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P1.2 Energy andefficiencyAppliances transfer energy but they rarely transfer allof the energy to the place we want. We need toknow the efficiency of appliances so that we canchoose between them, including how cost effectivethey are, and try to improve them.
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✓ compare the efficiency and cost effectivenessof methods used to reduce ‘energy consumption’✓ describe the energy transfers and the mainenergy wastages that occur with a range ofappliances✓ interpret and draw a Sankey diagram.
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P1.2.1 Energy transfers and efficiencya) Energy can be transferred usefully, stored, ordissipated, but cannot be created or destroyed.b) When energy is transferred, only part of it may beusefully transferred; the rest is ‘wasted’.c) Wasted energy is eventually transferred to thesurroundings, which become warmer. The wastedenergy becomes increasingly spread out and sobecomes less useful.
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d) To calculate the efficiency of a device using:efficiency = useful energy out (x100%)total energy inefficiency = useful power out (x100%)total power in
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More efficient so less electricity neededLess fuel burned, so less CO2 producedChanges in picture brightness and loudnessof sound affect energy transfer
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Sankey diagrams can be used to show efficiencies:An efficient machineAn inefficient machine
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Efficiency = (useful ÷ total) x 100%= (28 ÷ 50) x 100%= 56%More efficient so less electricity neededLess fuel burned, so less CO2 produced
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P1.3 The usefulness ofelectrical appliancesWe often use electrical appliances because theytransfer energy at the flick of a switch. We cancalculate how much energy is transferred by anappliance and how much the appliance costs torun.
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✓ compare the advantages and disadvantages ofusing different electrical appliances for a particularapplication✓ consider the implications of instances whenelectricity is not available.I can:
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Transferring electrical energya) Examples of energy transfers that everydayelectrical appliances are designed to bringabout.
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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…………………………………………….
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Draw a chain diagram to show the energy changes that occurwhen:A. An electric drill is switched onB. A Bunsen burner is ignitedC. A mass is lifted 1 metreD. A bullet is fired from a rifleE. A rubber ball is bounced on the floor.
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b) The amount of energy an appliance transfersdepends on how long the appliance is switched onand its power.c) To calculate the amount of energy transferred fromthe mains using:E = P x tE is energy transferred inkilowatt-hours, kWhP is power in kilowatts, kWt is time in hours, h
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ExampleHow much energy is transferred if a 1 kW fire isleft on for 2 hours?SolutionUsing E = P x tSubstituting in P = 1 kW and t = 2 h givesE = 1 x 2 = 2 kWh
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d) To calculate the cost of mains electricity given thecost kilowatt-hour.If 1 kWh cost 14p.How much does it cost to run a 1 KW fire for 2hours?We have already calculated the kWh aboveSo the cost will be 14 x 2 = 28p
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energy = power × timepower = 850 W = 0.85 kW,time = 6 minutes = 0.1 hEnergy = 0.85 x 0.1= 0.085 kWh
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Efficiency = (useful ÷ total) x 100%= (480 ÷ 800)x 100%= 60%Transferred to the surroundings
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Useful Power = 1200 x 0.8= 960 WWasted energy = 1200 – 960 = 240 WEnergy = Power x Time = 0.24 kW x 0.5= 0.12Cost = 0.12 x 15 = p
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Transferred to the surroundings as heat andsound
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Difference in kWh per year=350-225 = 125 kWhCost = 125 x 12= 1500p= £15
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Each year costs £15.So a reduction in (12-9) 3 years givesa saving of 3 x 15 = £45
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YESLess electricity used /energy neededLess fossil fuels burnedLess polluting gasesemittedNOOld freezer must bedisposed ofHazardous chemicals insidefreezer eg CFCsLot of Energy used inproducing new freezer
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