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How to Calculate KE Due to Stopping Voltages

Einstein got his Nobel Prize for explaining the photoelectric effect. A metal surface releases photoelectrons when it absorbs light. Einstein showed that the energy of the electrons depended not upon the brightness of the light, but upon its wavelength. In and of itself, that's an interesting if not so exciting fact. But Einstein's discovery implies that the world consists of tiny, discrete particles that can only have specific amounts of energy --- the foundation of quantum mechanics. The key to verifying Einstein's interpretation is measuring kinetic energy by applying stopping voltages. You can make that very measurement.

Things You'll Need

Photocell
Ammeter
Variable DC power supply
Single wavelength light source

Instructions

- 1
  Hook the photocell up to the ammeter and shine light onto it. Measure the current. The current is the result of light striking the foil cathode and freeing up electrons, which are absorbed at the anode.
- 2
  Hook the power supply up to the photocell with the positive lead connected to the photocathode and the negative lead connected to the anode. In this configuration, the applied voltage will tend to pull electrons back into the photocathode. Gradually turn the voltage up, monitoring the current. When the voltage gets high enough, the photocurrent will stop. That voltage is the stopping voltage.
- 3
  Record the kinetic energy. For situations like this, physicists invented a convenient energy unit called the electron-Volt, or eV. One way of delivering energy to a charged particle is by applying an electric field. The delivered energy is the charge times the voltage. For one electron, the charge is "e"; multiplied by a voltage, you get energy directly in eV. Since the current will come to a stop when the voltage gives each electron just enough energy to keep it from making the trip to the anode, the applied voltage is then equal to the kinetic energy. So if you find the current is stopped at 3.2 V, then the kinetic energy for each electron is 3.2 eV.
- 4
  Convert the kinetic energy to other units. If you'd like to compare the kinetic energy to that of other objects, you can convert it in eV to ergs or joules:
  
  1 eV = 1.602 x 10^-19 J
  
  1 eV = 1.602 x 10^-12 erg.
  
  So the electron with a kinetic energy of 3.2 eV would have an energy of 5.13 x 10^-19 J or 5.13 x 10^-12 erg.

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