Unit 1 Physics - Electrical Energy

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)

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)

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

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

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

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Working out the electricty bill

Total Cost = number of kWh xcost per kWh

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The cost per kWh is on the electricty bill

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

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?

Answers

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