Download Group Exercise #28

Physics 144 – Chowdary
How Things Work
Spring 2006
Group Exercise #28 – Cathode Ray Tube and Electron Beams
Goals:
 Learn how a Cathode Ray Tube (CRT) works.
 See how an electron beam deflects due to a magnetic field.
 Learn how to use the right hand rule for magnetic forces.
Instructions:
 Work in groups of 2 or 3 (there are only eight CRT setups around the room).
 Follow along with the class discussion on how the CRT works.
 Complete the rest of the exercises with your group.
(1) What is a Cathode Ray Tube? How does it work? Why is it useful?
We will discuss this as a class. The cathode ray tube (CRT) is the heart of the television and computer monitor.
The CRT causes a beam of electrons to be accelerated through an electric field, and the electrons are aimed by
electric fields. The electrons strike a fluorescent screen, and the energy of the electrons cause the screen to glow,
forming a pattern. The electron beam can move, and so the picture on the screen moves. A simplified sketch of a
CRT is shown in Figure 1.
The CRT is made of glass and is under a good vacuum in order to give the electron beam a clear path
where it won’t collide with gas molecules. The filament F is heated by running current through it, just like in a
light bulb. When the filament, also called the cathode, reaches a certain temperature, it emits electrons (historically,
the electron beam is called a cathode ray) by thermionic emission; the electrons are literally “boiled” off of the
cathode. Since the electrons are negatively charged, the back plate of A, which is also negatively charged, repels
the electrons, and the front plate of A, which is positively charged, attracts the electrons through the electric field.
The resulting electric force causes the electrons to accelerate from the back of the cathode ray towards the front (in
our drawing from left to right). A tight beam of energetic electrons passes through the hole in the front plate of A,
and eventually hit the screen. The inside of the screen is coated with phosphor (zinc sulfide) that converts some of
the kinetic energy of the electrons to visible light. The position of the beam on the screen is controlled by electric
fields applied to the vertical V plates and the horizontal H plates. (In this exercise, we won’t have any electric
fields on the V and H plates).
A
V
H
screen
electron beam
F
Figure 1: Simplified sketch of a Cathode Ray Tube
(2) Turn on the power at the power supply by flipping the switch. After a few moments, you should notice a green
spot appear on the screen. It likely won’t be at the center of the screen. On the CRT, you’ll notice a knob. Twist
that knob in both directions. What does that do? Adjust until the green spot is as small and tightly focused as you
can make it.
(3) Look inside the CRT itself. Do you see the little glowing filament? Turn off the power. What happens to the
glowing filament? What happens to the green dot on the screen?
(4) Make sure the power is off. Reverse the leads leading to the filament on the power supply (the green ports).
Turn the power back on. Is anything different? Does the filament still glow? Is there still a green dot on the
screen?
(5) Turn the power back off. Now, reverse the leads for the anode and the cathode. Turn the power back on. Is
anything different? Does the filament still glow? Is there still a green dot on the screen? Briefly explain your
results.
(6) Turn the power back off, and return the anode and cathode leads to their original configuration. Turn the
power back on. Now, bring the N pole end of your cylindrical bar magnet near the CRT, as directed by your
instructor. Which way does the green dot deflect? Repeat for different placement of the magnet, as well as
different poles. Which way does the green dot deflect? Repeat using the current carrying coil, again as directed by
your instructor.
(7) Discuss the right hand rule for magnetic force as directed by your instructor. Make sure to note that electrons
have negative charge, and we must keep track of that.