Download The Franck-Hertz Experiment Data Studio Version

File Franck-Hertz_11.doc
The Franck-Hertz Experiment
Data Studio Version
Objective: To measure the energy difference between quantized atomic energy levels in
mercury atoms.
INTRODUCTION:
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EXPERIMENTAL SETUP:
The Franck-Hertz apparatus used in this lab consists of the tube, a 5.0 to 6.3V (AC or
DC) filament power supply, a power supply to accelerate the electrons, a picoammeter to
measure the very small collector current and your ThinkPad to log and plot the data in an
XY plot. The tube contains a small amount of mercury, some of which vaporizes in the
heat of the oven (~180°C). The oxide-coated cathode emits electrons when heated by the
filament. These electrons are released by the accelerating voltage. These electrons pass
through the grid anode and then must overcome a retarding potential of -1.5V to reach the
collector. The tube itself is mounted on the rear side of the front panel in such a manner
that the entire tube including the connecting wires is heated to a constant temperature.
You can view the tube through the window on the side of the box.
First, we want to set up the experiment without the ThinkPad to visually check
that it is running properly. Figures 4 & 5 shows how to wire the experiment. Note that
neither side of the filament is grounded (i.e. it goes to + and – , NOT + and ground on the
power supply). The same is true for the 0-60VDC power supply. Only the ammeter
measuring the collector current is connected to ground on one side. Both the filament
power supply and the 0-60VDC power supply are said to be "floating". Floating allows
the component to maintain a set potential difference between its + and - terminals no
matter how much additional voltage (within limits) is added to the circuit.
Turn on the oven by setting the knob on the side of the unit to 180-200°C.
Connect the apparatus as shown in Figures 4 & 5. Adjust the 0-60V accelerating voltage
and verify that you get the expected result (see Fig. 3) in the picoammeter as V0 is
increased from 0 to 60 V. When you do, you are ready to move on.
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Fig. 5. Franck-Hertz wiring diagram w/opto-isolating oscilloscope probe
connecting to the Pasco interface.
RECORDING DATA:
We will use the Pasco interface and a ThinkPad to record the data directly as an XY
plot with X=accelerating voltage=analog port B and Y=tube current=analog port A as in
Figure 5. The Y input can be obtained easily, because the "recorder output" from the
picoammeter gives 0-3V as the meter reads 0-full scale. For the X input, we cannot use
0-60V directly because the Pasco interface is only capable of handling 10V across a
given input. An additional problem is that the Pasco input is referenced to ground while
the 0-60 V supply is floating. Thus, we will install an opto-isolated oscilloscope probe
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(Pomona Model 6731) and set it for 20x reduction in input voltage, which will output
approximately 0-3V for an input of 0-60V. It will not change the accelerating voltage to
the tube, but allow us to interface with the Pasco equipment.
1. Make sure the USB connector from the Pasco box is connected to your computer.
2. Start the DataStudio program on the ThinkPad. (Start Menu and then >All
Programs_WFU Academic Tools  Scientific Tools  DataStudio 1.9.8r7).
3. Connect the Pomona 6731oscilloscope probe to the 0-60 volt power supply as shown
in Figure 5. Make sure it is plugged in to its power supply and set it to 20x.
4. Plug two voltage sensors into the Pasco Interface:
connect the picoammeter output to Channel A.
connect the voltage from the Pomona 6731oscilloscope probe to Channel B
5. Start Data Studio…New Experiment.
6. Click on the Add Sensor or Instrument tab, then select ScienceWorkshop Analog
Sensors, and choose Voltage Sensor (last entry) from the analog sensor menu.
Repeat this for the second sensor.
8. Setup a Graph for the data:
a. From the Display Data menu in the lower left Drag the Graph icon to Voltage
(Ch A) entry in the Data window in the upper left corner.
b. Move the graph to a free part of the screen.
9. Now click on the Sampling option in the upper right section above the Data window.
Click on the time axis on the graph and change it to the other channel (Ch B).
10. Record data for accelerating voltages of 0 to 60 V, using START and STOP and the
manual adjustment of the power supply voltage VB.
11. Save the data by exporting it to a .txt file. You will be able to use this in Excel.
Analysis and Report:
0. You will need to convert your x-axis values to true power supply voltages multiplying by
20, which is the reduction factor of the isolation probe. The range of values for the
voltage should be 0-60 V.
1. Measure the voltage VB_true corresponding to the troughs in the current. Calculate the
energy differences E corresponding to the voltage differences V. Calculate the
average E between the ground state and the state(s) excited by the energetic
electrons. The accelerating potential at the first trough may be anomalous because of
contact potentials in the circuit.
Excited state
E2
 E=4.9eV
El
Ground state
Figure 1. The first excitation energy.
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2. Why there are several troughs? Explain.
3. Mercury arcs show strong photon emission at 578, 546, 436, 405, 365, 313, 302, 297, 280,
265, and 254 nm. Do any of these lines arise from transitions between the same two
states as in the F-H experiment?
4. In your lab report, calculate the fraction of its initial kinetic energy which an electron can
lose in a head-on, classical elastic collision with a stationary Hg atom. (Hint: the
energy lost is approximately 1 x 10-5Kei, where Kei is the initial kinetic energy of the
electron.) Does this explain why we can ignore elastic collisions?
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