Hints—Apparatus & Variables
If you have not done so already, see “What’s This?” under the Help menu. You may also find it useful to have the Thomson applet open on the screen as you read the following descriptions (if necessary, click on the link thomsonExp.swf). Try manipulating some of the variables while reading the text below and explore the Options menu.
Cathode Ray
A bluish coloured line that is observed originating at the cathode, moving toward the anode and then through an opening in the anode in a straight line to the end of the evacuated tube. The end of the tube is often coated with a fluorescent material that glows when struck by the cathode rays.
Cathode and Anode
These electrodes are connected to their own battery or power supply that is kept at a constant voltage in this simulation and also in Thomson e/m experiment. The potential difference between the cathode and anode produces the cathode rays and accelerates them to a final velocity. Assuming vi = 0,
The final speed, vf, is the speed determined by Thomson in the first part of his experiment, although he calculated it using balanced electric and magnetic forces.
Horizontal Plates & Deflecting Voltage
The cathode rays pass between two horizontal plates that are connected to a variable power supply. In the simulation, you control the potential difference across the horizontal plates using the Voltage slider at the bottom of the screen. You can drag the slider to the left or to the right, changing the voltage and the polarity of the plates. If you want to use a specific voltage, click on the box (underneath “Voltage (V)” label) beside the slider. A dialog box will pop up allowing you to type in a specific positive or negative voltage. Notice that as you change the voltage, the cathode ray beam deflects up or down.
Applying a potential difference to these plates sets up an electric field (vertical in the diagram).
, where d is the separation between the plates
This electric field produces an electric force perpendicular to the horizontal motion of the cathode rays. The negatively charged particles of the cathode ray are deflected vertically but their horizontal motion is not affected.
If you turn on Field Directions and Field Values under the Options menu, you can see both the direction and value of the electric field as you change the voltage across the plates.
Magnetic Coils and Current
Thomson created a magnetic field in the region between the horizontal plates by installing a pair of coils directly in front of and behind the horizontal plates. An electric current flowing through the coils creates an electromagnet. The strength of the magnetic field is directly proportional to the current flowing in the coils.
To adjust the strength of the magnetic field of the coils, move the Current slider at the bottom of the screen. When a current flows, you should notice a dotted circle around the horizontal plates. This is an outline of the end view of the coils. Just like with the Voltage slider, you can simulate currents in one direction or the opposite direction, and you can set a specific current value by clicking on the box below “Current (mA)” label.
If you turn on the Field Directions and Field Values in the Options menu, you can see how both the strength and the direction of the magnetic field change as you change the current in the coils. Notice that the magnetic field direction is either into () or out of () the plane of the screen. Use your left-hand rule for charged particles traveling in a magnetic field to verify that the direction of the magnetic force is either vertically up or down in the region between the horizontal plates. Therefore, just like the electric force, the magnetic force acts perpendicular the motion of the cathode ray, deflecting it vertically.
Key Design Feature of the Thomson Experiment
Thomson designed his apparatus so that he could create an electric field, magnetic field or both in the same region of space through which the cathode rays travel. Both fields created vertical forces perpendicular to the horizontal motion of the cathode rays. This means that the net force, acting vertically, can be any combination of electric and magnetic forces.
In the special case , non-zero electric and magnetic forces are balanced and no deflection occurs.
Try manipulating the voltage (changing the electric force) and the current (changing the magnetic force) to produce various deflections including an undeflected beam with non-zero fields.