Download Thomson`s Cathode Ray Tube Experiment

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Thomson’s Cathode Ray Tube Experiment
HONORS CHEMISTRY A
Introduction: When a beam of electrons, also known as a cathode ray, is passed
between two charged plates, it will deflect, or bend. By observing the deflection of a
cathode ray in a charged environment, the charge to mass ratio (q/me) for the
electrons can be determined.
Procedure
1. The simulation will open with a model of a cathode ray tube used by J.J.
Thomson in his experiment.
2. Sketch a picture of the cathode ray tube. Using Figure 4–5 on page 106 in
Chemistry, label the following parts of your sketch:
a. cathode
b. positive and negative plates
3. On the simulation screen there are two sliding bars. The one labeled E
controls the electric field. The one labeled B controls the magnetic field. Both
fields are set at zero. Click Play and observe the cathode ray as it passes
through the charged plates. What do you observe?
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4. Set the magnetic field to 0.2. You may use the slider or manually enter the
value into the box. Click Play and observe the cathode ray. What do you
observe? Set the magnetic field back to zero and set the electric field to 38.
Click play and observe the cathode ray. What do you observe?
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5. How should the cathode ray respond if the electric and magnetic forces are
balanced?
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6. Set the electric field to any value that causes the cathode ray to deflect away
from the center. Record the value you chose in Table 1.
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7. Increase the magnetic field strength until the behavior of the cathode ray
indicates that the magnetic force is balanced with the electric force. What
magnetic field creates a magnetic force that balances the electric force?
Record the value in Table 1.
Table 1
electric field (E)
magnetic field (B)
Two students conducted Thomson’s cathode ray tube experiment in a
laboratory. Their data are shown in Table 2.
Table 2
electric field (V)
magnetic field (B)
deflected distance (d)
Student A
13
44
4
Student B
10
49
5
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8. Using the student data, calculate the charge to mass ratio (q/me) by using
the following equation:
q ( 5.0826 ×10
=
me
B2
12
) ×V × d
where V is the electric field in volts, d the deflected distance from center in
centimeters, and B is the strength of the magnetic field in microtesla.
a. What is the calculated value for the charge to mass ratio using Student
A’s data?
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b. What is the calculated value for the charge to mass ratio using Student
B’s data?
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9. The accepted value for the charge to mass ratio of an electron is 1.76×1011.
Calculate each student’s percent error as follows:
=
% Error
student's value − accepted value
⋅100%
accepted value
a. What is the percent error for Student A?
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b. What is the percent error for Student B?
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c. Which student’s value was more accurate?
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