Download Title - jdenuno

Title: Thomson Cathode Ray Experiment
Purpose:

to duplicate the Thomson cathode ray tube experiment

to calculate the charge to mass ratio (q/m e) of an electron

to determine the percent error of the measurement
Background: As scientists began to examine atoms, their first discovery was that they could
extract negatively charged particles from atoms. They called these electrons. In order to
understand the nature of these particles, they wanted to know how much they weighed,
and how much charge they carried. Thomson showed that if you could measure how
much a beam of electrons were bent in an electric and magnetic field, you could figure
out the ratio of mass to charge for the particles. This lab will simulate some of Thomson's
experiments.
Materials: Virtual ChemLab Environment
Procedure:
1. Start Virtual ChemLab.
2. Enter the General Chemistry Laboratory (Click Guest)
3. Open the Quantum Lab by clicking on the lab table to the far right of the lab. There is a
starburst ---- symbol on the lab table.
4. READ THE ENTIRE EXPERIMENT BEFORE YOU START
5. You will need to set up the optics table for the Thomson experiment:
a. Go to SOURCES on the top shelf in the Stockroom area to enter the Stockroom.
b. Click on the clipboard that is on the right wall, just above the help bell.
c. Now Click on Preset Experiment 1: Thomson Experiment.
This will set up the lab for you!
d. Click the green Return to Lab arrow.
You should see a large green screen with a light green dot in the center. This is
the Phosphor Screen and will detect movement/deflection of an electron beam.
(If you hit the off button, the screen will disappear and you will see the entire
setup: Electron Gun, Magnetic and Electric Fields, and the Phosphor Screen. To
turn it on again, you need to click the on/off button on the top of the screen. It is
small, but as you move your cursor over the top, you will the words On/Off will
appear in red, indicating where you should click.
e. Move the cursor over the different items so you can identify them!
6. Now that the setup is complete, turn on the Phosphor Screen, move it to a convenient
location on your computer desktop, and click the grid button.
7. The electron gun’s energy should be set to 100 eV.
8. Now, set the Magnetic Field to 30 T, clicking the button over the 10s place 3 times.
What happens to the small green dot? _________________
(Make sure you change the magnetic field to 30 and NOT 300!)
9. Set the Magnetic Field back to 0 and set the Electric Field to 10 V.
10. What happens to the small green dot? _____________________
11. Play around a little with the electric and magnetic fields to see how and where the green
dot moves.
Note: Each box on the grid represents 1 cm.
Now you are ready to begin the experiment!
Judith Nuño
Chemistry
2006/2006
12. Reset both electric and magnetic fields to 0 and make sure the electron gun’s kinetic
energy reads 100 eV.
13. Use the electric field to deflect the beam to approximately 5 cm to the left of center by
increasing the electric voltage 1 V at a time.
14. Record the electric voltage in the Data Table below.
15. Now turn on the magnetic field, and increase it until the beam is back in the center of the
screen. (You may want to increase by 10s at first and then by 1s!)
16. Record the magnetic field strength in the Data Table below.
17. Record the deflection: This should be 5 cm, since you moved the dot 5 cm away with the
electric field and then 5.00 cm back to the center with the magnetic field.
18. Reset the electric and magnetic fields to 0 to center the dot.
19. Repeat, this time moving the green dot approximately 4.00 cm from the center.
20. Repeat with the dot at 3.00, 2.50, and 2.00 cm from the center.
21. Record the electric and magnetic field strength in the Data Table below.
22. Calculate the mass/charge ratio for each trial, using the following formula. Ask for help if
you need it!
q/me = (5.0826 x 1012) · V · d/B2
(V = electric field strength, d = deflected distance, and B = magnetic field strength)
23. Calculate the average mass/charge ratio for all 5 trials.
24. The modern accepted value for the q/m e ratio is 1.76 x 1011
Determine your percent error using the following formula:
% error = (average value – accepted value)/accepted value
Results:
Deflected Distance
(cm)
Electric Field Strength
(V)
Magnetic Field Strength
(T)
Charge/Mass Ratio
(q/me)
5.00
4.00
3.00
2.50
2.00
Average Charge/Mass Ratio
(q/me)
Discussion: Follow Lab Grading Guidelines
Conclusion: This should be your average value for the q/m e ratio ± the percent error
Reflection: Personal statement about what you learned from the simulation.
Save a copy of this lab writeup in your folder and in your Virtual Lab NB Folder on the Shared
Folder. (You may email this lab writeup to [email protected] and I will save it in your Virtual
Lab NB)
Print out a copy of this lab and tape it into your Lab NB!
Judith Nuño
Chemistry
2006/2006