P1. 3.3 Using electrical energybioteacher.weebly.com/uploads/9/9/1/9/9919780/wk_5...Cost of...
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P1. 3.3 Using electrical energy
Year 9
Week commencing 23rd April 2012
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Objectives
• I must be able to calculate energy transferred using the equation
• I should be able to carry out the equation with a change in units
• I should also be able to re-arrange the equation to find out power or time
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Do different appliances use
different amounts of electricity?
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© Boardworks Ltd 2003
How much energy?
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How much does electricity cost?
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How much does it cost every time I
watch TV?
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It is important to learn this
because: • When you leave home, you will need to know how
your electricity bill is worked out.
• When you buy new electrical appliances, you will know about power ratings.
• We need to make our fossil fuels last as long as possible. By knowing about how much electricity we use, we can achieve this.
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Last lesson you learnt
• About how to calculate power.....
• This lesson will teach you about how to
calculate the energy given to an appliance
and how to calculate the cost of that
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How do we measure energy?
• Energy is measured in Joules (J)
• We need to know how much energy something uses
over time = a rate
• We can use Joules per second (J/s)
• 1 Joule / sec = 1 Watt (W)
A 100W light bulb uses 100J of energy every second
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Power Rating
• Different electrical devices have different power
ratings
• They use different amounts of electrical energy over
a given time.
15W 100W
VS
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Write down the equation
• Energy transferred = power X time
• E = p X t
• E is energy transferred in kilowatt-
hours, kWh
• p is power in kilowatts, kW
• t is time in hours, h
• How could this be re-arranged?
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• A 0.5kW appliance is left on for 2
hours. How much energy is
transferred?
= 0.5 X 2
= 1kWh
energy transferred = power rating x time
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• A 100W light bulb on for 1 hour will use
100 Watt hours (Wh) of electricity
• This is usually measured in Kilowatt Hours
(kWh)
0.1 kWh
energy transferred = power rating x time
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On an electricity bill
1 unit of electricity = 1 kilowatt hour
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Cost of Electricity • At the moment, one unit (1 kWh) of electricity costs 12p.
• To run a normal 100W light bulb for 5 hours costs:
Energy used = power X time
100W x 5hours = 500Wh = 0.5kWh
Therefore, cost = energy used X cost per unit
= 0.5kWh x 12p = 6p
• To run a 15W eco bulb for 5 hours costs:
15W x 5hours = 75Wh = 0.075kWh
0.075kWh x 12p = 0.9p
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What about for a year?
• To run a normal 100W light bulb for 5 hours a day all year costs:
100W x 5hours = 500Wh = 0.5kWh
0.5kWh x 12p = 6p
6p x 365 days = 2190p = £21.90
• To run a 15W eco bulb for 5 hours a day all year costs:
15W x 5hours = 75Wh = 0.075kWh
0.075kWh x 12p = 0.9p
0.9p x 365 days = 328.5p = £3.29
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Meter reading on
Tuesday
Meter reading on
Wednesday
Electricity used in one day:
35 534.9 – 35 497.6 = 37.3 kWh or 37.3 units
If each unit costs 13 pence
The cost of that day’s electricity is 37.3 x 13 p = 484.9 p
OR £ 4. 85
OR £ 4.849
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Now carry out your worksheets on the sheet
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Plenary
Ultra-Elec has a standing charge of 20p per
day and a cost of 10 pence per kWh.
Top-Power has no standing charge but a
cost of 12p per kWh.
If the Jones family use 300kWh in a period
of 90 days, which company is cheaper for
them to use?