Unit 8. Energy production: Energy sources Note

12 | Energy Production

IB Physics Content Guide

Big Ideas

        Most energy sources can be traced back the sun, our ultimate primary source

        Energy sources must be compared based on many factors including energy density, cost, availability, politics, safety, and environmental impact

        No energy source can be converted to electricity with 100% efficiency

        All energy sources have advantages and drawbacks and it important to understand the complete picture

        Every object with a temperature above 0 K emits thermal radiation

        Radiation intensity is related to separation distance by the inverse square law (similar to force fields)

        The Earth’s climate relies on a delicate thermal energy balance where total energy in equals total energy out

Content Objectives

12.1 – Energy Sources Overview

 

I can list the top 6 most common sources in the global energy supply and general % of total

 

 

 

I can describe the different ways (sectors) that energy is used for

 

 

 

I can distinguish between primary and secondary energy sources

 

 

 

I can define power as a rate of energy usage in terms of watts

 

 

 

I can calculate the efficiency as the percentage of useful energy of the total

 

 

 

I can interpret energy flow from a Sankey Diagram

 

 

 

I can define specific energy and energy density with proper units

 

 

 

I can use specific energy to calculate the amount of fuel needed for a given amount of power

 

 

 

     

12.2 – Fossil Fuels

 

I can give an approximate number of years left in proved fossil fuel reserves

 

 

 

I can explain the Hubbert Peak and why it is important

 

 

 

I can discuss where the United States’ oil supply comes from

 

 

 

I can describe the process of fracking and how it is used to extract oil and natural gas

 

 

 

I can identify the primary uses for each of the primary fossil fuels

 

 

 

I can compare the transportation methods for the different fossil fuels

 

 

 

     

12.3 – Nuclear Power

 

I can describe the chain reaction that occurs to support a self-sustaining fission reactor

 

 

 

I can describe the concentration of U-235 as a sample is enriched into fuel-grade uranium

 

 

 

I can outline the process of enriching uranium

 

 

 

I can explain how a nuclear reactor transforms the energy of a fission reaction into electricity

 

 

 

I can describe the role of the moderator and control rods in a nuclear reactor

 

 

 

I can discuss the challenges of disposing of nuclear waste

 

 

 

     

12.4 – The Renewables

 

I can list examples of energy sources that are considered renewable

 

 

 

I can list examples of energy sources that are known carbon dioxide emitters

 

 

 

I can calculate the power produced by a wind turbine

 

 

 

I can compare the different styles of solar power and what each is used for

 

 

 

I can calculate the power from a solar panel from the panel area and solar intensity

 

 

 

I can describe the factors that affect the solar intensity in different locations on Earth

 

 

 

I can outline the operation of a hydropower generator

 

 

 

I can explain how a hydropower plant can incorporate pumped storage to store energy

 

 

 

I can list challenges that are facing a future of renewable energy

 

 

 

     

12.5 – Thermal Energy Transfer

 

I can provide examples of conduction, convection, and radiation

 

 

 

I can define black-body radiation in terms of absorption and emission of light

 

 

 

I can describe an object based on its emissivity

 

 

 

I can calculate the power emitted by a black body radiation using the Stefan-Boltzmann Law

 

 

 

I can describe the shape of the emission spectra in terms of radiation wavelength

 

 

 

I can mathematically relate peak wavelength and temperature using Wien’s displacement law

 

 

 

     

12.6 – Radiation from the Sun

 

I can define intensity with proper units

 

 

 

I can describe how intensity changes according to the surface area of a sphere

 

 

 

I can derive the Solar Constant from the sun’s power and distance from earth

 

 

 

I can calculate the average solar intensity on earth from the solar constant and earth’s radius

 

 

 

I can compare the properties of albedo and emissivity

 

 

 

I can list the gases that have the largest impact on the greenhouse effect

 

 

 

     

12.7 – Climate Change

 

I can describe the greenhouse effect as absorption and re-emission of thermal energy

 

 

 

I can describe the concept of thermal equilibrium and how it pertains to earth

 

 

 

I can recognize trends in the climate model based on different factors

 

 

 

I can describe the long term and seasonal trends in the carbon dioxide concentration

 

 

 

I can list examples of positive and negative feedback loops in terms of the climate discussion

 

 

 

I can engage in an evidence-based conversation about climate change

 

 

 

     

 

12 | Energy Production

Shelving Guide

Global Energy Usage

Rank

Energy Source

%

1

Oil

32%

2

Coal

28%

3

Natural Gas

22%

4

Biomass

10%

5

Nuclear

5%

6

Hydropower

2.5%

Efficiency

Sankey Diagram Rules:

Width of the arrow proportional to the amount of energy

Energy Density

 

Definition

Units

Specific Energy

Energy transferred per unit mass

J kg-1

Energy Density

Energy transferred per unit volume

J m-3

Primary and Secondary Sources

Primary Energy Sources

Secondary Energy Sources

Energy sources found in the natural environment

(fossil fuels, solar, wind, nuclear, hydro, etc.)

Useful transformations of the primary sources

(electricity, pumped storage for hydro, etc.)

Fossil Fuels

Number of years left in global reserves

 

Describe the process of Fracking:

Coal

~100-150 years

 

1.      Drill hole into shale rock

2.      Inject fracking fluid at high pressure to create cracks

3.      Extract newly released natural gas

4.      Seal fracking fluid in the hole

Oil

~50 years

 

Natural Gas

~50 years

 

Nuclear Power

 

% of U-235

 

Why is the concentration of U-235 important?

Only U-235 can undergo a fission chain reaction

Uranium Ore

0.7%

 

Fuel-Grade

3.5%

 

What is done with the nuclear waste?

Stored on-site in spent fuel pools and/or concrete dry cask storage

Weapons-Grade

90%

 

 

Moderator

Control Rods

Slows down neutrons to be absorbed by U-235

Made from Water or Graphite (carbon)

Absorbs neutrons to limit number of chain reactions

Made from Boron

Renewable Energy

 

Variable Symbol

Unit

 

Data Booklet Equations:

Power

P

W

 

Cross-Sectional Area

A

m2

 

Air Density

ρ

kg m-3

 

Air Speed

v

m s-1

 

 

Photovoltaic Cells

Solar Concentrator

Solar Heating Panel

Converts solar energy directly into electricity. Useful in solar panels on top of building or solar farms connected to the energy grid

Mirrors focus sunlight onto a central tower. The high thermal energy is converted to steam and runs turbines to produce electricity

Sun’s radiation is absorbed by black pipes that transfer thermal energy to the water flowing through them. Replaces hot water heater.

