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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?

106 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.

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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.

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 107

Virtual Physics Lab Workbook, by Brian F. Woodfield, Steven Haderlie, Heather J. McKnight, and Bradley D. Moser. Published by Prentice Hall.

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.

108 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.

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ISBN 0-558-83901-0

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ISBN 0-558-83901-0

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ISBN 0-558-83901-0