8.5+Greenhouse+effect+Notes+2011

Back to IB PHYSICS > ENERGY, POWER AND CLIMATE CHANGE > PHYSICS CLASS 2011 COLLABORATIVE NOTES PROJECT You should write notes on the section which is allocated to you. Make sure your notes are easy to understand and include pictures, links and examples where appropriate. Explain the formulas from the data booklet where necessary. On each page, you will see the syllabus references. To write notes, click on EDIT. You can then write, insert files and pictures or links. You could link to useful web resources, java applets etc. You can obtain information from the text books or the Internet. You must login to be able to edit and you must be a member of the wiki. Deon || 8.2 World energy sources Notes 2011 Zac || 8.3 Fossil fuel power production Notes 2011 Bevis || 8.4 Non-fossil fuel power production Notes 2011 Reilly, Luke, Ryan || 8.5 Greenhouse effect Notes 2011 Luka, Antoine || 8.6 Global warming Notes 2011 Ivy, Matteo || =8.5 GREENHOUSE EFFECT (Luka 1-7; Antoine 8-13)=
 * 8.1 Energy degradation and Power Generation Notes 2011
 * Aim 7: ** Computer simulation, spreadsheets and databases have a significant role here.
 * Solar radiation **

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

The Sun emits 3.6 x 1026 Js-1 of energy, and this energy spreads out in a sphere. The Earth is 1.5 x 1011 m away from the Sun, and therefore the intensity (power per unit of area) on the Earth is:

Power of the Sun/Area of sphere at distance to Earth = 1380 Wm-2

This same calculation can be made for any planet in the Solar system, the only difference is the distance from the Sun.

// 8.5.2 Define //albedo//.//

Electromagnetic radiation incident on a surface will be either absorbed or reflected. The ratio of reflected radiation to all incident radiation is called the //albedo.//

The albedo for snow is very high (90%), as it is a white surface that reflects most incident radiation, while for a dark forest it is around 10%. The average albedo for the Earth is around 30%, including the atmosphere.

// 8.5.3 State factors that determine a planet’s albedo. //

// Students should know that the Earth’s albedo varies daily and is dependent on season (cloud formations) and latitude. Oceans have a low value but snow a high value. The global annual mean albedo is 0.3 (30%) on Earth. //

//** The greenhouse effect **//

// 8.5.4 Describe the greenhouse effect. //

[|PheT simulation]

The radiation from the Sun passes through the atmosphere, where a fraction of it is reflected and a fraction is absorbed. A lot of that radiation passes through and reaches the Earth's surface where, again, a fraction is reflected (depending on the albedo) and a fraction is absorbed. The absorbed radiation will then be emitted by the ground and, according to Wien's Law (should be discussed by Antoine in black body radiation later on) and due to the low temperature of the ground, this radiation will be in the IR spectrum. Infrared just so happens to be the exact spectrum that is absorbed by molecules of greenhouse gases in the atmosphere, which causes them to get hotter and raising the temperature of the Earth.

Sankey diagram showing the greenhouse effect:

// 8.5.5 Identify the main greenhouse gases and their sources. //

// Methane CH4: fossil fuel mining and distribution, livestock (cow farts, faeces etc.), landfills //



Water Vapour H20 (most dominant greenhouse gas - 95% of all greenhouse gases): mostly natural causes, evaporation





Carbon (IV) Oxide CO2: fossil fuel combustion, almost all heavy industry and manufacturing processes, respiration...



Nitrogen (I) Oxide N2O (also laughing gas): soil fertilisation, manure and sewage management, biological processes mostly in wet tropical soil...

// The gases to be considered are CH4, H2O, CO2 and N2O. It is sufficient for students to know that each has natural and man-made origins. //

// 8.5.6 Explain the molecular mechanisms by which greenhouse gases absorb infrared radiation. //

As we know from the atomic physics chapter, electrons can absorb photons of light of specific discrete frequencies (and therefore specific energies) and be excited into a higher energy state, or they can emit a photon of the same energy and return to the previous energy level.

However, molecules can be excited too, not just atoms. Molecules are composed of 2 or more atoms bonded together by electromagnetic forces and can be made to oscillate or rotate. If an incident photon has the same frequency as the frequency of the oscillation of the molecule, the molecule will absorb the photon and gain kinetic energy i.e. it will increase its temperature. This is an example of resonance. The frequencies of photons that can do this are usually in the infrared region.

Absorption by solids:

As solids consist of many different atoms and molecules, their light spectra will be continuous as they will be a sum of the spectra of all the atoms and molecules. Therefore they can absorb photons of many different energy levels, which also makes it easier for energy to be transferred to atoms and raise their temperature. This means that shining light on a solid will cause it to emit IR radiation.

// Students should be aware of the role played by resonance. The natural frequency of oscillation of the molecules of greenhouse gases is in the infrared region. //

8.5.7 //Analyse absorption graphs to compare the relative effects of different greenhouse gases.//

Different gases will absorb different amounts of radiation of different wavelengths. Absorption graphs for different gases show this:



// __ Black Body Radiation __ // // Due to their atomic structure solids can absorb many different wavelengths of radiation: for the same reason, if a solid is heated up it will emit a wide range of frequencies. // // A black object is an object that absorbs all wavelengths, and if heated, will emit all wavelengths. However, not all frequencies will be equally intense. The intensity of the spectrum depends on the temperature of the body ( //T//), the hotter it is the shorter the wavelength will be. This wavelength (ʎmax) can be calcualted from Wien's displacement law:// // ʎmax = //b /t where b// = 2.89 × 10-3 mK



__ Stephan-Boltzman Law __ From the above graph we can see that as the temperature of a black body increases, the intensity of the radiation also increases. In other words, the amount of energy emitted from the surface increases. The Stephan-Boltzmann Law relates the power emitted per unit area to the temperature of the surface with the equation: Power per unit area = σT4 where σ = 5.67 × 10-8 W m-2 K-4

__ Emissivity (e) __ Emissivity is the ratio of the energy radiated by a body tothe enrgy radiated by a black body of the same temperature. Example: The average temperature of the Earth is 288 K. If the Earth was a black body it would radiate: 2.9 × 107 × 2884 W = 1.99 × 1017 W But since the temperature is stable the Earth must be radiating 1.23 × 1017 W.  So the emissivity is: 1.23/1.99 = 0.6

__ Surface Heat Capacity __ This is the amount of heat required to raise the temperature of 1 m2 of the ground by 1K. For the Earth this is 4 × 108 J km-2

__ Solving Problems on the Greenhouse Effect and the Heating of Planets __ The change in a planet's temperature over a period of time is given by: (incoming radiation intensity - outgoing radiation intensity) - time/surface heat capacity