Cathode Ray Tubes

A series of cathode ray tubes graphically demonstrate various properties of the electron. Cathode Ray Tube #12 provides evidence that cathode rays are not visible to human eyes. Cathode Ray Tube #14 - cathode rays cast a shadow behind a Maltese Cross. Cathode Ray Tube #16 - light shined on a pinwheel inside a CRT does not cause the pinwheel to move.  However, when cathode rays strike the pinwheel inside the CRT, the pinwheel begins to spin and move.  Cathode Ray Tube # 18 - in a darkened room, the electron beam (cathode rays) shows up on a phosphor screen as a bright blue line.  The electron beam is deflected by an electric field and a magnetic field.  Allow about 10 minutes for this demo.  A class activity sheet accompanies this demonstration.

Curriculum Notes 

Electrons travel in straight lines from the cathode towards the anode (Maltese Cross CRT).  Cathode Rays are particles and have mass because cathode rays cause a metal pinwheel inside a CRT to turn.  Cathode rays have a negative charge because J.J. Thompson's experiment showed the cathode rays being pulled toward the positive capacitor plate and away from the negative charge plate and a magnetic field deflects the cathode rays in a direction consistent with the rays being negatively charged particles.

Learning Objective

Students should be able to cite the contributions of the experiments performed by Thomson, Milikan, and Rutherford to scientists understanding of the structure of the atom.

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.

Deflection of Cathode Rays by an Electric Field - The application of high voltage to capacitor plates creates an electric field.  When a cathode ray is passed through this electric field, the negatively charged electrons are deflected toward the positive charged plate and away from the negatively charged plate.  Like charges repel, unlike charges attract.

Deflection of Cathode Rays by a Magnetic Field- 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 

Deflection of Cathode Rays (electron beam) by an Electric Field 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 capacitor 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.

Deflection of Cathode Rays by a Magnetic Field 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. 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 is close to the beam. Turning the magnet around so that the other pole is closest to the magnetic field should cause deflection of the cathode rays in the opposite direction. Turn down both power supplies all the way. 

Movement of a Pinwheel in a CRT by Interaction with Cathode Rays - cathode rays striking a pinwheel inside a CRT move the pinwheel. 

Cathode Rays striking a Maltese Cross inside a CRT, produces a shadow.  Provides evidence that the cathode rays originates at the cathode and travels toward the anode.

 

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