Topic 8 Energy Production

Question 1

the law of conservation of energy

Answer 1

“energy cannot be created nor destroyed, only changed from one form to another”

(usually to transfer energy, work must be done, exception: transfer of thermal energy)

Question 2

sankey diagram

Answer 2

show the flow of energy through a system and identify where energy is lost to the surroundings

Question 3

fuels:

Answer 3

• are sources of energy
• release energy by changing their chemical or nuclear structure
• in releasing energy their atoms lose Ep and gain Ek (heat)
• an engine or machine is needed if the heat produced is to do work eg. car engine (heat from petrol is used to move the car)
• some heat is always lost to the surroundings (second law of thermodynamics: “it is not possible to convert heat completely into work”)
• this lost energy is known as degraded energy - because this energy is no longer able to perform useful work

Question 4

specific energy in fuels

Answer 4

the energy liberated per unit mass of fuel consumed
unit: Jkg^–1 (MJkg^–1)
eg. coal = 30 MJkg^–1
uranium = 80 000 000 MJkg^–1

Question 5

energy density in fuels

Answer 5

the energy liberated per unit volume of fuel consumed
unit: Jm^–3 (MJm^–3)

Question 6

primary sources definition

Answer 6

has not been transformed or converted before use by a consumer

eg. burning coal to release its chemical potential energy for hearing or using the kinetic energy of the wind to turn a windmill to grind corn.

Question 7

secondary sources

Answer 7

results from the transformation of a primary source

eg. the electrical energy generated by coal-fired power stations or wind turbines is a secondary source of energy.

Question 8

non-renewable energy sources definition

Answer 8

energy sources that can be used up because they are used at a greater rate than they are produced

eg. coal, oil, gas, uranium

Question 9

renewable energy sources definition

Answer 9

energy sources that cannot be used up or are used at a rate that effectively means the sources will NOT run out
ie. rate of production is greater than the rate of usage

eg. hydro, wind, solar, tidal, wave, geothermal

Question 10

total global fuel use of coal

Answer 10

38%

Question 11

total global fuel use of coal to produce electrical energy

Answer 11

40%

Question 12

generating electrical energy: generator

Answer 12

• a conductor is moved relative to a magnetic field,
• then a force is exerted on the electrons in the conductor and a voltage difference will be induced across the conductor.
• If the conductor is part of a circuit a current will flow

Question 13

fluids typically used in turbines to convert the flow of a fluid into mechanical motion within the generator

Answer 13

• steam
• water
• hot gas
• wind

Question 14

production and usage of electrical energy measurement unit:

Answer 14

1 kilowatt-hour (kWh) = 3.60 x 10^6 J

a kilowatt-hour is the amount of energy used by a 1kW device in one hour

Question 15

advantages and disadvantages of fossil fuels

Answer 15

Advantages:
- very high energy density
- easy to transport
- cheap
- can be used directly or heating
- power stations can be built almost anywhere
Disadvantages:
- combustion products produce pollution (e.g. acid rain), “greenhouse” gases
- fuel extraction can damage the environment
- non-renewable
- power stations need large amounts of fuel

Question 16

how do nuclear power stations generate energy

Answer 16

nuclear fission that releases energy

Question 17

most common fuel source of nuclear power

Answer 17

uranium-235

splits into barium-141 and krypton-92

Question 18

how does uranium-235 undergo fission to produce energy

Answer 18

a neutron source such as americium-241 and beryllium-9 is used to initiate the fission process.
if the mass of uranium-235 is large enough (critical mass) then enough neutrons produced by fission will collide with further uranium nuclei to sustain fission before being lost from the surface of the fuel - this is know as a chain reaction

Question 19

what percentage of uranium is uranium-235 and what percentage does it need to be at to be used to produce power

Answer 19

0.7%

needs to be at around 5% before it can be used in a nuclear reactor

Question 20

what does a moderator do in nuclear power station

Answer 20

the high energy neutrons must be slowed down so that collisions with U-235 nuclei cause fission to occur.

(collisions between the moderator’s nuclei and the neutrons reduce the kinetic energy of the neutrons from several MeV to just several eV)

makes fission happen

eg. water, graphite

Question 21

what do control rods do in nuclear power station

Answer 21

the control rods absorb neutrons.
(by lowering the rods into, or raising them out of, the reactor ore the fission reaction can be controlled. an uncontrolled chain reaction is like totally bad - it’s a nuclear bomb)

control the rate of fission

eg. cadmium

Question 22

what does a heat exchanger do in nuclear power station

Answer 22

water heated by the fission reactions enters the heat exchanger and its thermal energy is transferred to cold water in a seperate circuit to convert it into steam to drive the turbines

Question 23

energy flow diagram of nuclear power generator

Answer 23

Ek of fission products ––> thermal energy of water ––> Ek of turbines ––> Ek of generator ––> electrical energy

