2.5 Efficiency
20 October 2010
What are the three types of heat
transfer. Which states of matter do
they work in?
Aims
• Understand the efficiency of energy transformations
• Calculate efficiency from given data
Efficiency
• Often a lot of the energy that goes into a system is wasted, mainly as heat
• To save energy and money, manufacturers are developing appliances that make better use of energy and therefore waste less
• We describe these as energy efficient
Efficiency equation
%)100(inputenergy total
outputenergy useful(%) efficiency
Worked example
• The energy input per second to a desk lamp with a standard tungsten filament is 100 J and the output is 5 J. Energy expressed as joules per second is the power, which has the unit of watts (W). How efficient is the lamp?
%5
%100100J
5J
%)100(inputenergy total
outputenergy useful(%) efficiency
Activity A: Write down the ways in which a petrol engine wastes energy
Activity B: what is the efficiency of a lamp if the useful energy given out each second is 60 J with 100 J of energy put it.
Saving our worlds resources
• Our sources of energy can be separated into renewable and non-renewable
• Renewable can be replaced, non-renewable cannot.
Type Source Energy Uses
Non-renewable Fossil FuelsPowering vehicles,
heating homes, generating electricity
Non-renewableThermal energy from
fissionGenerating electricity
Renewable WindKinetic energy transferred to
turbines
Biofuels
Crops are fermented to make ethanol.
Ethanol can then be burned to give thermal energy
Powering cars
Renewable SunThermal energy
captured by solar panels
Type Source Energy Uses
Non-renewable Fossil FuelsThermal energy from
burning fuels
Powering vehicles, heating homes,
generating electricity
Non-renewable NuclearThermal energy from
fissionGenerating electricity
Renewable WindKinetic energy transferred to
turbinesGenerating electricity
Renewable Biofuels
Crops are fermented to make ethanol.
Ethanol can then be burned to give thermal energy
Powering cars
Renewable SunThermal energy
captured by solar panels
Heating water inhomes, generating
electricity
Assessment activity 2.5
• See sheet
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