Topic 8: Energy Production

Question 1

Define The Law of Conservation of Energy

Answer 1

“Energy cannot be created or destroyed, only changed from one form to another.” (To transfer energy work is done, with the exception of thermal energy transfer)

Question 2

Define fuels

Answer 2

Sources of energy.

Question 3

Define energy density

Answer 3

The energy liberated per unit volume of fuel consumed.
(Jm-3 or MJm-3)

Question 4

Define specific energy

Answer 4

The energy liberated per unit mass of fuel consumed.
(Jkg-1 or MJkg-1)

Question 5

Define The Second Law of Thermodynamics

Answer 5

It is not possible to convert heat completely into work as some heat is always lost to the surroundings (known as degraded energy).

Question 6

Define primary sources of energy

Answer 6

Sources that have not been transformed or converted before use eg. directly burning coal to heat something.

Question 7

Define secondary sources of energy

Answer 7

Sources that result from the transformation of a primary source eg. electrical energy generated by coal-fired power stations

Question 8

Define renewable energy

Answer 8

Energy that cannot be used up.

Question 9

Define non-renewable energy

Answer 9

Energy that can be used up because they are used at a greater rate than they are produced.

Question 10

Give the advantages and disadvantages of fossil fuel usage

Answer 10

Advantages:
- Very high energy density
- Easy to transport
- Cheap
- Power stations can be built anywhere

Disadvantages:
- Combustion products are harmful to the environment (eg. acid rain and greenhouse gases)
- Fuel extraction can damage the environment
- Non-renewable energy source
- Power stations need large amounts of fuel

Question 11

Give the efficiency of fossil fuels —> electrical energy
(coal, oil and natural gas)

Answer 11

Coal (35%)
Oil (38%)
Natural gas (45%)

Question 12

Define degraded energy

Answer 12

The heat energy that is lost to the surroundings.

Question 13

How is electrical energy generated? (General process)

Answer 13

The flow of a fluid (eg. moving water, pressurised steam, wind) is used to spin a turbine which then spins a generator. The magnets spinning past copper coils in the generator produces electrical energy.

Question 14

Define a kilowatt-hour

Answer 14

The amount of energy used by a 1kW device in one hour.

Question 15

How are fossil fuels used to generate electricity?

Answer 15

Fossil fuels are burnt in power stations and the thermal energy released is used to boil water and create steam. This steam is used to spin turbines which are attached to generators that produce electricity.

Question 16

How do nuclear power stations generate electricity?

Answer 16

Naturally occurring uranium-238 is enriched to uranium-235.
A neutron source is used to initiate nuclear fission. The power stations use the energy released from nuclear fission to heat water, create steam and spin a turbine/generator to produce electricity.

Question 17

What are the three key components of a nuclear reactor?

Answer 17

1) The moderator (eg. heavy water)
This makes fission happen by slowing down the high-energy neutrons so that they are more likely to collide with the uranium-235.
2) Control rods
The control rods control the rate of fission by absorbing neutrons. By lowering them into or out of the reactor core the fission reaction can be controlled.
3) Heat exchanger
The water heated from the fission reaction enters the heat exchanger and its thermal energy is transferred to a separate circuit containing cold water to convert it into steam to drive turbines.

Question 18

Give the efficiency of nuclear power

Answer 18

30%

Question 19

Give the advantages and disadvantages of nuclear power

Answer 19

Advantages:
- High specific energy (a million times more coal is needed to produce the same amount of energy)
- No greenhouse gases produced
- Large reserve of uranium

Disadvantages:
- Radioactive waste is produced
- High risk
- Non-renewable source of energy (but large reserves of uranium so will last a long time)

Question 20

Give the safety issues with nuclear power

Answer 20

• Uranium mining is hazardous.
• If the reactor core (the fuel) is not constantly cooled then thermal meltdown can occur which will spread radioactive material.
• There is radioactive waste produced which must be stored for millions of years.
• Transportation of uranium and radioactive waste is hazardous.
• The by-products of nuclear power (eg. plutonium-239) can be used to make nuclear weapons.

Question 21

Why is nuclear fission used to generate electricity, not nuclear fusion?

Answer 21

Nuclear fusion occurs naturally in the stars but is too hard to create artificially. Very high temperatures and pressures are required for it to occur so the nuclei can overcome electrostatic repulsion and combine.

Question 22

What does the formula P=½Apv³ give?

