4.2.3 Domestic uses and safety

direct current

dc- the current from a cell

alternating current

ac-constantly changing direction

benefit of AC

it is very easy to use a transformer to increase or decrease the pd of ac

frequency of UK mains electricity

50Hz

pd of UK mains electricity

230V

how to see the pattern of an electrical current

using an oscilloscope

height of peak shows maximum potential difference

direct current on an oscilloscope

pd does not change

* -  DC, direct current, is the movement of charge in one direction only 
* Cells and batteries supply direct current

alternating current on an oscilloscope 

* -  AC is alternating current, which comes from the main so  Current continuously varies, from positive to negative( charge changes direction)
* mains electricity in the uk

how to calculate frequency from an oscilloscope

number of cycles in one second

number of cycles/ time period

how is electricity in the UK connected to the mains supply

using a three-core cable

what are wires made of

* copper which is a good conductor of electricity
* coatings are mad of plastic which does not conduct electricity

live wire

brown

right side

carries alternating pd from the supply (230V)

connected to a fuse in the plug

can be dangerous and Fatal if touched even if switch open because the first part of the live wire running to switch still has a pd of 230V

neutral wire

blue

left

pd is 0V

Completes the circuit

earth wire

green/yellow stripped

safety wire to stop appliances from becoming live

why would someone be electrocuted if they touched the live wire

can be dangerous and Fatal if touched even if switch open because the first part of the live wire running to switch still has a pd of 230V

a current would flow through the person into the earth due to the large (230V) pd

why can appliances with a metal case be dangerous

if the live wore becomes loose and touches the metal case, the metal case can become live

this can give someone an electric shock if they touch the live case

how earth wire works

metal case is attached to the earth wire and earth wire is connected into the ground with a metal rod

if the case becomes live, a huge current flows to the earth. The fuse melts and shuts off

prevents anyone from getting an electric shock when touch the case

what is the national grid

a system of transformers and high voltage cables

connects power stations to home

problems with getting electricity to homes

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

the bigger the difference between the power stations and the homes the greater the energy lost

how to reduce energy loss

building power stations near to homes

use transformers

how transformers work

step up transformers:

electricity passes through step-up transformers which increase the potential difference to several hundred thousand volts

because less energy is lost in the power cable when the potential difference is very high

* electricity passes through high voltage cables, this can’t be passed on to homes because pd is to high

step down transformers:

electricity passes through step-down transformers which decreases the potential difference to around 230V

before the electricity passes to home