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15 Cards in this Set
- Front
- Back
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Household electricity is usually measured in kilowatts. What is a kilowatt? |
1kW = 1000 W |
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What is the equation that shows the relationship between energy transferred, power and time? |
energy transferred (kilowatt-hour, kWh)
= power (kilowatt, kW) × time (hour, h) |
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A 10 kW electric shower is used for a total of 90 minutes during a week. How many kilowatt-hours of energy does it transfer in one week? |
energy = power (kilowatt, kW) × time (hour, h) = 10 kW x 1.5 h = 15 kWh |
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Which equation is used to calculate the cost of the electricity used?
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cost = power (kW) × time (hour) × cost of 1 kWh (pence) |
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An electric fire needs 2 kW. It is switched on for 3 hours. If each kWh costs 10p, how much does it cost to run the fire?
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cost = power × time × cost of 1 kWh
= 2 kW × 3 h × 10p = 60p |
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A TV needs 250 W. It is switched on for 30 minutes. If each kWh costs 8p, how much does it cost to run the TV? |
cost = power × time × cost of 1 kWh = 0.250 kWh × 0.5 h × 8p = 1p |
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Explain why there are two different units for measuring energy used. |
Joules and kilowatt-hours. Joules are used when the amounts of energy are small (or when working out energies for scientific investigations). Kilowatt-hours are used when working out how much energy appliances have used. |
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What is a Unit of electricity? |
The amount of energy equivalent to one kilowatt-hour. |
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An electricity bill says the current reading is 11 654 Units, and the previous one was 10 763 Units. What will the electricity bill be if electricity costs 14 pper Unit? |
Difference in readings = 11 654 – 10 763 = 891 Units (or kWh) Cost = 891 × 14 p = 12 474 p or £124.74 |
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A 3kW electrical heater transfers 15 kWh of energy. How long was it switched on for? |
Time = energy/power = 15 kWh/3 kW = 5 hours. |
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How much energy is transferred to a 100 W bulb left on for 2 hours 15 minutes? |
Energy = power × time = 0.1 kW × 2.25 h = 0.225 kWh (or 100 × 2.25 × 3600 = 900 000 J = 900 kJ). |
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An electric lawnmower has a power rating of 1.2 kW. It takes 30 minutes to mow a lawn and costs 9 p. What is the price of electricity? |
Energy = power × time = 1.2 kW × 0.5 h = 0.6 kWh Cost/kWh = cost/energy = 9 p/0.6 kWh = 15 p/kWh |
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The cost of running an aquarium pump continuously is 8 p per week. If the price per kWh is 12 p, what is the power of the pump (to the nearest watt)? |
Energy used per week = cost/cost per unit = 8 p/12 p = 0.666 kWh Power = energy/time = 0.666 kW/(7 × 24) hours = 0.0039 kW = 4 W |
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Explain how electricity is charged for, and how to calculate the energy used by an appliance and the cost of the electricity. |
Electricity is charged by the amount of energy used. The energy used by an appliance depends on its power rating, which is the energy used every second. The energy used is found by multiplying its power rating in kilowatts by the time for which it is used in hours. This gives the energy in kilowatt-hours. The cost is found by multiplying this energy by the cost per kilowatt-hour. |
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What are the key points to remember? |
The energy used by an appliance can be worked out using the power rating in kilowatts and the time in hours. A kilowatt-hour is the amount of energy transferred by a 1 kW device in 1 hour. It is also called a Unit. The cost of electricity is the energy used (in kWh) multiplied by the cost per kilowatt-hour. |
