Unit 1 Physics - Electrical Energy

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  • Created by: Becky2121
  • Created on: 25-03-16 21:18

Electrical Appliances

Electrical Appliance: A machine powered by electricity

  • Can transfer electrical energy into useful energy
  • For example lamps --> produce light
  • Uses of everyday electrical appliances include heating, lighting, making objects move and creating sound and visual images
  • Designed for particular purposes and to waste as little energy as possible
  • Energy is often wasted through thermal energy (heat)
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Electrical Power

Power: The rate at which an appliance transfers energy

The unit of power is the watt (W)

Joke to remember it by:

Watt is the unit of power? Watt is the unit of power

Power is often given in kilowatts (kW) --> due to watts being to small

1kW = 1000W

Power = energy / time 

(energy over time)

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P = E / T

Power = Energy / Time

(Power = energy over time)

P (power) = Watts = W (see the previous card)

E (energy) = Joules = J

T (time) = Seconds

Efficiency = useful power out / (over) total power in

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Using electrical energy

People have to pay for the amount of electricity they use

The amount of energy used is measued in kilowatt hours (kWh)

Kilowatt Hour: The amount of energy transferred by a one-kilowatt appliance when used for one hour

E= P x t <-- The equation to work out how much energy is transferred to a mains appliance

E = energy transferred

P = power of the appliance

t = time taken in hours for the energy to be transferred

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Electricity Bills

  • To work out how much electricty was used between readings you take the prevous reading away from the current reading


Working out the electricty bill

Total Cost = number of kWh xcost per kWh


The cost per kWh is on the electricty bill

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Cost Effectiveness

Cost Effectivity: getting the best value for money

When comparing the cost effectiveness of different appliances we need to consider several 

  • the cost of buying the appliance
  • the cost of installing it
  • the running costs
  • the maintence costs
  • environmental costs
  • interest charged on a loan (if necessary)

To reduce energy bills you can either buy a newer, more efficient appliance OR install materials designed to reduce energy wastage (see last chapter)

Payback time: the time it takes for an appliance or installation to pay for itself in terms of energy savings

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Test Questions

1) An electric motor transfers 48kJ of electrical energy into kinetic energy in 2 minutes. What is the useful power output?

2) How much electrical energy is transferred in kWh when a 9kW shower is used for 15 minutes?

3) The price of 1kWh of electrical energy is 9p. How much does it cost to use a 60W electric light for 4 hours?

4) Loft insulation costs £6000 but it saves £80 on the electricity bill per year. What is its payback time?

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1) 400w (1kJ = 1000J --> 48kJ = 48,000J --> 48,000 / 120 seconds)

2) 2.25kWh (15minutes = 1/4 of an hour = 0.25 --> 9 x 0.25)

3) 2.16p   4) 7.5 (600 / 80)

Step by step walk-through for question 3

1) 1kW = 1000W --> 0.60 

2) This is used for 4 hours so you need to times it by 4 --> 0.60 x 4 = 2.4

It uses 2.4kWh in 4 hours

3) Now you need to times it by 0.9 (it has to have the 0 in else it becomes £9)  

2.4 x 0.9 = 2.16

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