Photoelectric Effect

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  • Created by: victoria
  • Created on: 19-05-11 13:28
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Experimental setup:
Put a clean zinc plate (zinc has a lowish work function) on the top of a goldleaf electroscope.
Charge the zinc plate negatively (e.g. with the negative terminal of a very high PD power supply, also
known as an EHT supply). The gold leaf rises to show the presence of the negative charge.
Shine a bright visible light source on the zinc. Nothing will happen.
Shine a UV light source on the zinc and the gold leaf will fall, showing the loss of negative charge.
If you charge the zinc positively, neither visible nor UV light has any effect.
Visible light or UV
Zinc plate
Gold leaf
Electromagnetic radiation consists of packets of energy called photons.
The energy of a photon is proportional to its frequency, E = hf. {This is important}.
Metals have free electrons which can be removed from the metal if sufficient energy is supplied.
The work function is the minimum amount of energy required to release an electron from a metal.
Electrons that are deeper in the metal need more energy to release them than those on the surface.
This is why the work function is the minimum energy.
When electromagnetic radiation is incident on a metal surface, one electron interacts with one
photon. {This is a very important point to make. Without it, the whole explanation falls apart.}
If the photon has less energy than the work function then it is unable to free the electron. This
means that the photons need a minimum energy and hence minimum frequency (the threshold
frequency) for the photoelectric effect to occur.
If the photoelectric effect does occur, the photon is completely absorbed. The electron gains all the
energy of the photon. Some of this energy will be needed to remove the electron from the metal the
remainder will end up as the electron's KE.
E.g. photons hitting a 4eV work function metal
3eV photon 6eV photon 6eV photon
4eV metal 4eV metal
surface electron surface electron deeper electron needing
0.5eV to get to the surface
Result: electron stays Result: electron emitted Result: electron emitted

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KE = 2eV with KE = 1.5eV
The maximum KE of the emitted photoelectron will be equal to photon energy ­ work function.
KEmax = hf ­
At the threshold frequency, fo, KEelectron = 0 (the electrons are only just released). This means that the
photon energy is only just equal to the work function i.e. hfo = .
Factors affecting the photoelectric effect
Increasing the frequency of the EM radiation increases its energy (E = hf).…read more

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What can't be explained by a wave model?
A wave model can explain why the effect happens, as the wave supplies energy to releases electrons. It
can't explain:
The existence of a threshold frequency e.g. why bright visible light doesn't work but dim UV light
does. Bright visible light delivers more energy per second than dim UV and should be able to
release more electrons per second.
Why the maximum KE of the electrons is frequency dependent but not intensity dependent.…read more


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