Elmo Electron

Red Orange GreenYellow PurpleBlue

Parable of A Jailed Parable of A Jailed ElectronElectron

Elmo E. is jailed at the surface of a particular metal. The bail neededto him to get out happens to be $50.Each of the photon brothers carries exactly the amount shown. Only 1photon can enter the jail at a time.

$1 $5

$25$10

$100$50

Which photon(s) is(are) needed to post bail for Elmo?

$1 $5

$25$10

$100$50

Blue has exactly the right amount tobail Elmo out. When he goes into the jail, he gets Elmo out, but then Elmo hasno money to spend.What happens when Purple goes in?

$1 $5

$25$10

$100$50

Purple gets Elmo out and gives him $50 extra. Elmo gets out and has some cash to celebrate (responsibly).

$1 $5

$25$10

$100$50

No matter what, Red, Orange, Yellow, andGreen can’t get Elmo out. What if Redgets 50 of his friends (all exactly like him) to help Elmo out?Or what if Orange gets 10 of his identicalfriends to help out?

$1 $5

$25$10

$100$50

REMEMBER - Only one photon canenter the jail at a time, and they can’tpool resources. Elmo is stuck unlessBlue or Purple come by.

$1 $5

$25$10

$100$50

SummarySummary

Electron needs bail to get out Electron needs bail to get out ((work functionwork function))

Only photons with energy equal to Only photons with energy equal to or greater than the work function or greater than the work function can get the electron outcan get the electron out

Extra energy appears as kinetic Extra energy appears as kinetic energy of the electronenergy of the electron

Einstein’s Photoelectric Einstein’s Photoelectric EquationEquation

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

hf = + KE

Why so radical?Why so radical?

Classical physics said waves of any Classical physics said waves of any frequency (color) could gradually frequency (color) could gradually give energy to electron until it had give energy to electron until it had enough to escapeenough to escape

Classical physics said that an Classical physics said that an increase in intensity of red light increase in intensity of red light would give enough energy to would give enough energy to electronelectron

New theoryNew theory

Light in packets (“quanta” or “photons”)Light in packets (“quanta” or “photons”) Photons carry specific energy (hf)Photons carry specific energy (hf) Increasing Increasing intensityintensity increases increases number number

of photonsof photons of a particular f, but does not of a particular f, but does not increase energy increase energy per photonper photon

Increasing number of red photons does Increasing number of red photons does no good if 1 red doesn’t have enough Eno good if 1 red doesn’t have enough E

Important points for setupImportant points for setup

VoltageVoltage is the is the energy per chargeenergy per charge (Joules per coulomb)(Joules per coulomb)

Ammeter measures the Ammeter measures the currentcurrent, or , or flow of charges flow of charges

Electron has a charge of 1.6 x 10Electron has a charge of 1.6 x 10-19-19 C C Energy can be measured in Energy can be measured in electron-electron-

volts (eV)volts (eV) [The energy needed to [The energy needed to move one electron through one volt.]move one electron through one volt.]

Get into groups to study Get into groups to study the handout. the handout.

Discuss and answer the Discuss and answer the questions.questions.