Photoelectric Effect - Planck's Constant Computer Lab
Problem: To confirm the value of Planck’s constant using an applet which simulates the photoelectric
effect.
Materials:
Calculator
Graph paper
Straight edge
Pencil
Computer with Photoelectric Effect applet.
1. Open the Shared folder and locate the physics folder.
2. Open the physics folder and click on the Start file.
3. Scroll down the list of applets until you get to the Physics of Atoms section and click on
Photoelectric Effect.
4. Read through "The Photoelectric Effect." Complete the given table for each cathode type and do
the necessary graphs, calculations and questions. Note: When there is no photoelectric current,
record the Ek max as 0 eV.
Ek max = qV stop
Cathode
Metal
Cesium
Incident Light
Color
Wavelength (nm) Frequency (Hz)
Yellow
578
Green
546
Violet
436
UV – 1
365
UV – 2
254
Ekmax= hf- W
Work Function
eV
J
Planck’s Constant
Ek max
eV
J
h (J/Hz)
2.1
Potassium
2.3
Sodium
2.8
Average “h” _____________
Graphs
Using one grid only but three separate graphs, one for each metal, and with each cathode metal
(cesium, potassium and sodium) represented by a different color, graph Ekmax (eV) versus frequency of
radiant light (frequency is the manipulated variable). Make sure you have negative values on your
vertical axis, similar to the graph on the computer screen. Label both axes and provide a title as well as
a legend to explain your color code.
Calculations:
1. Rearrange the Ekmax formula to isolate Planck's Constant. Calculate the value for 'h' in each row
(where possible) and take an average. Calculate the percentage error of your estimate (use 6.626 x
10-34 Js as the actual value). (Note: Since Ekmax is zero for some frequency values, then an 'h' value
is not possible.)
2. Calculate the slope of each graph, making sure to include units, and record these values below .
In addition, be sure to label which slope is for which metal. What does this slope represent?
3. Show the comparison between your value(s) for the work function from your graph versus the
given work function in the table.
“h” from graph (Js)
“h” from table (Js)
Cesium
Potassium
Sodium
Questions:
1. Explain why the source of our incident light occurs only in 5 specific wavelengths?
2. What measurement does each meter represent?
a) blue meter_____________________________________________________________
b) red meter_______________________________________________________________
3. What do the negative values of the y-intercepts of the graphs represent? Why are they negative?
4. Suggest a reason(s) why every metal has a different work function and therefore would require a
different threshold frequency of the incident light for emission of photoelectrons to occur.
5. Why are the stopping voltages for some wavelengths zero?