User's Manual

SE-9639 Franck-Hertz Experiment
22
012-14264A
Analysis
Obtain the value of argon atom’s first excitation potential: V
0
= 11.44 V;
Calculate the value of Planck’s Constant, h
where e = 1.602 x 10
-19
C,  = 108.1 nm, and c = 3 x 10
8
m/s.
Based on the data, Planck’s Constant, h = 6.604 x 10
-34
J•s
Calculate the percent difference between the experimental value and the accepted value (h
0
=6.626 x 10
-34
J•s)
h = | (h - h
0
) / h
0
| x 100% = 0.3%.
Using V
0
= 11.44 + 0.50 V = 11.94 V gives h = 6.892 x 10
-34
J•s, which is 6.892 - 6.604 = 0.29 x 10
-34
J•s. Therefore, the
experimental value for h is (6.6±0.3) x 10
-34
J•s. So the answer is accurate to 0% to as many significant figures as we have, but
the precision is only ±4.5%.
Questions
1. Should you use the positions of the peaks or of the valleys to determine the excitation energy? Or both? Explain.
Use both. The average of the accelerating voltages matching peak positions and the valley positions is the voltage for the
approximate excitation energy, e•U
0
.
2. Why are the peaks and valleys smeared out rather than sharp?
The shape of the peaks and valleys in the curve is affected by the fact that there is a potential drop of 1.5 V at the cathode,
which is the source of the electrons. The cathode potential causes the peaks and valleys to occur over a space of 1.5 V, rather
than at a sharp point.
3. How precisely can you determine the peak/valley position? Explain and justify your estimates.
Note that the current fluctuations in the vicinity of the peaks, the width of the peaks, the steepness of the drop-off or rise, and
background height and shape all may play a role in this
4. How would molecular contaminants in the tube affect your results?
The molecular contaminant in the tube has a different first excitation potential (V
0
), so that the measurement of argon atom’s
first excitation potential would be affected.
Appendix C: Technical Support
For assistance with the equipment or any other PASCO products, contact PASCO as follows:
Address: PASCO scientific
10101 Foothills Blvd.
Roseville, CA 95747-7100
Phone: [phone] (worldwide)
877-373-0300 (U.S)
Web: www.pasco.com
Email: [e-mail]
he
V
0
c
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

=