Working Backward Task on Thomson's Method for Discovery of the Electron



magnetic field on            electric field on

Please wait for the animation to completely load.


Thomson's method for demonstrating cathode rays were small negatively charged partices was to send the rays through a region of both magnetic and electric field, such that each field deflected the rays in the opposite direction. If the fields are adjusted correctly the deflections will cancel each other out. From this Thomson could determine the speed of the cathode rays and this allowed him to determine the chare to mass ratio for the cathode rays.


1. Play the animation and observe the path of the charged particle.

2. Click on Restart.

3. Turn on the magnetic field by checking the magnetic field on box. The x's mean that the N pole of the magnetic field is directed into the page.

4. Click on set check box values and play. Observe the deflection of the charge.

5. What is the sign of the charge?

6. Click on Restart.

7. Turn on the electric field by checking the elctric field on box. The upper plate will be positively charged and the lower will be negatively charged. Predict the direction of deflection of the charged particle.

8. Check your prediction by clicking on set check box values and play. Were you correct? Resolve any discrepencies.

9. Compare the amount of deflection by the electric field to the amount of deflection by the magnetic field. Predict the path of the charge if both fields are turned on.

10. Test your prediction by turning on both fields and licking on set check box values and play. Was your prediction correct? Resolve any discrepencies.


Adapted from Physlet Physics problem 27.9
Problem authored by Melissa Dancy.

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Last Updated: August 21, 2009