 

 

Biomass

Coal

Geothermal

Hydropower

Natural Gas

Nuclear

Petroleum

Solar

Wind

Renewable

 

 

 

 

Produces CO2

 

 

 

 

 

 

Thermal Energy Transfer

Conduction

Convection

Radiation

Energy is transferred through molecular collisions

Energy circulates through the expansion and rising of hot fluids

Energy is transferred through electromagnetic radiation. Can travel through a vacuum

 

 

Emissivity

 

Black Body Radiation

Sun

~1

 

An idealized object that absorbs all the electromagnetic radiation the falls on it

Earth

~0.6

 

Black-Body

1

 

 

Power Emissivity

Variable Symbol

Unit

 

Data Booklet Equations:

Power

P

W

 

Emissivity

e

 

Surface Area

A

m2

 

Temperature

T

K

 

Max Wavelength

λmax

m

 

Solar Radiation and Climate Change

Intensity

Variable Symbol

Unit

 

Data Booklet Equations:

Intensity

I

W m-2

 

Power

P

W

 

Area

A

m2

 

 

Greenhouse Gases

 

Positive Feedback Loop

Negative Feedback Loop

Water Vapor (H2O)

 

Melting ice (decreases albedo)

Cloud formation (increases albedo)

Carbon Dioxide (CO2)

 

Melting permafrost (releases methane)

Increased photosynthesis (uses CO2)

Methane (CH4)

 

Rising ocean temp releases methane

Climate Change leads to renewables

 

8.1 Energy Production

Energy measurements

  • Specific energy (ES): “Amount of energy a fuel releases per unit mass”, in J kg^-1.

  • Energy density (ED): “Amount of energy a fuel releases per unit volume”, in J m^-3.

Both useful in the choice of a fuel, but other factors must be taken into account, such as transportation, availability, safeness etc.

Energy classifications

  • Primary energy: Found in nature, which has not yet been transformed or converted.

    • Examples: Fossil fuels (oil, coal and natural gas), Solar energy, Kinetic energy of air.

  • Secondary energy: Resultant from the transformation of a primary source.

    • Examples: Electricity and Kinetic energy from wheel.

  • Transformation process: From primary energy form into secondary energy form.

  • Renewable sources: “Rate of use is less than the rate of production of the source”.

    • Examples: Biomass, solar, hydroelectric, wind.

  • Non-renewable sources: “Rate of depletion is greater than the rate of production”.

    • Examples: Fossil fuels and nuclear. 

Energy sources

Fossil fuels (oil, coal and natural gas): Burning fossil fuels in thermal power plants. 

Process: 

  1. Initial chemical potential energy (decomposition of dead animals and plants)     

  2. Kinetic energy of moving steam          

  3. Kinetic energy of turbines        

  4. Electrical energy (generator).

Nuclear power  (uranium and plutonium): Induced fission in thermal fission reactors.

 

Fission reaction

  • Uranium enrichment: U-238 absorbs neutrons. Hence, uranium needs to be enriched, i.e. the percentage of U-235 needs to be increased to up to 3%, producing yellow cake, thus achieving critical mass and being able to have a chain reaction. 

2.gif

 

moderator

  • Moderator (graphite or H20): reduce the speed of neutrons, because very fast moving neutrons do not induce fission.

  • Heat exchanger: allows coolant (H20 or C02) to transfer heat from reactor to water.

  • Control rods: absorb neutrons, so that the reaction does not go out of control.

  • Safety issues:

    • Radioactive waste: buried deep underground in containers, supposed to avoid leakage

    • Fukushima: reactor explosion.

    • Chernobyl: thermal meltdown of the reactor.

Solar power

 

Solar heating panel: sunlight directly used to heat up water.

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Photovoltaic cells: sunlight directly converted into direct electrical current (dc), as the light incident on the panel releases electrons and establishes a pd across the cell.

4.png

 

Wind power

 

  • Formula: maximum theoretical value of the available power = ρAv3.

    • Assumptions: all wind is stopped, no friction or turbulence.

  • Ideal places: off-shore and top of hills, due to higher wind speeds.

 

turbine.png5.png
 

Hydroelectric power

  • Process: potential energy of a mass of water transformed into electricity.

  • Formula: P = mgh/∆t = ρ∆Vgh/∆t = ρQgh, where Q = volume flow rate and ρ the water density.

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  • Reverse process (pumped storage system): storing energy in large scale in the case of necessity (emergency), what requires more energy than it will be gained.

Sankey diagram

  • Use: represents energy and power flow, from left to right.

  • Representation: each energy source and “loss” is represented by an arrow.

    • Arrow drawn to scale (both vertical and horizontal).

    • Energy flow always from left to right.

    • Degraded/”lost” energy to the bottom.

Imagem1.pngEnergyGraph.png
 
 

8.2 Thermal Energy Transfer

There are three types of thermal energy transfer: conduction, convection and radiation. While conduction and convection will only occur if there is a difference in temperature, radiation will always occur. Temperature will be constant if radiant heat from surroundings equals heat lost by radiation. 

Conduction

  • Definition: moving electrons collide with neighboring molecules, transferring energy to them and so increasing their average kinetic energy, and, consequently, the temperature.

  • Occurrence: normally in solids, because fluids have weaker inter-atomic bonds and their atoms are further apart.

  • Materials: good thermal conductors are normally good electrical conductors (e.g. copper), since the mechanism is similar.

Convection

  • Definition: movement of groups of atoms or molecules within fluids that arise through variations in density, leading to thermal energy transfer.

  • Movement direction: Mass of fluid is bottom heated, what decreases density, causing mass to go up, while mass that goes down is heated, leading to the formation of a convection current.   

