Photoelectric Effect

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Transcript Photoelectric Effect

Parable of A Jailed
Electron
Red
Orange
Yellow
Green
Elmo
Electron
Blue
Purple
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
$50
$10
$25
$100
Which photon(s) is(are) needed to
post bail for Elmo?
$1
$5
$50
$10
$25
$100
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
$50
$10
$25
$100
Purple gets Elmo out and gives him
$50 extra. Elmo gets out and has
some cash to celebrate (responsibly).
$1
$5
$50
$10
$25
$100
No matter what, Red, Orange, Yellow, and
Green can’t 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
$50
$10
$25
$100
REMEMBER - Only one photon can
enter the jail at a time, and they can’t
pool resources. Elmo is stuck unless
Blue or Purple come by.
$1
$5
$50
$10
$25
$100
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

Einstein’s Photoelectric
Equation
Energy of photon (E = hf)
 Work function ()
 Kinetic energy of electron (KE)

hf =  + KE
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

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 doesn’t have enough E

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 electronvolts (eV) [The energy needed to move
one electron through one volt.]

Get into groups to study the
handout.
Discuss and answer the questions.