# Electrical energy

Physics - electrical energy

- Created by: Rosie
- Created on: 27-05-10 17:58

First 276 words of the document:

Electrical energy

Electrical devices

Device Useful energy Energy wasted

Light bulb Light energy Heat energy

Electric heater Thermal energy Light energy

Hairdryer Thermal energy and Sound

kinetic energy

Electrical power

The energy supplied to a device per second is the power supplied

to it.

The more powerful a device, the faster the rate at which it

transfers energy.

Power is measured in watts (W) or kilowatts (kW)

For any device its input power is the energy per second supplied to

it.

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

25s

Using electrical energy

The energy supplied to a 1kW device in 1 hour is 1 kilowatt-hour

(kWh).

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

(hours)

Example of energy transfer:

A 1 kW heater switched on for 1 hour uses 1 kWh of electrical energy.

Transfer = 1kW x 1 hour

= 1kWh

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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