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Lab 33: The Effect of a Magnetic Field on
Moving Charges
Problem
To investigate the effect of a magnetic field on moving charges
Background
Charged particles at rest are not affected by static magnetic fields. However,
when such charged particles are in motion, they are deflected by magnetic
fields. The discovery that flowing electrons are affected by magnets was a
pivotal discovery at the turn of the 20th century. Today, many common
technological applications, from electric motors to television screens, make use
of this interaction.
In cathode ray television tubes, for example, magnets are used to move a
stream of electrons. As the electrons hit the screen, the screen glows
momentarily where it was hit. Similarly, Earth’s magnetic field deflects charged
particles from the sun. In this lab, you will study the effect of a magnetic field
on different types of charged particles.
Skills Focus
Predicting, observing, developing hypotheses, drawing conclusions, applying
concepts
Procedure
1. Start Virtual Physics and select Effect of a Magnetic Field on Moving Charges
from the list of assignments. The lab will open in the Quantum laboratory.
2. The experiment will be set up on the table. An electron gun on the left side
of the table serves as the source of electrons. What type of charge do
electrons have?
3. There is a phosphor screen on the right side of the table to detect charged
particles. Turn on the phosphor screen by clicking on the green/red button.
What do you observe? What do you think that it shows?
The Effect of a Magnetic Field on Moving Charges
Virtual Physics Lab Workbook, by Brian F. Woodfield, Steven Haderlie, Heather J. McKnight, and Bradley D. Moser. Published by Prentice Hall.
Copyright © 2008 by Pearson Education, Inc.
ISBN 0-558-83901-0
106
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Date
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4. Drag the lab window down and to the left and the phosphor screen window
up and to the right in order to minimize the overlap between the two. Then
push the Grid button on the phosphor screen. Set the Magnetic Field to 30 μT
(microtesla) on the magnetic meter near the phosphor screen by clicking the
button above the tens place three times. (If you mistakenly click between the
digits, it will move the decimal point. To move the decimal point back again,
click where it was originally.) What happens to the spot from the electron
gun on the phosphor screen?
5. Predicting What do you think would happen to the spot if you increased
the voltage of the electrons leaving the electron gun? Why?
6. Observing Increase the voltage of the source by clicking above the
hundreds place on the electron gun voltage controller (the second meter
from the left). This does not change the number of electrons leaving the gun.
Rather, it just gives each electron more electrical potential energy. What
happens to the spot on the phosphor screen when the voltage is increased?
Why does this happen?
7. Predicting What do you think would happen to the spot now if you
increased the strength of the magnetic field that the electron beam is passing
through? Why?
8. Test your prediction, then zero out the Magnetic Field meter by clicking on
the appropriate digit buttons until the spot on the phosphor screen is
centered once again.
ISBN 0-558-83901-0
9. Double-click or click and drag the electron gun to move it to the Stockroom
counter. Enter the Stockroom by clicking inside it. Double-click the electron
gun to move it back to the shelf. Double-click on the alpha source to select it
and move it to the Stockroom counter. Click on the green Return to Lab arrow
to return to the lab. Drag the alpha source from the Stockroom counter and
place it on the table where the electron gun was originally placed (the middle
spotlight). Click on the front of the alpha source to open the shutter. What
appears on the phosphor screen? What charge do alpha particles have?
The Effect of a Magnetic Field on Moving Charges
Virtual Physics Lab Workbook, by Brian F. Woodfield, Steven Haderlie, Heather J. McKnight, and Bradley D. Moser. Published by Prentice Hall.
Copyright © 2008 by Pearson Education, Inc.
107
Name
Date
Class
10. Change the unit for the Magnetic Field from μT to mT (millitesla) by clicking
once above the unit. Click above the hundreds place three times to set the
Magnetic Field to 300 mT. This magnetic field is 10,000 times stronger than
the one you used for the electron gun. Which direction did the spot move
when you increased the magnetic field?
How does this direction of movement compare with the direction of
movement for the electron beam in the magnetic field?
11. Developing Hypotheses A charged particle at rest wouldn’t be affected
by a magnetic field at all, so why are moving charged particles affected?
12. Drawing Conclusions Why do you think it takes a significantly stronger
magnetic field strength to move the beam of alpha particles compared with
the beam of electrons?
13. Applying Concepts What technologies or applications of the connection
between electricity and magnetism can you think of? Describe the physical
processes.
The Effect of a Magnetic Field on Moving Charges
Virtual Physics Lab Workbook, by Brian F. Woodfield, Steven Haderlie, Heather J. McKnight, and Bradley D. Moser. Published by Prentice Hall.
Copyright © 2008 by Pearson Education, Inc.
ISBN 0-558-83901-0
108