 

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Thermal radiation

  • Definition: transfer of energy by means of electromagnetic waves, and so, it does not require a medium, like conduction and convection do.

  • Black body: at a certain temperature, black bodies absorb all incoming energy, while at other, it emits great amount of radiation.

  • Intensity (I) definition: “The power received per unit area from a radiating source”.

    • I = P/A

  • Stefan-Boltzmann’s law: P = eσAT^4, where σ = Stefan-Boltzmann’s constant = 5.67 x 10^-8 W m^-2 K^-4 and e = emmissivity of a body.

  • Emmissivity:

    • For a black body: e = 1.

    • Grey bodies: all other bodies, 0 < e < 1 

  • Albedo (α): the ratio of the power of radiation scattered from the body to the total power incident on a body

    • Relationship between albedo and emissivity: α + e = 1.

    • Albedo of a planet depends on cloud cover, ice, water, color and nature of the soil and latitude.

  • Solar constant (S): the solar radiation per second per m^2 at the top of the Earth’s atmosphere (= 1400 W m^-2).

  • Tip: Although the intensity of the Sun’s radiation at the position of the Earth is approximately 1400 W m^-2, the average power received per unit are on the Earth is 350 W m^-2. This occurs, because the solar radiation is captured by a disc of area πR^2, where R is the radius of the Earth, but it is distributed (when averaged) over the entire Earth’s surface, which has an area four times as large (4πR^2). The power received per unit area on the Earth may be further reduced if we take the effect of the albedo into account.

  • Graph: Radiation distributed over a range of wavelengths.

    • Wien’s displacement law: T x λmax = 2.90 x 10^-3 K m. 

2.pngwienlaw.png

Greenhouse effect

  • Definition: the warming of the Earth caused by infrared radiation, emitted by the Earth’s surface, which is absorbed by various gases in the Earth’s atmosphere and is then partially re-radiated towards the surface.

  • Greenhouse gases: The gases primarily responsible for this absorption are water vapor (H2O), carbon dioxide (CO2), methane (CH4) and nitrous oxide (NO2).

  • Temperature balance: energy input to Earth must be equal to energy output by the Earth.

    • Natural balance on the Earth, due to naturally occurring greenhouse gases.

  • Enhanced greenhouse effect: increased level of greenhouse gases, commonly attributed to human cause:

    • Water vapor (H2O): irrigation.

    • Carbon dioxide (CO2): burning fossil fuels in power plants and cars.

    • Methane (CH4): flooded rice fields, farm animals, processing of coal.

    • Nitrous oxide (NO2): burning fossil fuels, manufacture of cement, fertilizers.

The wavelength at which the curve peaks can be associated with temperature through Wien’s law

Earth’s Energy Balance

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Unit 8. Energy production Questions and Answers

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.
The conversion of energy needs to be cyclical in order to provide enough work for a machine. The cyclical conversion of energy also requires some of that energy to be released into the environment. An example would be the internal combustion engine, where chemical energy is converted to mechanical energy. In order for this process to work, it needs to be cyclical and some energy needs to be released.
8.1.2 Explain what is meant by degraded energy.
A typical power station may be only 35% efficient, so that 65% of the energy is degraded and transferred to the surroundings of the power station. This released energy is also known as degraded energy, it cannot be used to perform work. Degraded energy mostly comes in the form of heat. The higher the temperature of the input and the lower the temperature of the output, the higher the efficiency of the system. An example would be a light bulb, where 100J are put in, but not all are used. The light bulb heats up as a result of the degraded energy.
8.1.3 Construct and analyze energy flow diagrams (Sankey diagrams) and identify where the energy is degraded.

Picture

Sankey diagrams indicate how much energy remains after a series of transformations. The arrows pointing upwards indicate exhaust while the arrows pointing downwards indicate loss due to heat, friction or other factors. The thickness of the lines should be proportional to the percentage of energy being represented. Exercises with Sankey diagrams include identifying where energy is degraded, calculating overall efficiency and drawing diagrams to represent energy conversions.

Using Sankey diagrams, we can calculate the efficiency of a process. Efficiency can be calculated by:

(Useful Energy Output ÷ Total Energy Input) x 100

It is a percentage value with no units.


8.1.4 Outline the principle mechanisms involved in the production of electrical power.

Picture

Electricity can be produced mechanically, chemically or through photovoltaic cells. It can also be stored chemically in the form of batteries.

Mechanical energy is converted to electrical power through an electric generator. Electric generators are powered through steam, water or air. Generators come in the form of AC (alternating current) or DC (direct current). Most generators have a wired loop or coil that is rotated in a magnetic field to produce a current, this is called electromagnetic induction. The wired loop or coil is attached to a turbine that will turn when it is impacted by steam, wind or water, thus creating a current.

8.2.1 Identify different world energy sources
The main energy sources are:

Picture

Hydroelectric

Picture

Wave

Picture

Solar

Picture

Wind

Picture

Coal

Picture

Oil

Picture

Gas

Picture

Nuclear

Nearly all of the energy above comes from the sun. Electromagnetic radiation arrives at the Earth’s surface and although most of it is reflected back into space, a small amount is absorbed by plants and used to generate organic compounds. The sun is also responsible for creating wind and waves.
8.2.2 Outline and distinguish between renewable and non-renewable energy sources.

Renewable Resources

  • Hydroelectric Power
  • Solar Power
  • Wave Power
  • Wind Power
  • Other (ie. Biofuel, Tidal Energy, Geothermal)

Non-renewable Resources

  • Coal
  • Petroleum (Oil)
  • Gas
  • Nuclear Power
Non-renewable resources can become renewable if the rate of replenishment exceeds the rate of consumption.
8.2.3 Define the energy density of a fuel.
Energy density is the amount of energy liberated per unit mass of fuel consumed (J/kg). It is also known as the potential energy stored in a fuel per unit mass or volume. The following table shows the energy densities for various fuels:

Picture

 
8.2.4 Discuss how choice of fuel is influenced by its energy density.
Fuel is determined by the transportation costs, which depend on the energy density of the fuel. Some fuels have high energy density and low transportation costs, but are expensive themselves, such as uranium. Some fuels depend on application. For example, nuclear power can be used in submarines.
8.2.5 State the relative proportions of world use of the different energy sources that are available.

