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Name Date 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? _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ 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? _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ 5. How should the cathode ray respond if the electric and magnetic forces are balanced? © 2014 Connections Education LLC. All rights reserved. 1 _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ 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. _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ 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 © 2014 Connections Education LLC. All rights reserved. 2 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? ________________________________________________________ ________________________________________________________ b. What is the calculated value for the charge to mass ratio using Student B’s data? ________________________________________________________ ________________________________________________________ 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? ________________________________________________________ ________________________________________________________ b. What is the percent error for Student B? ________________________________________________________ ________________________________________________________ c. Which student’s value was more accurate? ________________________________________________________ ________________________________________________________ © 2014 Connections Education LLC. All rights reserved. 3