Photoelectric Effect

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Photoelectric Effect
Parable of A Jailed Electron
Elmo Electron
Red
Orange
Green
Yellow
Purple
Blue
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Elmo E. is jailed at the surface of a particular
metal. The bail needed to him to get out happens
to be 50. Each of the photon brothers carries
exactly the amount shown. Only 1 photon can
enter the jail at a time.
1
5
100
50
25
10
3
Which photon(s) is(are) needed to post bail for
Elmo?
1
5
100
50
25
10
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Blue has exactly the right amount to bail Elmo
out. When he goes into the jail, he gets Elmo
out, but then Elmo has no money to spend. What
happens when Purple goes in?
1
5
100
50
25
10
5
Purple gets Elmo out and gives him 50 extra.
Elmo gets out and has some cash to celebrate
(responsibly).
1
5
100
50
25
10
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No matter what, Red, Orange, Yellow, and Green
cant get Elmo out. What if Red gets 50 of his
friends (all exactly like him) to help Elmo
out? Or what if Orange gets 10 of his
identical friends to help out?
1
5
100
50
25
10
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REMEMBER - Only one photon can enter the jail at
a time, and they cant pool resources. Elmo is
stuck unless Blue or Purple come by.
1
5
100
50
25
10
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Summary

• Electron needs bail to get out (work function)
• Only photons with energy equal to or greater than
  the work function can get the electron out
• Extra energy appears as kinetic energy of the
  electron

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Einsteins Photoelectric Equation

• Energy of photon (E hf)
• Work function (?)
• Kinetic energy of electron (KE)

hf ? KE
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Why so radical?

• Classical physics said waves of any frequency
  (color) could gradually give energy to electron
  until it had enough to escape
• Classical physics said that an increase in
  intensity of red light would give enough energy
  to electron

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New theory

• Light in packets (quanta or photons)
• Photons carry specific energy (hf)
• Increasing intensity increases number of photons
  of a particular f, but does not increase energy
  per photon
• Increasing number of red photons does no good if
  1 red doesnt have enough E

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Important points for setup

• Voltage is the energy per charge (Joules per
  coulomb)
• Ammeter measures the current, or flow of charges
• Electron has a charge of 1.6 x 10-19 C
• Energy can be measured in electron-volts (eV)
  The energy needed to move one electron through
  one volt.

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Get into groups to study the handout.

• Discuss and answer the questions.