Elmo Electron RedOrangeGreenYellowPurpleBlue Parable of A Jailed Electron

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 $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 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 $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, 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 $25$10 $100$50

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 $25$10 $100$50

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

Einstein’s Photoelectric Equation n Energy of photon (E = hf) Work function (  ) Work function (  ) Kinetic energy of electron (KE) Kinetic energy of electron (KE) hf =  + KE

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

New theory n Light in packets (“quanta” or “photons”) n Photons carry specific energy (hf) n Increasing intensity increases number of photons of a particular f, but does not increase energy per photon n Increasing number of red photons does no good if 1 red doesn’t have enough E

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

Get into groups to study the handout. Discuss and answer the questions.