Posted: April 13th, 2023

Physics Lab essay

Lab 13: Photoelectric Effect

Equipment:

He-Ne Laser, BK 2705B Multimeter, Photoelectric device, Large Mercury Source, Spectrum Tube Power Supply, Helium Spectrum Tube, and colored filters [blue (round), green (round), and yellow]

Theory:

When light falls upon a surface, electrons can be emitted from the surface. This is the photoelectric effect. It was discovered by H. Hertz in 1887, explained in terms of light photons by A. Einstein in 1905, and verified by a series of careful experiments by R. Millikan in 1916.

In this experiment, photons from a light source, a laser, will be used to illuminate a photosensitive surface. According to the principle of energy conservation, the maximum kinetic energy gained by the electrons must equal the energy of the incident photons minus the energy needed to free the electrons from the surface. In symbols this relationship is

KE = hc/ – Φ,

where  is the macro-wavelength associated with the photons and Φ is the work function. The maximum kinetic energy can be expressed in terms of the retarding potential, i.e., in terms of the voltage needed to prevent the collection of electrons. Using that KE = eV, where V is the retarding voltage, the above relation can be written:

V = (hc / e) (1/) – (Φ / e).
Procedures:

Since we don’t have access to the lab, we will use a simulation of the photoelectric effect. The advantage of the simulation is that it will be able to give us more data points that our physical lab setup. The physical lab will give 4 data points, and the simulation will give 11 data points.

Here is the link to the simulation: https://www.thephysicsaviary.com/Physics/Programs/Labs/PhotoelectricEffect/
The directions below replace the procedure of Lab 12 and step 1 of the analysis.
1. Click on Begin
2. First, observe how the system operates.
a. If the plate on the right-hand side doesn’t say Potassium, click on the name until it does say Potassium.
You can lower or raise the voltage by clicking on the red arrows on the L’Enfant Corp DC Power Supply.
Set the voltage to zero, then set the brightness to 100 and the wavelength to 400 nm by using the
Black arrows.
b. The 400 nm photon (green squiggly line) give enough energy to the electrons (black dots) in the Potassium plate so that they can escape from the Potassium plate. Now increase the voltage by 0.1 V at a time. Notice that as you increase the voltage, the electrons are slowing down. That’s because the plate on the left is attached to the negative terminal of the power supply. As the plate become more negative it produces a larger and large stopping force. Eventually they slow down so much, that they can’t quite make it to the left-hand plate, and they return to the potassium plate. The current (flow of electrons) goes to zero. The voltage that causes this to happen is called the stopping potential. In our example, the stopping potential will be 0.8 V.
c. As the energy of the photons increases (wavelength decreases), it will take a greater and greater stopping potential to turn of the current.
3. Now, set the wavelength to 200 nm and find the stopping potential. Then fill in the chart on the next page.
Since this is a simulation, you only need to take one data point per wavelength. Once you have filled out the data chart, proceed to step 2 of the Analysis and complete the lab.
Wavelength in nm Stopping Potential
200
220
240
260
280
300
320
340
360
380
400

Analysis:

1. Calculate the average stopping potential for each wavelength, and record in your data table.
Wavelength in nm 1/Wavelength (m-1) Stopping Potential
200
220
240
260
280
300
320
340
360
380
400

2. Make a graph of stopping potential (V) vs. (1/m-1) and include trendline data — the slope and y-intercept. (To get the appropriate number of significant figures, Right-Click on the trendline equation, select “Format Trendline Label” then click “Scientific” under the “Number” category, then select 2 decimal places and click close). Copy your graph below.

V = (hc / e) (1/) – (Φ / e).

A plot of V vs. (1/) should yield a straight line with a slope given by (hc/e) and a y-intercept of (Φ/e). Using the accepted values for c (speed of light) and e (basic unit of charge), you will obtain an estimate for Planck’s constant (h) and for the work function (Φ). c=2.998 x 108 m/s e=1.602 x 10-19 C

3. Using the above equation and the details from the graph (Show your calculations below)
a) Establish a value for Planck’s constant.
b) Compare your value for h with the accepted value using percent error.
c) Determine a value for the work function.

Calculated value of h (Planck’s Constant)__________

h=6.626068 × 10-34 m2 kg / s

% error (Show work for this calculation) ____________

Work Function=____________
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