1. 20 October 2010
2.5 Efficiency
What are the three
types of heat
transfer. Which
states of matter do
they work in?

2. Aims
• Understand the efficiency of energy
transformations
• Calculate efficiency from given data

4. 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

5. Efficiency equation
useful energy output
efficiency (%)  (100%)
total energy input

6. 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?

7. useful energy output
efficiency (%)  (100%)
total energy input
5J
 100%
100J
 5%

8. Activity A: Write down the ways in
which a petrol engine wastes energy

9. 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.

10. Saving our worlds resources
• Our sources of energy can be separated into
renewable and non-renewable
• Renewable can be replaced, non-renewable
cannot.

11. Type Source Energy Uses
Powering vehicles,
Non-renewable Fossil Fuels heating homes,
generating electricity
Thermal energy from
Non-renewable Generating electricity
fission
Kinetic energy
Renewable Wind transferred to
turbines
Crops are fermented
to make ethanol.
Biofuels Ethanol can then be Powering cars
burned to give
thermal energy
Thermal energy
Renewable Sun captured by solar
panels

12. Type Source Energy Uses
Powering vehicles,
Thermal energy from
Non-renewable Fossil Fuels heating homes,
burning fuels
generating electricity
Thermal energy from
Non-renewable Nuclear Generating electricity
fission
Kinetic energy
Renewable Wind transferred to Generating electricity
turbines
Crops are fermented
to make ethanol.
Renewable Biofuels Ethanol can then be Powering cars
burned to give
thermal energy
Thermal energy Heating water in
Renewable Sun captured by solar homes, generating
panels electricity

13. Assessment activity 2.5
• See sheet