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