First 276 words of the document:
Device Useful energy Energy wasted
Light bulb Light energy Heat energy
Electric heater Thermal energy Light energy
Hairdryer Thermal energy and Sound
The energy supplied to a device per second is the power supplied
The more powerful a device, the faster the rate at which it
Power is measured in watts (W) or kilowatts (kW)
For any device its input power is the energy per second supplied to
For any device its output power is the useful energy per second
transferred by it.
1 watt = the rate of transfer of energy of 1 joule per second (J/s).
1 kilowatt = 1000 watts.
Power (W) = rate of energy transfer
= energy transferred (J)
time taken (s)
Example of power:
A motor transfers 10 000J of energy in 25s. What is its power?
Power (W) = energy transferred (J)
time taken (s)
Power (W) = 10 000J = 400W
Using electrical energy
The energy supplied to a 1kW device in 1 hour is 1 kilowatt-hour
Therefore, a 1kW heater uses the same energy in an hour as a 2
kW heater uses in half-an-hour.
Energy transferred (kWh) = power of device (kW) x time in use
Example of energy transfer:
A 1 kW heater switched on for 1 hour uses 1 kWh of electrical energy.
Transfer = 1kW x 1 hour
1 kilowatt-hour = 1000 joules/second x 60 x 60 = 3 600 000J
We use the kWh to work out the cost of electricity...
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Here's a taster:
Total cost = number of kWh used x cost per kWh
The National Grid
Electricity in the home reaches you through the National Grid.
The National Grid contains power stations and transformers.
Step-up transformers are used at power stations and increase the
voltage to the National Grid.
Step-down transformers are used at sub-stations near homes and
decrease the voltage to be used in homes.…read more