ATAR Notes: Forum
VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Physics => Topic started by: HarveyD on September 12, 2011, 09:28:18 pm
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If we were to vary the frequency while keeping the intensity of a light the same, are we just increasing the energy/speed of the electrons being ejected?
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The number of electrons get ejected depends only on the intensity. So greater intensity is more photons arriving per second (and therefore more photocurrent). Intensity doesn't change the stopping voltage.
Intensity does not increase the energy. This was one of the observations that the particle model could predict why, but the wave model could not. Particle model relates the energy to the frequency. The wave model relates energy to the intensity (which is proportional to the amplitude - not sure of the exact relationship).
Increasing the frequency will increase the energy of the electrons as: E=hf-W. I think this equation only applies to the first electron knocked out. Higher energy electrons would travel faster as E=hf-W=1/2mv^2.
Wikipedia's article on it is great: http://en.wikipedia.org/wiki/Photoelectric_effect#Experimental_results_of_the_photoelectric_emission
Remember for each of those, the particle model predicted the correct behaviour. Some of the observations listed there are beyond our level. Wikipedia also notes that: "In practice the number of electrons does change because the probability that each photon results in an emitted electron is a function of photon energy."
http://phet.colorado.edu/en/simulation/photoelectric - This is a demo simulation kind of thing that my teacher used to show the photoelectric effect, it needs Java on your computer to run.
You alter the frequency through the wavelength (shorter wavelength = higher frequency and longer wavelength = smaller frequency).
I think I made a few generalisations which are not true for some situations. I'll double check and then update my post if I stated anything wrong later in the day.