Flippy Posted April 7, 2009 Report Share Posted April 7, 2009 Hi there. I'm currently revising for Physics, Topic 8 (the Energy, Power and Climate Change unit) and I'm utterly stuck on one of the syllabus points. I've got my textbook here and I have no idea what it's talking about. I can't ask my teacher because my school is on holiday right now, and the rest of my class are all a bit stuck on this topic (we were asked to teach it to ourselves.) I was wondering if anyone out there could help a poor Physics SL student like me out? The following extract is from my textbook by Greg Kerr and Paul Ruth. I understand the first paragraph and I know what resonance is...it's just all the quantum stuff in the second paragraph that gets me a bit confused. Can anyone explain it in clearer terms? Doesn't have to be a lengthy explanation...just something to clarify it a bit."Explain the molecular mechanisms by which greenhouse gases absorb infrared radiation:Because ultraviolet radiation is more energetic than infrared radiation it tends to break bonds between atoms joined together. On the other hand, infrared radiation being less energetic tends to cause the atoms to vibrate in various ways. When the frequency of the infrared radiation is equal to the frequency of vibration then resonance occurs. This means that the frequency of the radiation is equal to the natural frequency of vibration of two atoms bonded together. It just so happens that the natural frequency of vibration of the molecules of the greenhouse gases is in the infrared region.If resonance occures and the moleculr dipole moment [what the heck is that!?] undergoes a chance, then the greenhouse gas will absorb energy from the albedo infrared radiation coming from a surface. Only certain energies for the system are allowed and only photons with certain energies will excite molecular vibrations. Therefore vibrational motion is quantized and transistions can occur between different vibrational energy levels. The absorbed energy can then be re-radiated back into the biosphere."Any help would be appreciated! Thanks! Reply Link to post Share on other sites More sharing options...
Ongfufu Posted April 10, 2009 Report Share Posted April 10, 2009 (edited) The GH gases absorb infrared radiation as a result of resonance. The natural frequency of oscillation of the bonds within the molecules of the gas is in the infrared region. If the driving frequency (from the radiation emitted from the Earth) is equal to the natural frequency of the molecule, resonance will occur. The amplitude of the molecules' vibrations increases and the temperature will increase. The absorption will take place at specific frequencies depending on the molecular bonds. [img]http://1.bp.blogspot.com/_wXDx9TVRBL8/Sc6YF7FBeOI/AAAAAAAAA1w/9xNkidL2Kns/s400/Picture%20e.png[/img] Consider a molecule of CO (one of the GH gases). You know from atomic physics that the energy of electrons within atoms is quantized. The same effect applies to the energy of molecules due to their vibrational and rotational motion. This energy is also quantized, and there are vibrational and rotational energy levels just as there are atomic energy levels. The big difference between the two kinds of energy level is that the difference in energy between the vibrational/rotational energy levels is approx. the same as the energies of infrared photons. This means that infrared photons traveling through these gases will be absorbed. Absorbing photons means that the gas molecules will now be "excited" to higher energy levels. But the molecules prefer to be in low-energy states, and so they will immediately make a transition to a lower-energy state by emitting the photons they absorbed. But these photons are not all emitted outwards into space. Some are emitted back towards the earth, therefore warming the earth's surface. The precise mechanism for photon absorption by the GH gases is complex and requires quantum mechanics. Consider two atoms forming a diatomic molecule. The force between the atoms may be very loosely modeled as a mass-spring system. The atoms may be thought to be connected by a spring, and simple harmonic oscillations take place when the atoms are disturbed from their equilibrium position. The frequency of oscillation is given by [img]http://upload.wikimedia.org/math/e/7/a/e7a22f418d95b8278ec3de0462f7cbd2.png[/img], where [i]m[/i] is related to the masses of the two atoms, [i]m1[/i] and [i]m2[/i], as [i]m=m1*m2/(m1+m2)[/i]. For CO, the "spring" constant has a value [i]k=1900 N*m^-1[/i] and [i]m=1.14*10^-26 kg[/i]. Therefore the frequency is: [i]f=1/2pi*(1900/(1.14*10^-26))^0.5=6.5*10^13 Hz[/i]. This frequency is the natural frequency of the molecule. Photons traveling through the gas will be in resonance with the molecule if they have a frequency equal to the natural frequency. A typical infrared photon has an energy of 0.25 eV and so its frequency is: [i]f=E/h=(0.25*1.6*10^-19)/(6.6*10^34)=6.1*10^13 Hz[/i]. This is approximately the same as the natural frequency of the molecule. This means that the photons will be absorbed by the molecule. Edited April 10, 2009 by ongfufu Reply Link to post Share on other sites More sharing options...
Flippy Posted April 14, 2009 Author Report Share Posted April 14, 2009 Thank you for your time! I really appreciate it. I think I get it now. Reply Link to post Share on other sites More sharing options...
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