Cathode Ray Tube

A cathode ray tube graphically demonstrates the electrostatic properties of the electron. In a darkened room, the electron beam shows up on the phosphor screen as a blue line that can be deflected by an electrostatic or magnetic field. 

Curriculum Notes 

Allow about 10 minutes for this demo.

Lead Time 
One day of lead time is required for this project.
Discussion 

The electrons are emitted by a heated filament and are accelerated by a positively charged screen in the electron gun. When the electrons strike the phosphorescent screen they excite the electrons of the phosphorescent material into higher energy states. As the electrons move back down into ground state they emit visible light.

Electrostatic DeflectionThe application of high voltage to the deflection plates on either side of the ray creates an electric field and the negatively charged electrons are deflected toward the positively charged deflection plate.

Magnetic Deflection - The magnet creates a magnetic field perpendicular to the electron beam and parallel to the plane of the tabletop. The magnetic field deflects the electrons according to the right hand rule.

Materials 
  • cathode ray tube
  • high voltage DC/ 120 VAC power supply for electron gun and heater coil
  • high voltage DC power supply for deflection plates
  • power strips
  • jumper cables with banana plugs to connect everything
  • a fairly powerful magnet
Procedure 

Electrostatic Deflection Turn on the power strip. Turn on the power to both high voltage sources. Dim the houselights. Turn up the power on the high voltage source connected to the electron gun. The trace of a cathode ray should appear on the phosphorescent screen. Turn up the power on the high voltage source connected to the deflection plates. The cathode ray should be deflected upward. Turn down both power supplies all the way. If desired, switch the leads going to the deflection plates and repeat the process. The cathode ray should be deflected downward.

Magnetic DeflectionTurn on the power strip. Turn on the power to both high voltage sources. Dim the houselights. Turn up the power on the high voltage source connected to the electron gun. The trace of a cathode ray should appear on the phosphorescent screen. Bring one end of the magnet close to the front of the tube perpendicular to the axis of the beam. The cathode ray should be deflected either upward or downward depending on which pole of the magnet was closest o the beam. Turning the magnet around so that the other pole is closest to the apparatus should cause deflection in the opposite direction.Turn down both power supplies all the way.

 
Safety Precautions 

Be careful not to touch any of the high voltage leads while the power is on. The glass tube is fragile and evacuated. Wear eye protection in case of implosion.

Prep. Notes 

Cathode Ray Tube

  • All materials are obtained from the physics demo prep room.
  • Connect the high voltage output from the high voltage DC/ 120 VAC power supply to the electron gun of the cathode ray tube (CRT). The negative lead goes in the jack in the center of the back of the electron gun part of the CRT. The positive lead goes in the jack on the side of the electron gun part of the CRT.
  • Connect the leads from the 120 VAC output of the high voltage DC/ 120 VAC power supply to the CRT. One lead piggybacks onto the back of the high voltage negative lead and the other goes into the other jack right beside it, off-center on the back of the electron gun part of the CRT.
  • Connect the leads from the other high voltage DC power source to the deflector plates at the top and bottom of the CRT. The positive lead should be connected to the top and the negative lead should be connected to the bottom.
  • Plug both of the power supplies into the power strip and plug the power strip into an outlet.

© Copyright 2012 Email: Randy Sullivan, University of Oregon Chemistry Department and UO Libraries Interactive Media Group