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P1.2 Energy and efficiency
Appliances transfer energy but they rarely transfer all of the energy to the place we want. We need to
know the efficiency of appliances so that we can choose between them, including how cost effective
they are, and try to improve them.
✓ compare the efficiency and cost effectiveness of methods used to reduce ‘energy consumption’
✓ describe the energy transfers and the main energy wastages that occur with a range of appliances
✓ interpret and draw a Sankey diagram.
Key wordsApplianceEfficiencyCost effectivenessSankey diagram
What would life be like without electricity?
P1.2.1 Energy transfers and efficiencya) Energy can be transferred usefully, stored, or dissipated, but cannot be created or destroyed.
b) When energy is transferred, only part of it may be usefully transferred; the rest is ‘wasted’.
c) Wasted energy is eventually transferred to the surroundings, which become warmer. The wastedenergy becomes increasingly spread out and so becomes less useful.
d) To calculate the efficiency of a device using:
efficiency = useful energy out (x100%) total energy in
efficiency = useful power out (x100%) total power in
Useful energy = Light + Sound 119 + 1 = 120 Efficiency = (useful ÷ total) x 100%= (120 ÷ 200)x 100%= 60%
More efficient so less electricity needed Less fuel burned, so less CO2 produced
Changes in picture brightness and loudness of sound affect energy transfer
Sankey diagrams can be used to show efficiencies:
An efficient machine
An inefficient machine
Least energy wasted as heat – 45% efficient
28 J/s
16 J/s
6 J/s
16 + 6 = 22 J/s
Efficiency = (useful ÷ total) x 100%= (28 ÷ 50) x 100%= 56%
More efficient so less electricity needed Less fuel burned, so less CO2 produced
P1.3 The usefulness of electrical appliances
We often use electrical appliances because they transfer energy at the flick of a switch. We can
calculate how much energy is transferred by an appliance and how much the appliance costs to
run.
✓ compare the advantages and disadvantages of using different electrical appliances for a particular application
✓ consider the implications of instances when electricity is not available.
I can:
Key wordsApplianceTransducerDynamoPowerKilowatt-hours
Transferring electrical energya) Examples of energy transfers that everyday electrical appliances are designed to bring about.
Name one transducer (device) that changes:A. Sound energy to electrical energy
…………………………………………….
B. Chemical energy to electrical energy …………………………………………….
C. Electrical energy to light energy …………………………………………….
D. Potential energy to kinetic energy …………………………………………….
Draw a chain diagram to show the energy changes that occur when:
A. An electric drill is switched on
B. A Bunsen burner is ignited
C. A mass is lifted 1 metre
D. A bullet is fired from a rifle
E. A rubber ball is bounced on the floor.
b) The amount of energy an appliance transfers depends on how long the appliance is switched onand its power.
c) To calculate the amount of energy transferred from the mains using:
E = P x t
E is energy transferred in kilowatt-hours, kWhP is power in kilowatts, kWt is time in hours, h
ExampleHow much energy is transferred if a 1 kW fire is left on for 2 hours?
SolutionUsing E = P x tSubstituting in P = 1 kW and t = 2 h givesE = 1 x 2 = 2 kWh
d) To calculate the cost of mains electricity given the cost kilowatt-hour.
If 1 kWh cost 14p.How much does it cost to run a 1 KW fire for 2 hours?
We have already calculated the kWh aboveSo the cost will be 14 x 2 = 28p
energy = power × time
power = 850 W = 0.85 kW,time = 6 minutes = 0.1 h
Energy = 0.85 x 0.1
= 0.085 kWh
20 J 60 J
20 J
Efficiency = (useful ÷ total) x 100%= (20 ÷ 80)x 100%= 25%
Light and sound
Efficiency = (useful ÷ total) x 100%= (480 ÷ 800)x 100%= 60%
Transferred to the surroundings
Useful Power = 1200 x 0.8 = 960 WWasted energy = 1200 – 960 = 240 WEnergy = Power x Time = 0.24 kW x 0.5= 0.12 Cost = 0.12 x 15 = p
Transferred to the surroundings as heat and sound
Difference in kWh per year =350-225 = 125 kWh
Cost = 125 x 12 = 1500p = £15
Each year costs £15.So a reduction in (12-9) 3 years gives a saving of 3 x 15 = £45
YES
Less electricity used / energy neededLess fossil fuels burnedLess polluting gases emitted
NO
Old freezer must be disposed ofHazardous chemicals inside freezer eg CFCsLot of Energy used in producing new freezer
Iron
HairdryerKettle
sound