Picture

 
(The above data is from the year 2009.)
8.2.6 Discuss the relative advantages and disadvantages of various energy sources.

The questions you need to ask yourself are:

  • Does this fuel release greenhouse gases and contribute to global warming?
  • Does this fuel pose a risk to human health or other living organisms?
  • Does this fuel pollute or pose a threat to the environment?
  • Is fuel source renewable or non-renewable?
  • Is this fuel expensive?
  • Does this fuel have a high or low energy density?
  • Is the fuel continuously available or is it dependent on factors such as weather or time of the day/night?
Picture
8.3.1 Outline the historical and geographical reasons for the widespread use of fossil fuels.
In the 1800s, heat engines were developed in the United Kingdom. Over the next century, intensive efforts went into improving the model and engines became widely used as an efficient source of energy. The change from physical labor to machinery is known as the industrial revolution. A demand for fuel began to arise and coal was the cheapest, most plentiful and most energy dense fuel at the time. Coal is an example of a fossil fuel — fuels that are formed underground by the action of high pressure and temperatures in the absence of air over millions of years. Coal is formed from dead plants, while oil and natural gas are formed from deceased marine organisms. Fossil fuels have very high energy densities compared to most other fuels. They are also widely available, so nearly every region in the world began using them.
8.3.2 Discuss the energy density of fossil fuels with respect to the demands of power stations.

The demand of a power station is the amount of joules needed in a certain period of time to comply with the required energy at that moment. In simpler terms, it is the amount of energy that needs to be generated in order to satisfy the demand of a group of people. Fossil fuels have a high energy density so they are able to meet the current demand, however, as the demand increases, more fossil fuels need to be burned.

Energy density can be used to calculate power through the following formula:

  • Power output = efficiency x mass of fuel burned every second x energy density

The mass of fuel burned every second can also be defined as the rate of consumption.

8.3.3 Discuss the relative advantages and disadvantages of the transportation of fossil fuels.

Advantages

  • Can be easily transported via pipelines, railroads, trucks and ships.
  • Oil refineries close to the sea have easy access to shipping.
  • Creates infrastructure jobs for the surrounding communities.
  • Easy transportation allows countries around the world to enjoy affordable  power.

Disadvantages

  • Pipelines spoil the natural beauty.
  • Risk of leakages and explosions.
  • Leakages and explosions cause environmental damage.
  • Risk of political issues and terrorism.
8.3.4 State the overall efficiency of power production fueled by different fossil fuels.

Picture

Natural gas power stations are the most efficient types of power stations. They are able to convert nearly half of the chemical energy in gas to electricity (45% efficient). Efficiency can be raised if the excess thermal energy is used to heat houses, or if surplus heat from a gas turbine is used to heat water and produce steam.

The factors that affect the efficiency of power plants include:

  • The type of fuel used
  • The load factor (ratio of average energy demand to maximum demand during a period)
  • Technology used

8.3.5 Describe the environmental problems associated with the recovery of fossil fuels and their use in power stations.

Problems with fossil fuels in power stations

  • Pollution (ie. carbon dioxide, acid rain, smog, heavy metals, volatile organic compounds, radioactive material)
  • Carbon monoxide released from powerplants is capable of poisoning people (haemoglobin cells begin reacting with carbon monoxide instead of oxygen)

Problems with retrieval of fossil fuels

  • Coal mining causes release of flammable gases, there is a risk of collapse and underground fires, gases are a threat to the lungs of miners
  • Crude oil and gas retrieval can lead to explosions, oil spills and leakages that pollute the environment and harm life
Methods to ‘clean’ the fuels, such as removing salt from dissolved gas or pumping water to oil deposits have proven costly and inefficient.

Nuclear Power

8.4.1 Describe how neutrons produced in a fission reaction may be used to initiate further reactions (chain reaction).
8.4.2 Distinguish between controlled nuclear fission (power production) and uncontrolled nuclear fission (nuclear weapons).

Picture

If for every three neutrons released, only one reacted with Uranium, the chain reaction would progress at a constant rate. This would be a controlled or self-sustained chain reaction. If, however, all three neutrons triggered more fissions (as presented by the diagram to the right) and the amount of fissions increased exponentially, the chain reaction would be uncontrolled and energy would be released in explosive amounts. This is what happens in a nuclear weapon. Nuclear reactors contain special materials that both slow down and capture neutrons in order to make sure the reaction moves at a constant rate. When these materials fail, nuclear runaways can occur.

8.4.3 Describe what is meant by fuel enrichment.

Picture

In order to gather enough Uranium-235, samples of ordinary Uranium must be enriched. Not all Uranium is fissile (able to sustain a chain reaction), therefore the fissile isotopes of Uranium must occupy a certain percentage of the overall sample in order to drive nuclear fission; a high percentage of non-fissile Uranium would cause the reaction to stop. A sample of Uranium typically contains 99.2% Uranium-238 and 0.72% Uranium-235. Enriched Uranium contains at least 2-5% Uranium-235, although higher percentages are possible. Nuclear weapons require a sample that contains 99% Uranium-235. Uranium is enriched through the formation of hexafluoride followed by the separation of Uranium-238 and Uranium-235 in a gas centrifuge. In more detailed terms, a type of Uranium gas is put into a centrifuge — a series of cylinders with a rotor inside of them. The mixture of isotopes is spun around at high rates (up to 70,000 rev/min). The heavier Uranium-238 requires a larger centripetal force to stay in the circular path and thus moves to the outside of the centrifuge, while the Uranium-235 requires a smaller force and collects together in the enter. The Uranium-238 (now called “depleted Uranium”) can then be scraped off the edges while the remaining Uranium is spun a few more times. This process is repeated until the Uranium has the desired percentage of 3% Uranium-235. It requires very technical and challenging engineering, partly because of the tremendous forces involved.

The critical mass is the minimum amount of Uranium needed to sustain a controlled reaction. As the surface area to volume ratio of an object increases, the higher the probability of a neutron colliding with it, thus a greater percentage will cause fission. Uranium that contains 20% Uranium-235 has a critical mass of 400 kg.


