Paper 1 Electricity in the home Flashcards
Direct current
Current travels in one direction
e.g. simple circuit
Alternating current
Current repeatedly reverses its direction
Frequency = number of waves per second
Oscilliscope measures these waves
What supplies a d.c
Cells and batteries supply current which always passes in the same direction
What supplies an a.c
Mains electricity is an a.c supply.
The live wire alternates between -325V and +325V
How does the current flow in a wire
Metal wire has millions of atoms, each with delocalised electrons. Wire made up of a lattice of positive ions surrounded by delocalised electrons
What happens to the electrons when a battery is attached to the wire
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’
What happens to the electrons in an a.c
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
How does a main powered appliance get energy if electrons don’t travel all the way to the appliance from the power source
The power source cause all electrons in a circuit loop to vibrate at once. energy is transformed from electrons to appliance
Green and yellow wire
Earth wire, prevents the appliance from becoming ‘live’
Brown wire
Live wire, alternates between +325V and -325V
Blue wire
Neutral wire, completes the circuit
Why is the casing of a plug made of plastic
Because it doesn’t conduct electricity and is a good insulator
Why do the cables contain copper wires
Because copper is a good conductor
Why is the copper made of lots of thin stands twisted together
Makes cables more flexible instead of being one thick wire
Why are the pins made from brass
Because it’s a good conductor and is stiffer and doesn’t oxidise (unlike copper)
Where and why does the plug contain a fuse
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
Which pin is longest and why
Earth pin to make contact with the earth wire of the socket before contact with the live wire
What is the purpose of different thickness of wires
Low current (e.g. lighting) will be thinner than high current (e.g. cooker). Thicker cables less likely to overheat.
What is a short circuit
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
Why do bulbs ‘blow’ when they have just been switched on
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
How do electrical companies bill their customers
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
What does the cost of using a particular device depend on
How fast it uses up the energy and how long the device is used for
Equation for working out the cost of electrical energy
No. of units used (kWh)= power x time
total cost= no. of units used x unit price
What’s a fuse and how does it work
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
What happens if the current rating of the fuse is too large
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
Calculating which fuse rating to use
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
Why is earthing important
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
How does the national grid work
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)
Advantages and disadvantages of overhead cables
Cheap to build and supply/repair, up high so out of reach
Look ugly, more likely to get damaged in storms
Advantages and disadvantages of underground cables
Out of sight, safe from weather conditions
Not as far from people, digging roads, danger expensive to install difficult to reapir
