# Electrical Devices - AQA GCSE Physics Core

Revision note on electrical power, using electrical energy and the national grid

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• Created by: Sophie
• Created on: 03-01-12 17:09

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Electrical Devices
Electrical devices are used everyday. They turn electrical into useful energy
Electrical Power:
The energy we supply to a motor per second is the power supplied to it.
The more powerful a device is, the faster the rate at which it transforms energy.
The Power of a device is measured in Watts (W) or Kilowatts (kW)
For any device:
The input power is the energy supplied per second
The output power is the useful energy transferred per second
1 watt is the rate of transfer of energy of 1 joule per second
1 kilowatt = 1000 watts
Power (Watts) = energy transferred (Joules)
Time taken (seconds)
Eg: A motor transfers 10000J in 25s. What is its power?
Power = 10000J / 25s = 400W
Using Electrical Energy:
The energy supplied to a 1kW device in 1 hour is 1 kilowatthour.
Kilowatthour is used as the unit of energy supplied by mains electricity.
You can use this equation to work out the energy, in kilowatts used by a mains device in a certain
time:
Energy transferred = power of device x time in use
(kWh) (kW) (h)
Eg: a 0.5 kW heater is turned on for 6 hours uses 3kWh of electrical energy.
An electricity meter in your home measures how much electrical energy you use.
We use kilowatthours to work out how much the cost of electricity is:
Total Cost = number of kilowatthours used x cost per kilowatthour
Eg: 1442kWh x 7p = £100.94
National Grid:
The national grid is what supplies our homes with electricity.
Power stations produce electricity at 25000 Volts.
Stepup transformers are use to step this voltage up to grid voltage which is 132 kV. The voltage
is increased because it will decrease the current which reduces heat loss in the cables.
Stepdown transformers are used in sub stations to reduce the voltage to 230V which is safer to
use in home and offices.
Over ground or Underground?