Measuring Planck's constant V2

Resource: https://www.scienceinschool.org/2014/issue28/planck

Teacher notes

What is a diode and how can it be used to measure Planck's constant:

Build the following circuit:

Draw the circuit in your book using the correct symbols for components.

Method:

• 1. Turn the dial on the potentiometer to the max resistance as marked

  2. Connect the the battery

  3. Connect the circuit to the first diode

  4. Turn the dial anti clockwise to gradually increase the voltage across the diode until you observe the light from the diode.

  5. Determine the voltage that represents when the diode just turns on (activation voltage)

  6. record your result and turn the resistance back to the mark.

  7. Repeat for each diode

LEDs use the properties of semiconductors to produce light. The light produced is a result of electrons losing energy as they change energy levels within the atoms of the semiconducting material. Different materials are doped to enable different energy level jumps and thus different frequencies/wavelengths of light. In this experiment if we can determine the activation voltage, that is the first voltage the LED produces light, for a variety LEDs of known wavelength, then we can plot this data to determine Planck’s constant. This activation voltage is equated to the energy required to release the first photon thus we can apply the relationship.

E = hf - ω

Add a column to your data table and calculate the frequency for each wavelength using c = 2.9979 x 108 m/s

A plot of V vs f this should reveal a linear relationship with the gradient m = h

The work function ω, is assumed to be equivalent for all the diodes.

If we use the voltages only we will calculate the eV version of planck’s constant, which can be simply converted to the the Joule version using e = 1.6022 x 10-19 C

Discussion points:

• 1. How close to the known value do they get? (h = 4.135 667 696... x 10-15 eV s)

  2. What are the sources of error?

  3. Speed of light is usually taken as 3.0 x 10 m/s, why do we use the value above?

  4. This experiment can be refined to get a better measure of the activation point by taking a series of measurements of current and voltage as the diode activates. This gives a V-I graph that can be interpolated back to find exactly where the trend cuts the axis…

• • 1. Explain why this method would yield more accurate results compared to the method you used.

• 1. This experiment assumes the value of ω is the same for all the diodes. Is this a fair assumption? Why/why not?

If time: complete a set of results of V-I for one of the diode and upload you results to the class sheet provided to combine the data.