8.4.5 Discuss the role of the moderator and the control rods in the production of controlled fission in a thermal fission reactor.

Picture

The thermal fission nuclear reactor or Magnox reactor utilizes nuclear fission to create energy.

  • The control rods, held by electromagnets, take in neutrons in order to control the reaction and ensure that for every fission, one more fission is triggered at a constant, manageable rate. Control rods can be made of boron or cadmium steel, since these materials absorb neutrons without fissioning. When the reaction goes out of control, the control rods are released from their magnets in order to stop the fission reaction completely.
  • Fast neutrons (neutrons at around 1-2meV) will end up bursting through the nucleus, so they need to be slowed down by the moderator. Moderators can come in the form of graphite, water, deuterium monoxide, beryllium and liquid sodium. These materials have small atoms with masses that correspond with the neutrons. The neutrons are slowed down from a speed of 10^6 ms-1 (“fast neutrons”) to 10^4 ms-1. At this speed, they are known as thermal neutrons. Thermal neutrons have an energy of around 0.2MeV.

8.4.6 Discuss the role of the heat exchanger in a fission reactor.

Picture

The pump is a body of water that consistently flows throughout the system, collecting thermal energy from the reactor core. This water is used to heat up another body of water — the heat exchanger. The heat exchanger is under high pressure; the heat from the pump causes the water within the heat exchanger to turn to steam. The steam pushes turbines, which generate electricity.

8.4.7 Describe how neutron capture by a nucleus of Uranium-238 results in the production of Plutonium-239.

Picture

Non-fissile Uranium can also be converted to a fissile material. Uranium-238, when bombarded with a neutron, turns into Uranium-239 through neutron capture (it absorbs the neutron). It then undergoes beta decay to turn into Neptunium. This element, still radioactive, undergoes another beta decay to turn into Plutonium-239. Plutonium-239 is a fissile element and can be used in the nuclear reactor. Plutonium-239 is attractive due to it being more easily fissile and producing slightly more energy than Uranium. Plutonium is made in a breeder reactor, a reactor that produces more fissile material than is consumed. Breeder reactors were particularly popular in the 1960s, however, as more Uranium reserves were found, Uranium enrichment became less costly.

8.4.8 Describe the importance of Plutonium-239 as a nuclear fuel.

Since Plutonium-239 has a half-life that is significantly higher than that of Uranium and Neptunium, it is easily stored. The short amount of time required to produce Plutonium also makes it an attractive source of fuel. Plutonium-239 can also be used in a nuclear weapon if it is pure enough.

Uranium enriched with 20% Plutonium-239 can be used in a fast breeder reactor. In this reactor, the fast neutrons released from the fission of Plutonium-239 are capable of triggering more fissions, therefore a moderator is not required. This saves spacing and costs. The overall process operates at a very high temperature that requires liquid sodium to cool it down. The liquid sodium can be contained with electromagnetic pumps.

8.4.9 Discuss safety issues and risks associated with the production of nuclear power.

Picture

Some problems associated with using nuclear power include:

  • thermal meltdown can occur when a component fails and the reactor core overheats. This usually occurs due to a lack of coolant or a power surge that overrides the coolant’s abilities. It can also be caused by failed control rods. A prime example would be the disaster at Chernobyl.
  • Radioactive waste can be produced from nuclear power; it is generally costly to maintain. This waste can be either high-level or low-level, depending on its radiation per mass or volume. High-level waste is converted to a rock-like form and placed underground, while low-level waste is buried (20 ft) in shallow depths in soil. Incidents can occur with material that is not disposed of appropriately, leakages during transport, abandoned waste or stolen waste. Inappropriately handled waste can harm humans and other living organisms
  • Mining uranium can cause toxic radon gas emissions, which decay to Radon-222, a carcinogen. Mining Uranium can also contaminate air, water and soil.
  • Nuclear weaponry is possible. The Plutonium-239 generated in breeder reactors is very pure and Uranium enrichment is another process that can produce fuel for nuclear weapons.

8.4.10 Outline the problems associated with producing nuclear power using nuclear fusion.

Although nuclear fusion theoretically seems to be the cleanest and most efficient energy source, it is very difficult to manage with current-day science.

  • The energy expended to undergo nuclear fusion far exceeds the amount received.
  • Tritium and Deuterium cannot fuse until a very high temperature is achieved. The plasma circulating the core must be at a temperature of around 10^8K and cannot come into contact with anything (otherwise it would cool down and damage the surroundings), requiring energy to both maintain the temperature and power the magnetic field.
  • Even if this temperature were achieved, there must still be a critical density of ions for the reaction to be maintained.

Overall, the difficulty of confining high density plasma is what makes nuclear fusion unsustainable. Below — the Russian Tokamak thermonuclear reactor core.

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8.4.11 Solve problems on the production of nuclear power.

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Solar Power

8.4.12 Distinguish between a photovoltaic cell and a solar heating panel.
Photovoltaic Cell

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  • Made of semi-conducting materials such as silicon or gallium
  • Facilitates the transformation of solar energy to electrical energy
  • Can be used to power a calculator or watch

Photovoltaic cells or solar cells create a direct conversion from light energy to electrical energy. Silicon, gallium and other semi-conducting materials are able to emit electrons when excited by photons. These electrons can only move in one direction, creating a potential difference.

Solar Heating Panel

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  • Made of numerous thermally stable metals, including aluminum, steel or copper
  • Facilitates the transformation of solar energy to heat energy
  • Can be used to heat water in a home

A solar panel uses solar heat to heat up water or air. Solar panels are usually connected to heat exchangers; the heated water or air can be provided to homes or used to produce steam and turn turbines. Most solar cells operate at less than 20% efficiency, with future hopes aiming at 40%.

