Download
1 / 39
430 likes | 833 Views
Physics Topic Thermal Radiation. By the end of this session students should Describe which surfaces are best absorbers of thermal radiation. Explain variations in radiated heat from sides of Leslie's Cube. Energy Production Pros and Cons for differing fuels. Thermal Radiation.
E N D
Physics Topic Thermal Radiation By the end of this session students should Describe which surfaces are best absorbers of thermal radiation. Explain variations in radiated heat from sides of Leslie's Cube. Energy Production Pros and Cons for differing fuels.
Thermal Radiation • Cooler objects emit lower frequency EM (Infra red) waves with low amounts of power transmitted • Hot objects emit higher frequency EM waves (Visible Light) with much higher amounts of power transmitted • Red hot 1000°C • White Hot Tungsten light bulb 2500-3000°C
Black Bodies and Reflectors • A mirrored surface reflects the majority of incident radiation • A matt black surface absorbs the majority of incident radiation • A hot matt black surface will radiate-transmit more energy than a hot mirrored reflective surface. • Matt & Black good at absorbing or transmitting radiant heat • Shiny, Silver or White poor at absorbing or transmitting radiant heat
Leslie's Cube • Leslie's Cube helps demonstrate the large differences in emissivity of the surface finish of metals. One side is matt black Remember a matt black body is very good at radiating (transmitting) heat. One side is silver & shiny. Remember a silver mirrored body is quite poor at radiating (transmitting) heat.
Crookes radiometer The Crookes radiometer, also known as the light mill. Radiometer turns when the amount of EM radiation it absorbs differs from the amount it transmits.
Why do you think it turns? • One side of the radiometer vanes are matt black! • One side of the radiometer vanes are shiny white! • There is low pressure gas inside the sphere This is not a trivial question!
Why do you think it turns? • Albert Einstein showed a change in pressure should exist (between white and dark sides) because of the temperature difference. The force predicted by Einstein would be enough to move the vanes, but not fast enough. • Osborne Reynolds found that if a porous plate is kept hotter on one side than the other, the interactions between gas molecules and the plates are such that gas will flow through from the cooler to the hotter side. • Einstein's and Reynolds's forces appear to cause a Crookes radiometer to rotate, although it still isn't clear which one is stronger.
Radiant Heat from Leslie's Cube • With radiometer a fixed distance from face of hot cube • How will it turn next to shiny silver side • How will it turn next to matt black side
Radiators • Central heating “radiators” don’t really radiate much heat energy!!! They mainly give out their heat by Convection. • An electric radiant heater is a much better example of a thermal radiator.
Thermos Vacuum Flask Note Mirrored Finish to prevent transmission of EM radiation
Thermos Vacuum Flask Flask has a hollow jacket. Air pumped out of the jacket to prevent convection Only conduction through glass happens in small area at the lid.
Power Generation • At the moment most of UK electricity is generated in power stations using steam • This includes Coal Fired - Oil Fired – Nuclear - Power stations • However it is likely other methods of power generation will become increasingly important in the future as “Renewable Energy” sources are developed.
Fossil Fuels • Coal, Oil and Natural gas are Fossil Fuels • Coal is one of the most widely used fuel used to generate electricity. • Burning coal releases the greatest mass of carbon dioxide into our atmosphere (per unit energy produced) • There are two strong areas of scientific evidence that suggest using Fossil Fuels in this way is not “sustainable”.
Sustainability & Fossil Fuel • Burning hydrocarbons from fossil fuels releases mainly two chemicals into the atmosphere H2O and CO2. • H2O (Water) condenses as rain which causes few problems. • CO2 is a gas that traps low frequency infrared EM radiation from radiating into space. • This is called “Greenhouse Effect”
Sustainability & Fossil Fuel Burning hydrocarbons is an easy way to release a lot of energy quickly. To turn released CO2 back into a trapped carbon rich solid requires a great deal of energy. At least same amount as was released when created combustion.
Is Nuclear Power Safe and Sustainable? • Nuclear accidents can happen. • Windscale fire in Britain (Windscale now renamed Sellafield) • Three Mile Island reactor melt down in the USA • Chernobyl reactor melt down Ukraine (At the time part of the Union of Soviet Socialist Republics) • Concerns exist about long term processing and storage of Radioactive waste. • Reactors and Uranium enrichment technology enable production of atomic weapons.
Is Nuclear Power Safe and Sustainable • Nuclear Power generation can be scaled up to levels similar to Coal fired generation. • Safety concerns over nuclear industry are starting to be considered more acceptable (by Governments) as many climate predictions predict severe environmental problems with continuing use of Fossil Fuels. • Cost? Estimates Vary!
Hydroelectric • Often requires large dams and flooding of areas that could be used for farming. • Dams can burst! • Does not cause atmospheric pollution when in operation. • HEP projects typically have very long lifetimes. Longer than coal fired power stations. • Big HEP projects produce large scale cheep electricity. • Easy to adjust output to meet demand.
Energy From The Wind • Some people have concerns over the appearance of wing turbines on the landscape. • However land can still be used for agriculture. • North Atlantic (around Britain) is a very windy place making it ideal for wind generation. • Does not cause atmospheric pollution when in operation. • Initial cost higher than coal fired power generation. • High barometric pressure weather systems can develop. No wind = no power generation.
Wave Power • Hostile sea environment and maintenance problems have so far prevented large scale Wave Power Generation Projects. • At the moment it is very costly per unit of power produced • Does not cause atmospheric pollution when in operation. • Sometimes the sea can be calm and little power can be harvested.
Solar Energy • Probably more suited to sunnier climates. • However building design in UK climate can utilise Solar Energy to dramatically cut buildings energy requirements. • Expensive to produce electricity using current technologies. • Does not cause atmospheric pollution when in operation. • The sun doesn't always shine, especially at night.
Bio Fuels • Requires use of agricultural land that could otherwise grow food. • Although CO2 is released when fuel is burnt CO2 is retrieved from atmosphere again as the crop is re-grown. (Carbon Cycle) • In UK coppiced woodland has provided heating for buildings for century's. • Bio diesel and Alcohol are used for fuelling some vehicles. • A very expensive option for producing Electricity.
