Transcript 10. Electrical powerx - E

Electrical Power
Learning objectives
Use the electrical power formula in
everyday situations
 Calculate the cost of electricity

Recap
What is power?
 What are the units?
 What is the formula?

With electricity

There is an easy way to work out power
if you know the voltage and current for a
particular device
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Power = Current x Voltage (P=IV)
What do you think?

What do you think the power of the
following would be?
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A mobile phone
An LED bulb
An incandescent bulb
An LED TV
A laptop
Task 1 answers
Device
A mobile phone
An LED light bulb
An incandescent bulb
An LED TV
A laptop
A hair dryer
A clothes dryer
Power
1W
7W
60 – 100W
80W
80W
1400W
3000W
Task 2 answers
Device
Current / A
Voltage / V
Power / W
A mobile phone
0.24
5
1.2
An LED light
bulb
0.02
230
4.6
An incandescent
bulb
0.43
230
100
An LED TV
0.33
230
75.9
A laptop
4.3
19
81.7
A hair dryer
6.5
230
1500
A clothes dryer
13.9
230
3200
Paying for electricity

How does your electricity company know
how much electricity you have used?
Paying for electricity

How does your electricity company know how
much electricity you have used?
There is a meter in your house that measures
how much power has been used for a how long
 The units are kilowatt hours (kWh)

Paying for electricity
Device
Power
Time
kWh
Cost
Laptop
76W
14 hours
1.064
€0.30
Dishwasher
450W
1.5 hours
0.675
€0.19
Oven
2700W
2 hours
5.4
€1.54
Smoke alarm
1.3W
4 weeks
0.8736
€0.25
LED light
8W
1 day
0.224
€0.0
9kW
3 hours
27
€7.68
180W
20 mins
0.06
€0.02
TV
95W
5 hours
0.475
€0.14
Clothes iron
1800W
45 mins
1.35
€0.38
Electric car
charging
Hair
straighteners
Paying for electricity problem 1
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A family has an old clothes drier which uses
3200W. They use the drier for 5 hours a week.
Electricity costs €0.2846 per kWh. How much
money would they save per year if they changed
to a drier that uses 1900W?

Old machine:
◦ 3.2 x 5 x 52 x 0.2846 = €236.79

New machine:
◦ 1.9 x 5 x 52 x 0.2846 = €140.59

Difference = 236.79 – 140.59 = €96.20
Paying for electricity problem 2
A family has an old clothes drier which uses 3200W.
They use the drier for 5 hours a week. Electricity
costs €0.2846 per kWh. How much money would
they save per year if they changed to a drier that
uses 1900W?
 How much money would the family save if they
then changed to a cheaper electricity tariff that
costs €0.2654 per kWh?

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kWh: 1.9 x 5 x 52 = 494
Old tariff = 494 x 0.2846 = €140.59
New tariff = 494 x .2654 = €131.11
Paying for electricity problem 3
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A family uses electricity to heat water. How much would it
cost to heat the water for a bath?
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Water start temperature = 10°C
Water final temperature = 40°C
Specific heat capacity of water = 4200J/kg°C
Bath capacity = 230 litres
One Watt = one Joule per second
Cost of electricity = €0.2846 per kWh
◦ Energy used = (40-10) x 4200 x 230 = 28.98MJ
◦ If this was heated in 30 minutes:
 Power = 28,980/(30 x 60) = 16.1kW
 kWh = 16.1 x 0.5 = 8.05kWh
 Cost = 8.05 x 0.2846 = €2.29
Learning objectives
Use the electrical power formula in
everyday situations
 Calculate the cost of electricity