8.4.13 Outline the reasons for seasonal and regional variations in the solar power incident per unit area of the Earth’s surface.
The power of radiation per unit area can be defined as intensity:

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The intensity of the sun on the surface of the earth is approximately 1400W/m^2 — also known as the solar constant. This value can vary by as much as 7%. The main reasons for variation in intensity include:

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  • Weather and climate will affect the intensity of the sun. Some areas are more cloudy than others.
  • Due to the earth’s spherical shape, the sunlight is more spread out near the poles because it is hitting the earth at an angle, as opposed to hitting the earth straight-on at the equator. There is also less atmosphere at the equator, allowing more sunlight to reach the earth. Hence, the intensity varies depending on the geographical latitude of the location.
  • Due to the earth’s rotation, all areas are not consistently exposed to sunlight. Areas that are experiencing ‘nighttime’ are not receiving a lot of the sun’s power, therefore the time of the day or night will affect the solar constant.
  • The angle of the surface to the horizontal at that location

8.4.14 Solve problems involving specific applications of photovoltaic cells and solar heating panels.
I=P/A

Hydroelectric Power

8.4.15 Distinguish between different hydroelectric schemes.

When water is able to freely flow downwards, the gravitational potential is converted to kinetic energy, which can be used to drive a turbine. In order for this process to work, it must be cyclical.

There are three different types of hydroelectric power:

  • Water storage in lakes
  • Tidal water storage
  • Pump storage
Water storage in lakes

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Water storage in lakes is when water flowing down from mountains or from the sky (as rain) is stored in artificial lakes and run through dams, turning turbines and generating electricity. These lakes are called reservoirs. The processed water usually continues to flow into a river. Some of the water in the river evaporates due to convection currents and forms clouds, which then release either rain or snow, allowing more water to be stored in the reservoir. The largest dam in the world is the three gorges dam on the Yangtze river.

Tidal water storage

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Tidal water storage is when dams take advantage of the gravitational attraction between the earth and the moon. As the earth spins on its axis, this gravitational attraction causes some parts of the ocean to rise and others to fall. Oceans rise and fall approximately twice every day. Water is stored behind a dam through sluices at a high tide and then released at a low tide to generate electricity. The gravitational potential energy of the high tide is used to produce electrical energy by spinning turbines.
 
Pump storage
When the demand for power is low, electricity can be expended to pump water back to a higher location. At night, power stations operate at a lower efficiency and use some of the energy to pump water back up then release it during the day for extra power.

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8.4.16 Describe the main energy transformations that take place in hydroelectric schemes.

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The main energy conversions are gravitational to rotational kinetic energy and kinetic to electrical energy (through electromagnetic induction). Energy is usually lost due to friction and heat.
8.4.17 Solve problems involving hydroelectric schemes.
The amount of energy available is directly proportional to the flow of the water and height at which the water falls.

Wind Power

8.4.18 Outline the basic features of a wind generator.

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Radiation from the sun causes differences in temperature that result in changes in air density. These changes produce convection currents and differences in air pressure, which create wind — a transfer of energy between areas of high pressure and low pressure. A wind generator converts the kinetic energy of wind to electrical energy.
The wind turbine is composed of a tower, rotating blades, a generator and a storage or grid system. When the blades of the wind turbine move, the axle rotates. This axle is connected to coil of wire in a generator — this is called the armature. The armature is then rotated in a magnetic field and creates a current.

The blades move in a vertical circle and are designed to be impacted by wind moving parallel to the earth’s surface. The number of blades, their width and the angle to the wind are all carefully chosen to maximize the amount of power produced. Winds flowing close to the ground will lose energy due to friction from objects, so generators are usually located in open areas. Placing wind generators in shallow water has many advantages, but these are expensive to construct. The largest wind farms are located in the UK and Denmark
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:

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The mass can be defined as the volume of air hitting the blades at each given second. This is written as the volume of the air multiplied by its density. The volume can be redefined as the area multiplied by the velocity, since the amount of wind passing each second is equal to the length of the wind ‘cylinder’ hitting the blades:

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Since this is an energy per second value, it can be rewritten as a power value:

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A common misconception is that all of the kinetic energy from the wind is transferred to the generator. This is impossible because if all of the wind were converted to electricity, there would be no wind left — the wind would be completely absorbed by the generator. Other factors that make this impossible include the friction between the moving parts, the collisions of air molecules and degraded heat energy (resistive heating in wires).
8.4.20 Solve problems involving wind power.
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Wave Power

8.4.21 Describe the principle of operation of an oscillating water column (OWC) ocean-wave generator.
 

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Oscillating wave column (OWC) generators can be moored into the ocean floor or built into cliffs. As a wave enters the chamber of the generator, the air inside the chamber is compressed and provides enough kinetic energy to spin a turbine which generates a potential difference. As the water drops, the air decompresses and flows in the opposite direction and an opposite potential difference is generated. The turbine is designed to continue to turn in the same direction regardless of the flow of the air. The best places for OWC generators are the north and south temperate zones, where the westerly winds are strongest in winter. There are OWC stations in England and Japan.

8.4.22 Determine the power per unit length of a wavefront, assuming a rectangular profile for the wave.

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Solar Radiation

8.5.1 Calculate the intensity of the sun’s radiation incident on a planet.

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The intensity of the sun’s power can be calculated at various points through the following formula:

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If we imagine that the sun produces a sphere of radiation, the radius of this sphere would be the distance from the sun to the target object. The P in this case would be the luminosity of the sun (power emitted by the sun due to fusion reactions), which is defined by 3.84 × 10^26W. The radiation incident on the target object is hence represented by the intensity I.

The solar constant represents the average amount of incoming solar electromagnetic radiation per unit area on the earth’s surface. This constant constitutes all types of solar radiation, including UV and infrared. It is thought to be approximately 1.361kW/m^2. The accuracy of the solar constant is questionable due to the following generalizations:

  • This radiation is assumed to be incident on a plane perpendicular to the earth’s surface.
  • It is assumed that the earth is at its mean distance from the sun.
8.5.2 Define Albedo.
Albedo is the measure of reflection on a surface. Albedo in latin refers to “whiteness” or reflected sunlight. It can be calculated by the ratio of reflected radiation from the surface to the incident radiation upon it. Being a dimensionless fraction, it can also be expressed as a percentage and is measured on a scale from zero (0%) for no reflective power to 1 (100%) for perfect reflection. An object that has no reflective power and completely absorbs radiation is also known as a black body.

