Learning about Appliances and Power Use at Home
In our modern “developed households”, there abound energy-hungry machines and electrical appliances. It is only 200 years or so since we learnt to use fire to drive steam engines.
These appliances are designed to make our life easier and comfortable by keeping us warm or cool, by helping us do many daily jobs around the house that might otherwise be tedious and time-consuming or by keeping us entertained. Examples include dishwashers, toasters, washing machines, irons, food blenders, stoves, ovens, TV’s, CD and DVD players, computers, reverse cycle air conditioners, clothes dryers, refrigerators and freezers, vacuum cleaners, just to name a few.
While some of these machines need our attention, most are designed to take away some of the human effort by using other sources of energy to do the work. However, there is a financial and environmental cost to pay for our modern way of living, of which we must be aware.
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In every modern house there is a myriad of electrical appliances to make life easy and comfortable. In our “consumer society”, many of these appliances are considered necessary for our way of living…. but are they really? |
Student Task
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Do you have too many electrical appliances at home? Do you think you could get rid of some of these without really affecting your quality of life? What could and couldn’t you throw out …. Discuss. |
How is energy and power calculated?
Most households in Australia use appliances which use electricity or natural gas. Natural gas is primarily used for heating, hot water systems and cooking.
Basic Units of Energy
The basic energy unit is the Joule (J). All forms of energy, whether it be chemical, potential, nuclear, kinetic, sound, electrical, solar, geothermal, wind, or tidal can be measured by this unit.
Basic Unit of Power
The rate at which energy is transformed from one form to another is the power, P; being developed. The basic unit of power is the Watt (W).
Formula:
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Units: 1W / 1J/s |
1 Watt is the rate of energy conversion and is given by one Joule per second.
Rearranging the formula for power gives:
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Energy Converted = Power x time |
E = P x t |
Units: 1J = 1W.s |
1 Joule = 1 Watt times second
We can summarise the above by using the triangle formula which will rearrange or transpose the quantities for us.
E = Energy (J)
P = Power (W)
t = time (s)
Examples of using energy/power formula:
Example 1: An incandescent light globe “uses” (converts) 600 Joules of electrical energy into heat energy in 10 seconds. What is the power of the light globe?
| P = |
| = |
| = 60 J/s | = 60W |
Example 2: The power rating of an electric kettle is 2200 Watts. It takes 2.5 minutes for one litre of water to boil. How much energy (in Joules) has been “used”? (Which really means converted from electrical to heat energy).
E = P x t
E = energy (J)
P = power (W)
t = time (s)
Note we must convert minutes to seconds:
2.5 minutes = 2.5 x 60 = 150 s
E = P x t = 2200 x 150 = 330,000 J
Measuring household energy consumption
A Joule of energy is a very small amount. More often we measure electrical energy consumption in mega Joules (MJ) or more commonly in kilowatt-hour (kW.h).
Note the symbols used (some lower case and some upper case).
1 MJ = 1,000,000 J = 106 J
1 kW.h = 1000 W x 3600 s = 3,600,000 W.s = 3,600,000 J = 3.6 MJ
So one kiloWatt.hour is equivalent to 3,600,000 Joules of electrical energy. It is often easier to use kiloWatt.hour to measure electrical appliance energy consumption as an appliance power rating is usually in kW. You are charged in kW.h on your electricity bill. Currently this varies between AU$0.15 to AU$0.20 per kiloWatt.hour. So if you have an appliance rating, simply multiply this with the number of hours the appliance is on and this will give the energy consumption in kW.h
Example 3: A clothes dryer with power rating of 600 Watts is left on for 45 minutes. How much electrical energy has been used? Give the answer in:
(i) kW.h (kiloWatt.hour)
(ii) J (Joules)
Answer (i):
| 600W = |
| 0.600kW |
| 45 minutes = |
| 0.75 hours |
Therefore; E = P x t = 0.600 x 0.75 = 0.45 kW.h
Answer (ii)
1 kWh = 3,600,000 J
Therefore; 0.45 kW.h = 0.45 x 3,600,000 = 1,620,000 J
Example 4: A 1500 W clothes iron is left on while “The Smith” family goes on holidays interstate. They are away for 2 weeks. Luckily the house doesn’t burn down! The Electrical Retailer charges Au$0.20 per kW.h.
Find:
(a) The amount of electrical energy consumed
(b) The cost of the electricity used.
(a) E = P x t = 1.50 x 336 = 504 kW.h
1500 W = 1.50 kW
2 weeks = 14 x 24 = 336 h
(b) cost = kW.h x $/kW.h = 504 x $0.20 = $100.80 (Expensive. Don’t leave appliances on!)
Student Task
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1. An electric pump rated at 5kW is on for 45 hours pumping water from a river to a storage dam. Calculate how many kWh of energy has been used and also give the answer in Joules of energy. If the cost of electricity is Au$0.20 per kWh, how much will it have cost to operate the pump? 2. At a party there are 6 large spotlights to light up the back yard. Each light has a power rating of 1500 W. How many Joules of energy have been used if the lights are on for 8 hours? What is the cost of the electricity if the Electricity Retailer charges $ 0.18 per kWh? (give answer to nearest dollar) |