The Basic Idea of Photoelectric Effect (page 2)

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Author: Jerry Silver

Photoelectric effect apparatus

This approach uses a metal target in a vacuum tube. Because the currents that need to be measured are so small, it is helpful to have the detector very close to the source of the current. This procedure goes through the generic steps to make this measurement with specific references to the EP-05 operation (more detailed instructions are available with that apparatus):

  1. Set up the fluorescent lamp to focus on the detector (photodiode).
  2. Attach a voltmeter to read the stopping voltage (stopping potential) across the photodiode. (The connections are the red and black banana jacks on the EP-05.)
  3. Place the blue filter over the opening going into the photodiode. The apparatus should be set up as shown in Figure 121-2.
  4. Darken the room. If necessary, construct a light shield from a cardboard box to protect the photodiode from stray light.
  5. Adjust the stopping potential, so all the electrons are turned back and there is no photocurrent. (This is accomplished by turning the "voltage knob" to the full clockwise position.)
  6. Now, adjust the stopping potential to its minimum value. (This can be done by turning the voltage knob as far in the counterclockwise position as possible.)
  7. Adjust the radiation intensity by changing the distance between the light source and the detector to read about 10 on the intensity scale. You are now calibrated and ready to make some measurements.
  8. Measure the current reading and write down the reading on the voltmeter.
  9. In several steps, increase the stopping potential and record the current reading at each step. Five readings should be sufficient to define a linear relationship.
  10. Photoelectric Effect

  11. The data should produce a linear relationship similar to the one shown in Figure 121-2 between stopping potential and voltage.
  12. The voltage required to produce zero current is a key point that determines the value of the work function for the metal (in the photodiode).
  13. Repeat the previous steps using the green filter.
  14. Replace the fluorescent lamp with a tungsten incandescent lamp. Install the red filter and repeat the previous steps until current versus voltage curves for the red filter.
  15. A laser or LED of known wavelength can also be used as a source of illumination. A diverging lens (biconcave) may be helpful in spreading the laser beam to fill the opening area of the photo diode.
  16. For each color, plot the current versus voltage and extrapolate the curve to find the threshold stopping voltage that results in zero current.
  17. Plot the stopping voltage versus the frequency for each of the frequencies (colors) for which you took data.

Expected Results

Ultraviolet light shining on a piece of zinc results in a charge separation. This charge causes the leaves of a negatively charged electroscope to separate further and causes the leaves of a positively charged electroscope to come together. This indicates the charge is negative or, more specifically, consisting of electrons. Visible light does not result in this charge being developed in the zinc.

Using the photoelectric effect apparatus, we find that:

  1. The greater the frequency, the greater the stopping voltage required to limit the current flow. This relationship is linear. This means the kinetic energy of the freed electrons is proportional to the frequency of the light.
  2. Below a certain threshold frequency, no current is generated.
  3. Increasing the intensity of the light increases the current (for a given stopping potential and light frequency). However, increasing the light intensity does not have any effect on the kinetic energy of the freed electrons.
  4. The slope of the stopping voltage versus the frequency graph represents Planck's constant divided by the charge on one electron. The equation for this is:
  5. Because the wavelength of light is usually more readily available, the frequency can be determined from the equation: frequency = speed of light / wavelength

  6. From the slope of the voltage versus frequency graph, Planck's constant can be determined from the slope multiplied by the electronic charge: q = 1.6 × 10–19 C. A slope of 4 = 10–5 gives the expected value of Planck's constant.
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