Electrical Energy


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Electrical Appliances

Every electrical appliance can waste energy in some way; usually by heating the untargeted parts of an appliances, or sound energy. E.g:

Electrical heater: heat, light from the glowing element

Toaster: heating the toast, heating the toaster itself, wire and surroundings

Hairdryer: KE of the fan and heating the filament/ anything inside the fan, sound and heating the hairdryer itself

Computer disc: energy stored in magnestic dots on the disc, energy heating the motor that drives the disc

An electrical appliance is designed for a particular purpose and should waste as little energy as possible

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E= P/t

Power is the rate at which energy is transferred.

The more powerful an appliance, the fater the rate at which energy transfers

W = J/s, P = E/t

1 KW = 1000 W, 1 MW = 100,000 W

The energy needed to lift a weight of 1 N 1m off the ground is 1 J

So to calculate the energy needed to lift 30kg 1m off the ground:

1kg = 10N so 30kg x 10N = 300 N

300N x 1m = 300 J 

If the weightlifter takes 0.5 seconds to lift this, then to calculate the his power:

P = E/t (W= J/s) so

P= 300J / 0.5s = 600 W

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Cost of electricity

Kilowatt-hours (kWh)s are the units of energy supplied by mains electricity

Energy = Power x Time

      J  =  W x  s   > scaled up

 kWh = kW x h

Cost of electricty = Number of kWhs x cost per kWh

Cost effectiveness means value for money. There are three things to take into account when comparing this:

1) Buying + Installing costs                                2) Running costs     3) Environmental costs

As well as the payback time, efficiency and how effective it is at doing its job e.g. U Values

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