Renewable sources of energy: Cannot be used up (within a lifetime, especially for nuclear). E.g. hydroelectric (gravitational energy of the Sun and Moon, tides), solar (Sun’s radiated energy), wind (Earth’s internal heat energy), biofuels, photovoltaic cells (nuclear energy stored within atoms)

Non-renewable sources of energy: Can be used up and eventually run out (within a lifetime), rate of consumption exceeds rate of production. E.g. coal, oil, natural gas, nuclear

Energy density: Energy liberated per unit mass of fuel consumed, Jkg-1.

Energy degradation: Energy converted to thermal energy that is lost to the surroundings, and thus less energy made available to do useful work.

Know to state that in a Sankey diagram, width of the arrow is relative to magnitude of energy consumed.

Main fuel currently is

FOSSIL FUEL.

Development of history of fossil fuel: Dead matter is accumulated over hundreds of millions of years, then converted into fossil fuels due to exposure to very high temperatures and pressure beneath the Earth’s surface. During industrial revolution and invention of steam engine, factories became established around existing deposits of fossil fuels, hence becoming city centres.

Advantages:

- High energy density

- Relatively easy to transport

- Still cheap

- Readily available in the short term

Disadvantages: Produces pollution (damages environment) and greenhouse gases, non-renewable, large amounts required.

Reasons for widespread use (in addition to advantages):

- Many transport systems rely on fossil fuels

- Power stations can be built close to energy sources

- Health considerations still not treated as a major issue

In coal fired power station: Energy lost via,

1. Hot gases from burning of coal into chimney, heating losses

2. Radiation and convection from the boiler, heating losses

3. Friction losses in mechanism of the dynamo

Coal, oil, natural gas are around 40-50% efficient (natural gas more efficient). Note this came out for IB in mcq before.

NUCLEAR

Production: Uranium-235 bombarded with neutrons under very high temperatures to cause fission. When critical mass of fuel present, chain reaction occurs, releasing large amounts of energy. (As size of fuel increases, chance of neutron causing further reaction increases. Critical mass depends on nature of fuel and shape of assembly.)

Moderator: Collisions between neutrons and nuclei of moderator slow down neutrons created by fission process, allowing further reactions to take place.

Control rods: Readily absorb neutrons, control the chain reaction. If not, explosion and thermal meltdown of core would take place.

Heat exchanger: Allow the nuclear reaction to occur in a sealed off environment. Also allow thermal energy released to heat up the core, heat up the water and produce steam to turn turbines.

As temperature of sample rises, fuel will eventually melt (thermal meltdown of core. Why?

The bombarding particles transfer KE to the fuel atoms. The average KE of the fuel atoms increases. Since temp is a measure of the average KE of the atoms, temp of the fuel rises.

When temp reaches melting point of the fuel, transfer of energy increase PE of the fuel atoms, while keeping the KE of the atoms constant, thus allowing the metal to melt at a constant temp.

Advantages: High energy density, large excesses of uranium compared to coal

Disadvantages:

- Produces radioactive nuclear waste (remains dangerously so for millions of years, currently buried in geologically secure sites) à poses severe health risks and causes problems of disposal of the nuclear waste

- High risk (especially for workers in uranium mines)

- By-products of civilian use of nuclear power can be used to produce nuclear weapons

Enrichment: Process to increase percentage composition of uranium-235 in naturally occurring uranium in order to make nuclear fission more likely (plutonium-239 can also be obtained via fusion of uranium-238 first to produce uranium-239, then beta-decay).

Reprocessing: Treating of fuel waste from nuclear reactors to recover uranium and plutonium (e.g. a fast breeder reactor)

Warfare: Threat of nuclear weapon deployment has been used as deterrent to non-nuclear aggressive acts against countries with nuclear capability. Non-proliferation treaties limit nuclear power technologies to a small number of nations.

SOLAR

Production:

1) Use of a photovoltaic cell to convert portion of radiated energy directly into a p.d. using a semiconductor microchip (electrical energy). Used in series to generate high voltages and parallel to generate high current.

2) Use of an active solar heater to capture thermal energy in water.

Solar constant: The amount of solar energy that falls per second (i.e. power) on an area of 1m2 above the Earth’s atmosphere. More than the power that arrives on 1m2 of Earth’s surface, due to scattering and absorption in the atmosphere. Varies at different parts of earth’s surface (radiation has to travel through greater depth of atmosphere at high latitudes, and is also spread out over a larger area).

Advantage: Clean/no harmful by-products, renewable, free

Disadvantage: Unreliable, low energy density

Inverse square law: Intensity of received radiation is inversely proportional to the square of the distance from point source to observer.

Albedo: Fraction of radiation received by a planet that is reflected back into space

Mean albedo of the earth is 0.3.