Answer 22

The theoretical maximum power available in the wind. In reality, only 30% will be converted to electrical energy because not all kinetic energy in the air can be converted as some air must pass through unused to turn the rotors and there are thermal energy losses due to friction in the turbine/generator.

Question 23

Give the advantages and disadvantages of wind power

Answer 23

Advantages:
- Clean (no greenhouse gases produced)
- Renewable source of energy
- Free

Disadvantages:
- Unreliable (amount of wind changes daily)
- Low energy density so lots of wind turbines are needed
- Need to be built far from population centres

Question 24

Give the efficiency of wind power

Answer 24

30%

Question 25

What is the original source of wind power?

Answer 25

The Sun.

Solar energy from the sun gives the land and atmosphere thermal energy which creates wind.

Question 26

Give the formula for efficiency

Answer 26

Useful power out/total power in x 100

Question 27

What are the three ways water can be stored for hydroelectric power?

Answer 27

1) By using a dam
2) Capturing water at high tide and releasing it at low tide
3) By pumping water from a low to a high reservoir (pumped storage)

Question 28

Give the efficiency of hydroelectric power

Answer 28

90%

Question 29

What are the energy losses of hydroelectric power?

Answer 29

• Friction losses due to the turbulence of water in the penstock
• Friction in the turbines and generator
• Electrical heating losses due to resistance of wiring

Question 30

Give the advantages and disadvantages of hydroelectric power

Answer 30

Advantages:
- Clean (no greenhouse gases produced)
- Renewable
- Free

Disadvantages:
- Only available in areas with large bodies of water
- Building dams involves large areas of land being covered by water

Question 31

What are the two uses of the Sun’s radiation in terms of solar power?

Answer 31

Solar heating panel (converts solar energy -> thermal energy and is used to heat water in houses)

Photovoltaic cell (used to produce small amounts of electrical energy)

Question 32

Give the advantages and disadvantages of solar power

Answer 32

Advantages:
- Clean (no greenhouse gases produced)
- Renewable
- Free

Disadvantages:
- Only available during the day (when electrical needs are low)
- Unreliable (depends on cloud cover)
- Low energy density (large solar panels are needed)
- Sunlight varies depending on the season

Question 33

Give the formula for the area of solar panels

Answer 33

Area = power input/power per m2

Question 34

Give the formula and units for specific energy

Answer 34

Specific energy = Ep or Ek/mass
Units Jkg-1

Question 35

W -> MW -> GW

Answer 35

W -> MW ÷ 10⁶
MW -> GW ÷ 1000

Question 36

What is conduction?

Answer 36

The transfer of thermal energy without any overall movement of the molecules/atoms that make up the substance. It is more effective in solids (particularly metals). It can occur in gases and liquids but to a much lesser extent because of the weaker interatomic/molecular bonds and increasing distance between atoms/molecules.

Question 37

What is convection? And, how are convection currents formed?

Answer 37

The transfer of thermal energy by bulk movement of the atoms/molecules that make up the substance. It only takes place in liquids and gases.

The heated region of the fluid expands (molecules move further apart). This region has a lower density than surrounding fluid so rises. A colder, denser region of fluid fills the space left behind, is heated and the process repeats. This is called a convection current.

Question 38

What the model used for thermal radiation?

Answer 38

A black-body. It is a “perfect” emitter/absorber of radiation. The amount it emits/absorbs does not depend on the nature of its surface but on its temperature.

Question 39

Define emissivity.

Answer 39

The power radiated by an object per unit area divided by the power radiated per unit area by a black body at the same temperature.

Question 40

What is the global annual mean albedo for Earth?

Answer 40

0.3 (therefore this means 30% of solar radiation incident on Earth is reflected back into space and 70% is absorbed).

Question 41

Define the solar constant.

Answer 41

The amount of solar energy that falls per second on an area of 1m² above the Earth’s atmosphere.

Question 42

Give examples of greenhouse gases.

Answer 42

Methane (CH₄)
Water vapour (H₂O)
Carbon dioxide (CO₂)
Nitrous oxide (N₂O)

Question 43

What do greenhouse gases do?

Answer 43

They absorb the infra-red radiation emitted by the Earth’s surface and re-emit it in all directions. Some of this will be reabsorbed by the surface causing the Earth’s average temperature to rise.

*Note: greenhouse gases are able to absorb the infra-red radiation because the frequency of this matches the resonant frequency at which the greenhouse gases vibrate. This is a phenomenon called resonance.

Question 44

Why do polar ice caps have the highest albedo?

Answer 44

Because they have multiple layers which reflect radiation whereas a body of water only has one.