Photo Electric Effect

+-V

Reverse the voltage

Photon Energy = Work + Kinetic Energyhf = + Emax hf = hfo + eV

- Work function (Depends on material)

fo - Lowest frequency that ejectse - Electron chargeV - The uh stopping potential

Example 1: A certain metal has a work function of 3.25 eV. When light of an unknown wavelength strikes it, the electrons have a stopping potential of 7.35 V. What is the wavelength of the light?

(1.17059E-07 m = 117 nm)

Example 2: 70.9 nm light strikes a metal with a work function of 5.10 eV. What is the maximum kinetic energy of the ejected photons in eV? What is the stopping potential?

(12.4 V would stop the electrons.)

Ag (silver) 4.26Au (gold) 5.1Cs (cesium) 2.14Cu (copper) 4.65Li (lithium) 2.9Pb (lead) 4.25Sn (tin) 4.42Chromium 4.6Nickel 4.6

Metal Work Function

6.00 eV

Photon energy = Work function + Kinetic energy of electronPhoton energy = 2.15 eV + 3.85 eV = 6.00 eV

Photons of a certain energy strike a metal with a work function of 2.15 eV. The ejected electrons have a kinetic energy of 3.85 eV. (A stopping potential of 3.85 V) What is the energy of the incoming photons in eV?

147 nm

E = hf = hc/,Photon energy = Work function + Kinetic energy of electronPhoton energy = 3.46 eV + 5.00 eV = 8.46 eVE = (8.46 eV)(1.602 x 10-19 J/eV) = 1.3553 x 10-18 J

E = hf = hc/, = hc/E = (6.626 x 10-34 Js)(3.00 x 108 m/s)/(1.3553 x 10-18 J) = 1.4667x 10-07 m = 147 nm

Another metal has a work function of 3.46 eV. What is the wavelength of light that ejects electrons with a stopping potential of 5.00 V? (2)

6.67 V

E = hf = hc/, 1 eV = 1.602 x 10-19 JPhoton energy = Work function + Kinetic energy of electronE = hf = hc/ = (6.626 x 10-34 Js)(3.00 x 108 m/s)/(112 x 10-9 m)E = 1.7748 x 10-18 J E = (1.7748 x 10-18 J)/(1.602 x 10-19 J/eV) = 11.079 eV

Photon energy = Work function + Kinetic energy of electron11.079 eV = 4.41 eV + eVs

11.079 eV - 4.41 eV = 6.6688 eV = eVs

Vs = 6.67 V

112 nm light strikes a metal with a work function of 4.41 eV. What is the stopping potential of the ejected electrons? (2)

3.70 eV

E = hf = hc/, 1 eV = 1.602 x 10-19 JPhoton energy = Work function + Kinetic energy of electronE = hf = hc/ = (6.626 x 10-34 Js)(3.00 x 108 m/s)/(256 x 10-9 m)E = 7.7648 x 10-19 J E = (7.7648 x 10-19 J)/(1.602 x 10-19 J/eV) = 4.847 eV

Photon energy = Work function + Kinetic energy of electron4.847 eV = Work function + 1.15 eV11.079 eV - 1.15 eV = 3.70 eV

256 nm light strikes a metal and the ejected electrons have a stopping potential of 1.15 V. What is the work function of the metal in eV? (2)