In this simulation of a photoelectric effect experiment, light of a particular frequency shines on a metal plate. If the energy of the photons is larger than the work function of the metal, electrons are ejected. By graphing the maximum kinetic energy (KEmax) of the electrons as a function of frequency, you can determine Planck’s constant from the slope and the work function from the y-intercept.
Here is the procedure:
|1. ||Choose the material to be used as the plate. Set the voltage to zero, and start at a low frequency. If the current is zero, the frequency is below the threshold frequency required to eject electrons. Press the “Plot data” button to record the voltage (zero) as a function of frequency.|
|2. ||Increase the frequency, recording points on the graph at a few frequencies. When you increase the frequency above the threshold, you will see a non-zero current reading. This means electrons are being ejected from the metal. To determine their maximum kinetic energy (KEmax), slowly increase the battery voltage until the current returns to zero. Press the “Plot data” button to record the maximum kinetic energy for this frequency.|
|3. ||Plot a few more points, increasing the frequency and finding the maximum kinetic energy each time by finding the minimum battery voltage required to bring the current to zero.|
|4. ||When you have several points for frequencies above the threshold frequency, press the “Fit the data” button. Your points should follow a straight line, because the equation governing the process is KEmax = hf-W, where h = Planck’s constant, f = frequency, and W is the work function of the metal. With KEmax on the y-axis and f on the x-axis, what should the slope of the line be equal to? What should the y-intercept be equal to? Note that the fit only includes the data points with non-zero KEmax values. Why are the others ignored?|
|5. ||When you are satisfied with your graph and the values for the first material, choose a new material for the plate and repeat the process. What are the similarities between the two graphs? What are the differences? Explain these with reference to the equation KEmax = hf-W.|
To explain the photoelectric effect Einstein used the photon theory, which predicts that increasing the intensity of the light has no impact on the maximum kinetic energy of the emitted electrons. This contrasts with the wave theory of light, which predicts that increasing the intensity increases KEmax. In the simulation, does changing the intensity affect KEmax?