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Power Calculations Noadswood Science, 2011

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**Power calculations To be able to calculate the cost of electricity**

Wednesday, April 19, 2017 Power calculations To be able to calculate the cost of electricity

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Back to the future

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Back to the future

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Energy All the electricity you use has to be paid for – different appliances are more expensive to use than others, depending on how much electrical power (Watts) they require (and for how long they remain on) E.g. lights need 100W of power, whilst some microwaves need 850W (but the lights tend to be running much longer than the microwave)

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**energy transferred (kWh) = power (kW) × time (h)**

Electrical The amount of electrical energy transferred to an appliance depends on its power and the length of time it is switched on The amount of mains electrical energy transferred is measured in kilowatt-hours, kWh - one unit is 1kWh The equation below shows the relationship between energy transferred, power and time: - energy transferred (kWh) = power (kW) × time (h)

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**energy transferred (kWh) = power (kW) × time (h)**

Electrical energy transferred (kWh) = power (kW) × time (h) Power is measured in kilowatts instead of the more usual watts - to convert from W to kW you must divide by 1000 E.g. 2000W = 2000 ÷ 1000 = 2kW Time is measured in hours instead of the more usual seconds - to convert from seconds to hours you must divide by 3600 E.g. 1800s = 1800 ÷ 3600 = 0.5 hours

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**total cost = number of units × cost per unit**

Electricity meters measure the number of units of electricity used in a home or other building - the more units used, the greater the cost The cost of the electricity used is calculated using: - total cost = number of units × cost per unit E.g. if 5 units of electricity are used at a cost of 8p per unit, the total cost will be: - 5 × 8 = 40p

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Electricity costs Complete the electricity costs worksheet…

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Electricity costs

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Electricity meters In your home there will be an electricity meter – it records how much electricity you use in units Your electricity bill is calculated from the number of units you use: - How many units have been used? 28

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Questions A heater uses 45.2 kWhr of energy - if electricity costs 10p per unit, how much does it cost to use the heater? Number of units = 45.2 Cost = 45.2 x 10 = 452p (£4.52)

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Questions An iron that operates at a power of 3 KW for 4 hours uses electricity that costs 8p per unit - how much does it cost for the electricity that it uses? Number of units: kWhr = 3kW x 4hr = 12 kWhr Number of units of electricity is 12 units Cost = 12 x 8 = 96p

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Appliance energy The power rating of an appliance is simply how much energy it uses every second (1 watt = 1 joule per second) For example, this engine puts out a power of 2600J every second (2.6kW)

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