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Unformatted text preview: blem. PREDICTION
If you bring the north end of a magnet
near the side of the CRT, which arrow
represents the deflection of the electron
beam on the screen? ? N ? screen ? ? ?
? Does the size of the deflection increase or decrease as the magnet gets closer to the CRT?
As you increase the size of the magnetic field? Does the size of the deflection depend
on the speed of the electrons? Explain your reasoning. 157 MAGNETS AND MOVING CHARGE – 1302Lab5Prob7 EXPLORATION
WARNING: You will be working with equipment that generates large electric
voltages. Improper use can cause painful burns. To avoid danger, the power should
be turned OFF and you should WAIT at least one minute before any wires are
disconnected from or connected to the power supply. Never grasp a wire by its
Connect the CRT according to the directions in the Equipment appendix and review the
problem Deflection of an Electron Beam by an Electric Field in your lab journal. If
known, select the accelerating voltage that gave the largest deflection for the smallest
electric field. Record the location of the non-deflected beam spot using the selected
NOTE: In this experiment we are interested in understanding the effects of ONLY a magnetic
field and NOT an electric field. Do not use the deflection plates.
Determine which pole on your bar magnet is the north magnetic pole. Make a
qualitative field map of your magnet to make sure it is a simple dipole. If it is not, you
should re-magnetize it following the instructions in the appendix. Describe the
magnetic field at the end of the magnet.
Place the magnet near the side of the CRT. Did the deflection match your prediction?
Why or why not? Repeat this procedure for the south pole. Should there be any
difference? In which direction did the beam spot deflect?
Put the bar magnet perpendicular to the screen of
the CRT, do you see a deflection? Try this with both
poles of the magnet. Record your results. Were
they what you had expected?
Can you orient the bar magnet so that it attracts or repels the electron beam? Place the
north pole of your magnet a fixed distance away from the side of the CRT near the
screen. Record the deflection. Increase the speed of the electrons by increasing the
accelerating voltage if possible. Calculate the increase in speed. How does the
deflection change? Try this with both poles of the magnet. Record your results. Were
your results what you had anticipated?
Place the north pole of your magnet a fixed distance away from the side of the CRT near
the screen. Record the deflection. Increase the magnetic field by adding a second
magnet. How does the deflection change? Try this with both poles of the magnet.
Record your results. Were your results what you had anticipated? 158 MAGNETS AND MOVING CHARGE – 1302Lab5Prob7 What effect does the Earth’s magnetic field have on the electron beam of a CRT? What
is the direction of the Earth’s magnetic field in your laboratory room? Arrange the CRT
to see the maximum effect. Arrange it to observe the minimum effect. By measuring
the electron deflection, what would you say is the relative strength of the magnet and
the Earth’s magnetic field in the lab? Remember to take account of the distance that the
electron travels through each magnetic field. What is the effect of the Earth’s magnetic
field on the CRT beam relative to the Earth’s gravitational field?
Devise your own exploration of the effect of a magnetic field on electrons using the CRT
and the bar magnets. What variables can you control with the magnets and the CRT?
Record your questions that will guide your exploration. ANALYSIS
Draw a picture relating the three vectors representing the velocity of the electron, the
magnetic field, and the force on the electron that is consistent with your results. CONCLUSION
Did the electron beam deflection, in the presence of a magnetic field, agree with your
prediction? Why or why not? What was the most interesting thing you learned from
this exploration? 159 MAGNETS AND MOVING CHARGE – 1302Lab5Prob7 160 PROBLEM #8: MAGNETIC FORCE ON A MOVING CHARGE
You are working with a team to design a better electron microscope. To precisely
control the beam of electrons, your research team decides to try a magnetic field. For
your study of electron control you decide to use a Cathode Ray Tube (CRT) with a
magnetic field perpendicular to its axis. From your work with Helmholtz coils in the
earlier problem, Measuring the Magnetic Field in Two Parallel Coils, you know that
the magnetic field between these parallel coils is fairly uniform, so you decide to use
them for your test. Before you can evaluate the sensitivity of the electron microscope
design, you need to determine how the magnitude of a constant magnetic field affects
the position of the beam spot.
Instructions: Before lab, read the laboratory in its entirety as well as the required reading in the
textbook. In your lab notebook, respond to the warm up questions and d...
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- Spring '14