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The earth’s albedo is 0.3, meaning, on average, 30% of the radiation incident on the earth is directly reflected or scattered back into space.
8.5.3 State factors that determine a planet’s albedo.
  • Seasons have different cloud formations and latitude. In thin clouds, albedo varies from 30-40% whereas in thicker clouds it can be up to 90%.
  • Light surfaces such as deserts will tend to have a higher albedo while dark surfaces, such as seawater, will absorb more radiation.
  • Areas of ice and snow have a high albedo compared to the rest of the planet and will reflect most of the radiation.

The Greenhouse Effect

8.5.4 Describe the greenhouse effect.
The radiation coming from the sun is approximately 50% visible light and 50% infrared, with small amounts of UV light present. When UV light enters the atmosphere from the sun, it is reflected back from the earth at a lower frequency. The light, now Infrared, is no longer able to penetrate the gaseous layer in the troposphere and becomes ‘trapped’. This causes an overall increase in global temperature, affecting a number of things, including arctic permafrost, climate, animal behavior and frequency of natural disasters.
8.5.5 Identify the main greenhouse gases and their sources.

Carbon dioxide

Source: Consumption of fossil fuels ie. petroleum, cellular respiration, decaying matter/fossilization

Effect: The most abundant manmade greenhouse gas, contributes significantly to the greenhouse effect

Methane

Source: Agricultural land (soil fermentation, animals), burning of biomass, coal mining, gas drilling

Effect: 5x more effective in causing the greenhouse effect than carbon dioxide, the oxidation of methane gives off carbon dioxide and water vapour

CFC’s

Source: Aerosols, solvents, air conditioners, refrigerants

Effect: Responsible for 15-20% of global warming. When CFC’s are released into the atmosphere, UV radiation breaks them down into chloride ions. The chloride ions then react with ozone and break it down to oxygen; one chloride ion is capable of destroying 100,000 ozone molecules. The thinning of the ozone layer allows more UV radiation to enter the atmosphere, which results in more InfraRed being reflected back and heating up the atmosphere. 

Nitrous oxide

Source: Burning of forests, burning of grassland, burning of biomass, nitrogenous fertilizers

Effect: 230x more effective in causing the greenhouse effect than carbon dioxide, responsible for ozone depletion

Water vapor

Source: Transpiration in plants, evaporation

Effect: N/A, water vapour is a natural process and does not need to be controlled

* The two major components of the atmosphere — oxygen and nitrogen — are so tightly bound together that they do not absorb heat and therefore do not contribute to the greenhouse effect.
8.5.6 Explain the molecular mechanisms by which greenhouse gases absorb infrared radiation.

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High-frequency light (UV) is energetic and able to break bonds within molecules. Infrared light, on the other hand, causes atoms to vibrate. The greenhouse gases have a natural frequency that falls in the infrared region, so when they are hit by infrared light, they begin to resonate, creating a change in molecular dipole moment. They absorb the infrared radiation and “re-radiate” it back into the biosphere. Essentially, the greenhouse gas molecules are just the right size for the infrared radiation to resonate with the molecules, causing the molecules to heat up.

8.5.7 Analyse absorption graphs to compare the relative effects of different greenhouse gases.
In order to examine the vibrations of greenhouse gases, an infrared spectrophotometry graph is created:

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The peaks in the graph to the left represent asymmetric vibrations that create a change in the molecular dipole moment of the various molecules. This state is also known as “infrared active”, while the troughs represent times when the molecules are “infrared inactive”.
8.5.8 Outline the nature of black-body radiation.

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A black-body is an object that is capable of absorbing all electromagnetic radiation within its surrounding area. The black-body is at thermodynamic equilibrium; its temperature is constant. The radiation emitted by a black body has a special spectrum and intensity that depends on the temperature of the body.

A black body in thermal equilibrium has two notable properties:

  • It is an ideal emitter: it emits energy that is more or equal to any other body at the same temperature, at every frequency
  • It is a diffuse emitter: the energy is radiated isotropically, independent of direction

8.5.9 Draw and annotate a graph of the emission spectra of black bodies at different temperatures.

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Since an object cannot emit radiation over all frequencies at once, black-body radiation is shown as the intensity distribution of the frequencies emitted. As an object gets hotter, it emits more radiation, therefore the graph shifts upwards. The area under the graph indicates the total power emitted per unit area. As temperature increases:

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The peak wavelength is defined by the wavelength where the most energy is being emitted. Wien’s Law describes the relationship between the temperature of the black body and its peak wavelength:

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Where B is Wien’s constant — approximately 2.8977721(26) × 10^−3 mK.
8.5.10 State the Stefan-Boltzmann law and apply it to compare emission rates from different surfaces.

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This formula calculates the amount of power radiated by an object. A black body would have an emissivity of 1, therefore the e would be omitted. If a question asks for the power radiated per unit area, the A value can be omitted.

8.5.11 Apply the concept of emissivity to compare the emission rates from different surfaces.
Since the earth is not a black body, it has a certain emissivity value. Emissivity is defined as the power radiated by a surface divided by the power radiated from a black body of the same surface area and temperature. In simpler terms, it is the relative ability of a surface to emit energy by radiation. A true black body would have an emissivity of 1 while highly polished silver could have an emissivity of around 0.02. Emissivity is a dimensionless quantity.

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8.5.12 Define surface heat capacity.
Surface heat capacity is defined by the energy required to raise the temperature of a unit area of a planet’s surface by one degree Kelvin. It is measured in joules per meter squared per Kelvin.

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(where Q is the thermal energy transferred, A is the area and T is the temperature)

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8.5.13 Solve problems on the greenhouse effect and the heating of planets using a simple energy balance climate model.
This formula is used to predict future changes in the earth’s climate. Consider the following example:

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In this example, a reduction of outgoing intensity by 2W per meter squared (the current average intensity is 342) would result in an increase of 0.16K. This equation inaccurately suggests that reduced activity would continue to increase global temperature. By using this equation to model climate change, we would be making the following mistakes:

  1. Any increase in surface temperature would also increase the outgoing radiation, according to the Stefan-Boltzmann law.
  2. The calculation ignores possible changes to the Earth’s climate due to increasing temperatures (ie. increased cloud cover, melting ice etc.)
  3. The model does not consider changes in human activity
  4. It does not take other natural processes that periodically affect the earth’s temperature into account

The accuracy of climate projection could potentially be increased by redoing the calculation every week, day, minute or even second. The problems involved in predicting future temperature and climate result from the numerous interlinked unknowns. For example, scientists are still unsure about the role of clouds in climate change. The surface temperature of the earth is also difficult to determine with a high degree of accuracy and there is some uncertainty regarding the current value of average outgoing radiation intensity.

