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Power Calculations

Power Calculations. Noadswood Science, 2011. Power calculations. To be able to calculate the cost of electricity. Back to the future. Back to the future. Energy.

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Power Calculations

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  1. Power Calculations Noadswood Science, 2011

  2. Power calculations • To be able to calculate the cost of electricity

  3. Back to the future

  4. Back to the future

  5. 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)

  6. 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)

  7. 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

  8. Cost • 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

  9. Electricity costs • Complete the electricity costs worksheet…

  10. Electricity costs

  11. 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: - 06895674 06895702 • How many units have been used? • 28

  12. 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)

  13. 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

  14. 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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