WAVE POWER

Production: Use of an oscillating water column, relative motion of different sections of a pelarmis or vertical motion of buoys due to passing waves, turns turbines and generates electrical energy.

Outline how the energy of a wave can be converted to electrical energy:

The mechanical energy of a wave in a turbine/ oscillating water column can be converted to electrical energy via a dynamo/electrical generator.

Using square waves of amplitude A (height 2A), speed v, wavefront L and wavelength :

HYDROELECTRIC

Production: In power station, tidal power schemes to trap water at high tide, or pumping of water to a high reservoir allows gravitational potential energy of water to be stored, then converted to kinetic energy when water allowed to flow downhill (at low tide, or to low reservoir).

Advantages: Clean, renewable, free

Disadvantage: Can only be utilized in particular areas

WIND

Production: Different parts of atmosphere are heated to different temperatures, temperature differences cause pressure differences, allowing air to flow.

Solar à KE of wind à KE of turbine à electrical energy

For air travelling at speed v and of density p, and turbine of radius r,

Advantages: Clean (no carbon dioxide emission, does not contribute to enhanced greenhouse effect), renewable, FREE =)

Disadvantages: Unreliable, low energy density (not 100% efficient), requires large amounts of land, high initial costs of capital, noise pollution

Planets: Radiation

Blackbody: Perfect absorber and emitter of radiation

Radiation: Infra-red region of EM spectrum

Light and shiny surfaces are poor radiators and poor absorbers. Rough and dark surfaces are good radiators and good absorbers.

Area under intensity-wavelength graph = intensity emitted in that wavelength range = measure of the total power radiated

Hot stars emit all frequencies of visible light, thus appear white. Cooler stars give out longer wavelengths and lower frequencies of visible light, thus appear red. Radiation emitted from planets peaks in infra-red region, hotter stars will peak at higher frequencies and shorter wavelengths.

Stefan-Boltzmann Law: Total power radiated is directly proportional to temperature of a black-body raised to the power of four,

Wien’s displacement Law: Product of the wavelength at which the intensity of the radiation is a maximum and the temperature of the black body is a constant,

Emissivity: Ratio of the power radiated by an object to the power radiated by a black body at the same temperature (no units) and of the same dimensions.

Surface heat capacity: Energy required to raise the temperature of 1 unit area of a planet’s surface by 1 degree,

Thermal equilibrium for planets: Power absorbed = power radiated into space

(note area by which power absorbed )

Assumptions:

- Ignore any feedback processes (resulting change as a further change of one of the constants)

- Take whole planet as a single black body

Greenhouse effect:

Earth receives short wavelength radiation from the sun, warms up. Earth surface emits longer wavelength infra-red radiation which is readily absorbed by greenhouse gases in the atmosphere and re-radiated in all directions (as a result of resonance, natural frequency of oscillation of bonds within greenhouse gas molecules is in infrared region). Thus prevents radiation from escaping into space. Hence upper atmosphere and surface of the earth are warmed.

Greenhouse gases absorb infrared radiation, but transmit ultraviolet radiation.

Can lead to global warming as a result of enhanced greenhouse effect due to increase in greenhouse gases in the atmosphere. Increased greenhouse gases such as carbon dioxide due to increased combustion of fossil fuels.

Other causes of global warming/enhanced greenhouse effect: Increased solar flare activity, cyclical changes in the earth’s orbit and volcanic activity, deforestation (reducing carbon fixation, increasing CO2 levels)

Greenhouse gases: methane, water, carbon dioxide, nitrous oxide (livestock and industries e.g. nylon production), ozone (absorbs high energy UV photons), chloroflurocarbons (from refrigerants, propellants and cleaning solvents; deplete ozone layer)

Calculations: Assume all radiated energy in infrared region, gas absorbs all the radiation, no change in radiating power of Earth due to temperature change.

Evidence of global warming due to increased levels of greenhouse gases: Isotopic analysis to estimate temperature and measurement of atmospheric concentrations of greenhouse gases in air bubbles trapped in ice cores, show

evidence of increasing temperature with increasing levels of carbon dioxide.

Effects of global warming:

Solutions to greenhouse effect:

- Advances in technology to increase efficiency of power production and decarbonising of exhaust gases

- Reduction of energy requirements by using thermal insulation in homes and energy efficient hybrid vehicles

- Use alternative energy sources besides coal and oil, such as nuclear power or natural gas (less harmful by-products)

- Reforestation, maintaining existing forests

- International efforts to reduce greenhouse gas emissions e.g. Kyoto Protocol, Intergovernmental Panel on Climate Change (IPCC), Asia-Pacific Partnership on Clean Development and Climate (APPCDC)

	Creado por ibukunadeleye66 hace más de 10 años

(Notes on Chapters) Physics Apunte sobre Environmental Physics: Energy, power and climate change, creado por ibukunadeleye66 el 15/01/2014.

Environmental Physics: Energy, power and climate change