8.6.1 Describe some possible models of global warming.
Milankovitch cycles

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  • Orbit cycle variations (Milankovitch cycles) indicate that the earth has regularly moved between highly eccentric cycles (oval-shaped) to highly oblique cycles (circular). These cycles influence the amount of solar radiation incident upon the north and south poles. Analysis of deep-sea sediments has shown that these regular cycle variations are closely associated with climate change.
Volcanic activity

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  • Volcanic eruptions have a significant effect on climate. They release a large amount of greenhouse gases, most of which (up to 97%) is water vapor. The next most abundant gas is carbon dioxide. Eruptions can also cause the warming of local areas.
Solar flare activity

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  • Rises in solar flare activity have occurred alongside increases in temperature. Although the solar activity tracks temperature better than variations in carbon dioxide, recent years have shown a divergence between the two variables. Solar flares are also criticized for not having enough power to cause climate change.
Carbon dioxide emissions

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  • The most well-known cause of climate change is an increase in carbon dioxide emissions due to industrial activity.This fact is further supported by the Vostok ice core data, which indicates that both carbon dioxide concentrations and temperature are currently higher than ever for the past 420,000 years.
8.6.2 State what is meant by the enhanced greenhouse effect.
The enhanced greenhouse effect is the increasing of the greenhouse effect due to mankind.
8.6.3 Identify the increased combustion of fossil fuels as the likely major cause of the enhanced greenhouse effect.
Although debatable, it is highly likely that human activity — namely the burning of fossil fuels — is responsible for the enhanced greenhouse effect. The use of fossil fuels increases the amount of greenhouse gases released into the atmosphere, which are known for contributing to the greenhouse effect.
8.6.4 Describe the evidence that links global warming to increased levels of greenhouse gases.

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According to Vostok Ice Core data and various climate models, the current temperature and carbon dioxide levels are higher than they have ever been for the past 420,00 years. There is a clear correlation between the amount of carbon dioxide in the atmosphere and global temperature.

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8.6.5 Outline some of the mechanisms that may increase the rate of global warming.

The enhanced greenhouse effect has mostly been attributed to:

  • The burning of fossil fuels, which release greenhouse gases into the atmosphere.
  • The reduction of ice/snow cover due to global warming, which reduces albedo and increases the rate of heat absorption.
  • Reductions in the solubility of carbon dioxide in the sea due to increases in temperature, which increase atmospheric carbon dioxide concentrations.
  • Deforestation reduces carbon fixation by plans, which in turn increases carbon dioxide concentrations.
8.6.6 Define coefficient of volume expansion.
The coefficient of volume or cubical expansion is the fractional change in volume per degree change in temperature, given by the equation:

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Where V0 is the original volume, delta V is the volume change, delta T is the temperature change and ß is the coefficient of volume expansion. This value is typically expressed in 10^-4 per degrees Celsius.
8.6.7 State that one possible effect of the enhanced greenhouse effect is a rise in mean sea-level.
One predicted effect of the enhanced greenhouse effect is a rise in mean sea level.
8.6.8 Outline possible reasons for a predicted rise in mean sea-level.

A rise in mean sea level can occur due to:

  • Atmospheric pressure
  • Plate tectonic movements
  • Wind
  • Tide
  • Flow of rivers into the sea
  • Changes in water salinity

A change in sea level is normal and occurs naturally, however, the enhanced greenhouse effect is a newly developed cause. The change in sea level has, as a result, reached an unnatural rate. Some reasons why this prediction is difficult to make include:

  • The anomalous expansion of water (water will expand above 4 degrees Celsius and contract below 4 degrees Celsius, hence, rises in sea level only occur at a certain temperatures)
  • Differences between ice melting on water as opposed to land (when ice on the sea (ie. icebergs) melts, the water replaces the area where the ice used to be, therefore there is no change in sea level)
8.6.9 Identify climate change as an outcome of the enhanced greenhouse effect.
Due to the rises in temperature, the earth’s climate has changed significantly.
8.6.10 Solve problems related to the enhanced greenhouse effect.
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8.6.11 Identify some possible solutions to reduce the enhanced greenhouse effect.

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Some steps have been taken to reduce the enhanced greenhouse effect. These include greater efficiency of power production, replacing the use of coal with natural gas, use of combined heating and power systems (CHP) — where excess heat from electricity is used to heat the home, increases in use of renewable energy sources and nuclear power, carbon dioxide capture and storage and use of hybrid vehicles.

8.6.12 Discuss international efforts to reduce the enhanced greenhouse effect.

Political steps have also been taken to aid in the reduction of global climate change. These include the intergovernmental panel on climate change (IPCC), kyoto protocol and asia-pacific partnership on clean development and climate (APPCDC).

  • The IPCC is the leading international body for assessment of climate change. It was established by the united nations environment program (UNEP) and world meteorological organization (WMO) to provide the world with scientific reasoning on climate change and its potential impacts. It reviews and assess the most recent scientific data produced worldwide to further understand climate change. It is open to all members of the UN and currently 195 countries are signed.
  • The Kyoto Protocol is an international agreement linked to the united nations framework convention on climate change. This agreement sets binding targets for the 37 industrialized countries and the european community regarding greenhouse gas emissions. Kyoto mechanisms include emissions trading (“carbon market”), clean development (CDM) and joint implementation (JI).
  • The APPCDC was an international, voluntary public-private partnership between Australia, Canada, India, Japan, the People’s Republic of China, South Korea and USA in 2005. These members account for over 50% of the world’s greenhouse gas emissions, energy consumption, GDP and population. Unlike the Kyoto protocol, which imposes mandatory limits on greenhouse gas emissions, this partnership engages member countries to accelerate the development and deployment of clean energy technologies, with no mandatory enforcement mechanism. 

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