance path for the current to take
if there’s a fault in the appliance, causing the live wire to touch the metal case then a large current surges down the earth wire and the fuse blows
If the fuse doesn’t blow the current will flow down the earth wire and the user wouldn’t be electrocuted by touching the case

Question 28

How does the national grid work

Answer 28

At power stations the voltage is increased using a transformer (20kV). this reduces the current in the power lines which reduces energy loss in the cables, as it reduces the heat energy produced
At consumers end another transformer is used to decrease voltage to safer levels (230V)

Question 29

Advantages and disadvantages of overhead cables

Answer 29

Cheap to build and supply/repair, up high so out of reach

Look ugly, more likely to get damaged in storms

Question 30

Advantages and disadvantages of underground cables

Answer 30

Out of sight, safe from weather conditions

Not as far from people, digging roads, danger expensive to install difficult to reapir