Study Guide Answer Key
Topic 8. Energy, Power, and Climate Change
8.1. Energy Degradation and Power Generation
8.1.1. State that thermal energy may be completely converted to work in a single process, but
that continuous conversion of this energy into work requires a cyclical process and the transfer of
some energy from the system.
8.1.2. Explain what is meant by degraded energy.
 Degraded energy: energy that has become less useful, i.e. cannot perform mechanical work
due to being transformed into other forms of energy, e.g. thermal energy.
8.1.3. Construct and analyze energy flow (Sankey) diagrams and identify where the energy is
degraded.
8.1.4. Outline the principal mechanisms involved in the production of electrical power.
Electrical energy may be produced by rotating coils in a magnetic field.
8.2. World Energy Sources
Fuel: a substance that can release energy by changing its chemical or nuclear structure.
8.2.1. Identify different world energy sources.
 Fossil Fuels emitting CO2
o Coal
o Oil and gas
o Wood and biomass
 Nuclear fuel
 Energy derived from the Sun
o Solar energy
o Hydroelectric power
o Wind power
o Wave power
 Energy sources not derived from the Sun
o Tidal power:
o Geothermal energy
 The Sun is the prime energy source for the world’s energy.
8.2.2. Outline and distinguish between renewable and non-renewable energy sources.
8.2.3. Define the energy density of a fuel.
 Energy density: the amount of energy that can be extracted per kilogram of fuel. Unit:
8.2.4. Discuss how choice of fuel is influenced by its energy density.
8.2.5. State the relative proportions of world use of the different energy sources that are
available.
 Oil: 38%; Coal: 26%; Gas: 23%; Hydroelectric: 6%; Nuclear: 6%; Renewables: 1%.
 Renewable energy: Solar: 44%; Wind: 27%; Geothermal: 17%; Biofuels: 12%.
8.2.6. Discuss the relative advantages and disadvantages of various energy sources.
8.3. Fossil Fuel Power Production
8.3.1. Outline the historical and geographical reasons for the widespread use of fossil fuels.
 Industrialization led to a higher rate of energy usage, leading to industry being developed
near large deposits of fossil fuels.
8.3.2. Discuss the energy density of fossil fuels with respect to the demands of power stations.
Estimate the rate of fuel consumption by power stations .
8.3.3. Discuss the relative advantages and disadvantages associated with the transportation and
storage of fossil fuels.
8.3.4. State the overall efficiency of power stations fuelled by different fossil fuels.
 Coal: 40%
 Gas: 59%, up to 80% if waste heat is used to heat houses.
8.3.5. Describe the environmental problems associated with the recovery of fossil fuels and their
use in power stations.
8.4. Non-Fossil Fuel Power Production
Nuclear Power
8.4.1. Describe how neutrons produced in a fission reaction may be used to initiate further fission
reactions (chain reaction).
 Energy is required to split a U-236 nucleus. This can be supplied by adding a neutron to the
U-236 nuclei, which increases the binding energy and causes the nucleus to split in two.
 Extra neutrons are produced, which can go on to react with other U-236 nuclei in a selfsustaining chain reaction. However they must be first slowed down to less than 1 eV.
 Critical mass: the minimum mass required for a chain reaction.
8.4.2. Distinguish between controlled nuclear fission (power production) and uncontrolled
nuclear fission (nuclear weapons).
8.4.3. Describe what is meant by fuel enrichment.
 Uranium comes naturally as 99.3% U-238. However only U-235 is used in the reaction
process.
 The process of increasing the percentage of U-235 in the material is called enrichment.
 3% U-235 must be reached in order to be power a nuclear reactor.
8.4.4. Describe the main energy transformations that take place in a nuclear power station.
8.4.5. Discuss the role of the moderator and the control rods in the production of controlled
fission in a thermal fission reactor.
 Moderator: material whose atoms slow down neutrons to make them suitable for reaction.
 Control rod: material that absorbs excess neutrons to prevent an uncontrollably large
release of energy.
8.4.6. Discuss the role of the heat exchanger in a fission reactor.
 To transfer heat from the reactor to the turbine, the medium through which nuclear energy
converts to heat and then to motion and electricity.
8.4.7. Describe how neutron capture by a nucleus of uranium-238 results in the production of a
nucleus of plutonium-239.
8.4.8. Describe the importance of plutonium-239 as a nuclear fuel.
 Plutonium-239 is used as a fuel in other types of reactors.
8.4.9. Discuss safety issues and risks associated with the production of nuclear power.
 Thermal meltdown:
 Nuclear waste:
 Mining of uranium: environmental damage, workers safety hazards.
 Possibility that a nuclear power program may also be used to produce nuclear weapons.
8.4.10. Outline the problems associated with producing nuclear power using nuclear fusion.
8.4.11. Solve problems on the production of nuclear power.
Solar Power
8.4.12. Distinguish between a photovoltaic cell and a solar heating panel in terms of energy
transfers and utility.
 Solar panel is used for central heating or for making
hot water for household use, placed on roofs of
houses, consisting of metal absorber, water pipes,
and glass. Energy is merely converted from solar
power, electromagnetic waves of light, to heat.
 A photovoltaic cell converts solar radiation into
electrical energy.
8.4.13. Outline reasons for seasonal and regional
variation sin the solar power.
 The power per unit area received at a distance r from
P
the sun is called the intensity I: I 
.
4r 2
 On Earth, this amounts to roughly 1400 W/m2, the
solar constant, the power received by one square
meter placed normally to the path of the incoming
 1011 m from the sun.
rays at a distance of 1.50
 Due to the time of the day, this may vary 1.5%
daily; due to Earth’s elliptical orbit, this may vary an additional 4.0% seasonally.
8.4.14. Solve problems
involving specific applications of photovoltaic cells and solar heating

