IB Photoelectric PHET

Click Here to run the Photoelectric PhET. It runs the old Java applet in a browser environment, so it takes a bit to load.

You will be set on 400 nm light with a sodium target (small work function - easy to eject electrons), and 0.00 V potential. Slide the intensity to 50% and watch the light eject photoelectrons. (or type into the text box replacing the 0 with a 50 - but being careful not to replace the %) The energy of the photons striking the surface is being turned into electrical energy, and a current is flowing in the circuit.

Slide the potential to +1.00 V. and see how the photoelectrons accelerate after they leave the metal. Any electron that is ejected is now accelerated toward the right side. Turn the light on and off and see the current turn on and off. (This is what photoelectric cells were designed to do - they were used as light detectors)

Turn the intensity all the way up to 100%. Next, slide the voltage to -0.80 V. Watch the photoelectrons decelerate as they try to cross the vacuum. *Are any able to make it? Notice that they don't all go the same distance as they try to cross the vacuum. This is because there is a random amount of the photon energy from the light that is being turned into thermal energy. Watch for awhile and try to see that the fastest electrons all stop in pretty much the same place.

With the wavelength still 400 nm, and the voltage still -0.80 V, try changing the intensity from 10% to 100%. What effect does changing the brightness have? Do the photoelectrons go faster, (farther?) or are there just more of them?

With the wavelength still 400 nm, *find the minimum negative voltage you can make it that stops all electrons. This is the stopping potential. The slider lets you change the voltage by 0.20 V increments, so you will have to try typing numbers into the text box instead. When you hit "enter" the simulation will change the voltage. Be patient, and wait after you change the voltage until you can see the effect of the change in the long term.

Now change the wavelength to 350 nm (shorter wavelength, higher photon energy) *Find the new stopping potential Notice that the stopping potential is a measure of the photon energy.

With the wavelength at 350 nm, switch the target to Calcium (A higher work function.) What happens to the energy of the photoelectrons?

With the intensity at 50%, and the target Calcium, set the potential to +1.00 V. Start at 350 nm, and slowly increase the wavelength of light (reducing the photon energy) until no electrons are ejected. You will have to type into the text box. Type a number, and hit "enter" and it will go to that wavelength. What is the wavelength? Does turning up the intensity cause electrons to be ejected? This wavelength is called the cutoff wavelength. A photon with this wavelength has an energy just equal to the work function of the metal. Calculate the work function of Calcium in eV. (i.e. calculate the energy of photons of the cutoff wavelength)

Find the cutoff wavelength of sodium, and calculate its work function in eV.