Question

Photoelectric Lab - Use stopping voltage and wavelength to find kinetic energy (NO work function). I have the values for stopping potential in V, and the values for wavelength in nm. I can't for the life of me figure out how to find the KE from this information alone. For example, wavelength 400 nm, stopping voltage -0.80 V. Please solve for KE and show me your steps. We haven't "learned" about the work functions yet, and I assume they can't be used in this.

Answer 1

In the photoelectric effect, the kinetic energy of the electron that is ejected from the material is equal to the energy of the incoming photon, minus the work function. The work function is simply the energy required just to get the electron ejected from the material.

Your stopping voltage is a voltage used to create an electric field to actually stop the electrons from reaching the other half of the circuit. That is, you induce an electric field which exerts a force on the ejected electrons, pushing them back toward their point of origin. We slowly ramp up the voltage, and eventually electrons cannot reach the other side. The minimum voltage that achieves this is the "stopping voltage."

Now, when the electron is ejected, it has some kinetic energy. It uses this kinetic energy to move outward, some going across the gap to the other side of the circuit. If the voltage created just barely prevents them from making it across the gap, then the voltage used was enough to convert all the electron's kinetic energy into potential energy, before it was forced back.

If we work in electron volts, it's very easy:

-0.8 V stopping voltage -> 0.8 eV of kinetic energy.