panels.

Hydroelectric Power

8.4.15. Distinguish between different hydroelectric schemes.
 Pumped storage schemes: when energy from nearby coal plants are used to pump water up a
reservoir by night.
 Run-of-the-river power stations: stations that use water diverted from a fast-flowing river
without damming the river.
 Harnessing tidal power:
8.4.16. Describe the main energy transformations that take place in hydroelectric schemes.
The rate of change of the potential energy converted into kinetic energy is
mgh Vgh
V
P


gh  Qgh , where Q is known as the volume
t
t
t
flow rate (volume per second).

8.4.17. Solve problems involving hydroelectric schemes.
Wind Power
1. Basic wind: Sun heats the air, which
rises. Cool air moves into low-pressure
area. Movement is wind.
2.
Coastal wind: water
has larger specific heat
capacity than land and therefore does not rise in temperature as much as land.
Hot air from land rises, cool air from sea moves in.
3.
Katabatic wind: high pressure from cold air presses down at the top of
a mountain, resulting in airflow downhill.
8.4.18. Outline the basic features of a wind generator.

Consists of a horizontal axis with two blades.
8.4.19. Determine the power that may be delivered by a wind generator, assuming that the wind
kinetic energy is completely converted into mechanical kinetic energy, and explain why this is
impossible.
Consider the above tube of air with density  , velocity v , cross-sectional area A . The
1
1
1
kinetic energy of this air tube is given by: E k  mv 2  Avt  v 2  Atv 3 .
2
2
2
Ek 1
3

 Kinetic energy per unit time gives 
power: P 
  Av .
t 2
 The underlying assumption of this calculation is that the wind is stopped by the wind turbine,

which is not the case.
8.4.20. Solve problems involving wind power.


Wave Power
8.4.21. Describe the principle of operation of an oscillating water column (OWC) ocean-wave
energy converter.

The waves make the water alternately rise and fall, causing the air within the column to
move out and in, turning the turbine.
8.4.22. Determine the power per unit length of a wavefront, assuming a rectangular profile for
the wave.
P pvgA2

L
2
8.4.23. Solve problems involving wave power.

8.5. The Greenhouse Effect
Solar Radiation
The Greenhouse Effect
8.6. Global Warming