Question 24

advantages and disadvantages of nuclear power

Answer 24

advantage:
- uranium has a very high specific energy (need 10x more coal)
- relatively large reserves of uranium
- no greenhouse gases are produced during energy production
disadvantage:
- radioactive waste is produced (safe storage is needed)
- high risk if things go wrong (radioactive contamination)
- non-renewable (but uranium reserves will last a long time)

Question 25

safety issues with nuclear power

Answer 25

• the reactor core needs to be constantly cooled, otherwise a thermal meltdown may occur causing an explosion that sends radioactive material over a wide area
• nuclear reactors produce radioactive waste which must be stored for a long time (millions of years)
• uranium mining is hazardous
• transportation of both uranium and the radioactive waste is hazardous
• the by-products of nuclear power (eg. plutonium-239) can be used to make nuclear weapons

Question 26

why can’t we perform nuclear fusion right now

Answer 26

include maintaining:
- the very high temperatures required so that the nuclei have enough energy to overcome their electrostatic repulsion and combine
- the magnetic field needed to contain the plasma
- the plasma at a high enough density ie. at high pressure, so nuclei are close together to increase the chance of collision between them

Question 27

energy flow diagram of wind power generator

Answer 27

Ek of wind ––> Ek of turbines (blades) ––> Ek of generator ––> electrical energy

Question 28

advantages and disadvantages of wind power

Answer 28

advantages:
- “clean” - no harmful by-products eg. CO2
- renewable
- free source of energy
disadvantages:
- some consider wind turbines visually undesirable
- often need to be build far from population centres
- unreliable (dependant on amount of wind)
- low energy density so wind turbines need to be large and numerous

Question 29

energy flow diagram of wind power from sun

Answer 29

solar energy of sun ––> thermal energy of land and atmosphere ––> Ek of air molecules (wind)

Question 30

hydroelectric power generation uses

Answer 30

gravitational potential energy of water. this water is stored in one of three ways:
- in a lake using a dam
- by capturing water at high tide and releasing it a low tide
- **by pumping water from a low to high reservoir when energy demands are low (pumped storage)

Question 31

energy flow diagram of hydroelectric power generator

Answer 31

Ep of water ––> Ek of water ––> Ek of turbines ––> Ek of generator ––> electrical energy

Question 32

energy losses of hydroelectric power due to:

Answer 32

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

Question 33

pumped storage hydroelectric systems

Answer 33

• when demand for electricity is high, water is allowed to run from the upper to the lower reservoir, through the turbines, and electrical energy is generated
• when demand for electricity is low, the turbines operate in reverse (using electrical energy which is cheap at this time) to pump water back up from the lower to the higher reservoir

pumped storage systems can meet a rapid increase in demand for electrical energy by releasing large volume of water through the turbines

Question 34

advantages and disadvantages of hydroelectric power

Answer 34

advantage:
- “clean” - no harmful by-products eg. CO2
- renewable
- free source of energy
disadvantage:
- only available in areas where suitable bodies of water exist
- building dams involves land being covered in water

Question 35

solar power can be used in ___ ways:

Answer 35

two ways: photovoltaic cell and solar hearing panel

Question 36

how does photovoltaic cell work

Answer 36

• converts solar energy to electrical energy (using photoelectric effect)
• only produce small voltage so only used for electrical devices that do not need much energy eg. TV, radio, lighting, phone charger
• for higher VOLTAGES connect in SERIES
• for higher CURRENTS connect in PARALLEL

Question 37

how does solar hearing panel work

Answer 37

• converts solar energy to thermal energy (in water)
• water in black copper pipes is heated by the sun
• used to provide hot water for domestic use (it uses the most energy to heat water)

Question 38

the average amount of solar energy falling on the outer edge of the Earth’s atmosphere ____ __________ _________ ____ _____ ____ is approximately:

Answer 38

the average amount of solar energy falling on the outer edge of the Earth’s atmosphere at right angles to the Sun’s rays is approximately:

1400W m^–2

Question 39

why does only half of the average amount of solar energy falling on the outer edge of the Earth’s atmosphere at right angles to the Sun’s rays reach the earth’s surface?

Answer 39

• the atmosphere itself absorbs solar radiation
• clouds reflect solar radiation
• during different seasons the angle at which the Sun’s rays are incident on different parts of the Earth vary; the shallower the angle, the more the energy is spread out

Question 40

advantages and disadvantages of solar power

Answer 40

advantages:
- “clean” - no harmful by-products eg. CO2
- renewable
- free source of energy
disadvantages:
- only available during the day (when energy demand is low)
- unreliable (dependant on the amount of cloud cover)
- low energy density (large area needed)
- seasonal variations in sunlight