Activity Description

Activity 1 - WBT Charge in a magnetic field

This is the first in a sequence of three activities using the same simulation to illustrate Thomson's discovery of the electron. Because of improved vacuum technology Thomson was able to show that cathode rays could be defelcted by an electric field, something that Crooke's ws unable to do. Thomson's determintion of e/m was actually made by finding equilibrium in the Loretnz force to detemine the speed of the particle and then using the B-field deflection to dtermine the e/m ratio. The animation shows the path of a charged particle in a region of uniform magnetic field. The students have to determine the sign of the charged particle using the right hand rule.

Activity 2 - PET Charged Particle in an Electic Field

In the second part of the activity, the same charge particle travels in a region with a uniform electric field created by positively charging the upper plate and negative charging the lower plate. The charge on the plates is shown to the students and they then have to predict the direction that the charge will be deflected.

Activity 3 - PET Charged Particle in Crossed Electric and Magnetic Fields

In the third part of the activity, both the electric and magnetic fields are turned on. The students are presented with evidence from the first two activites to conclude that the force exerted by the fields are equal in magnitude and opposite in direction and consequently equilibrium will result. The students have to predict the deflection of the particle in the crossed fields.

Target Conceptual Difficulty

Inability to understand the discovery of the electron

PowerPoint

Thomson's Discovery of Electron

Simulation

Working Backwards Task - Discovery of Electron

Presentation Notes

Initially no fields are turned on. When you click on the button "set check boxes and play", the red charged particle will pass btween the two plates undeflected. Click on the Restart button to reset the simulation. Then click on the "magnetic field on" check box. Once you click on the "set check boxes and play" x's will appear showing the direction of the B field. The particle will be defelcted upwards indicating that it is a positively charged particle. You also want to point out to students the amount of vetical deflection that occured.

For the second part of the activity, the students should be shown what the arrangement of charges should be so prior to running the simulation. They should then be asked to predict the defelction of the charged particle as it passes between the plates on their own, followed by a group discussion and a second prediction. The question is resolved by showing the simulation. Currently the PowerPoint doesn't have a slide asking for reasons. That defect will be repaired in a future revision of the activity. You again want to point out to students the amount of vetical deflection that occured and the change in verical component of the velocity.

For the third part of the activity, the students should be shown the arrangement of charges on the plates as well as the magnetic field arrangement prior to running the simulation. They should then be asked to predict the defelction of the charged particle as it passes between the plates on their own, followed by a group discussion and a second prediction. The question is resolved by showing the simulation. Currently the PowerPoint doesn't have a slide asking for reasons. That defect will be repaired in a future revision of the activity.

Student Results

The first part of this task went well. Students understood that it was necessary to use the right hand rule to resolve the task. Initially, on their own 60% of the students correctly determined the sign of the charge.

1.) What is the sign of the charge?
Responses
(percent) (count)
+ 60% 12
- 40% 8
Insufficient Information 0% 0
Totals 100% 20

Following Small group discussion, the number of correct responses increased to 90%.

2.) What is the sign of the charge?
Responses
(percent) (count)
+ 90% 18
- 10% 2
Insufficient Information 0% 0
Totals 100% 20

The second activity also went very well. 80% initially predicted the correct direction the charge would be defelcted in the electric field. After samll group discussion, 95% of the students made the correct prediction.

3.) Predict the direction the charge will be deflected by the electric field
Responses
(percent) (count)
Up 20% 4
Down 80% 16
Totals 100% 20

4.) Predict the direction the charge will be deflected by the electric field
Responses
(percent) (count)
Up 5% 1
Down 95% 19
Totals 100% 20

The difficulties in this activity ocurred in the third step. Students initially haddifficulty in understanding that the upwards force exerted by the magnetic field and the downwards force exerted by the electric field were equal in strength. Even when they understood that theey had difficulty in determining the resulting motion.

5.) Predict the path of the charge when both fields are
Responses
(percent)(count)
Will wiggle back and forth 20% 4
Will deflect first one direction then the other 30% 6
Will go straight 40% 8
Some other answer 10% 2
Totals 100% 20

6.) Predict the path of the charge when both fields are
Responses
(percent)(count)
Will wiggle back and forth 25% 5
Will deflect first one direction then the other 40% 8
Will go straight 25% 5
Some other answer 10% 2
Totals 100% 20

Pen and Paper Actvivities

Pen and Paper version of this activty

Follow up exercises (non-tiper) from homework assignments, test reviews and test items