1. Module 4

1.9. Lesson 2

Lesson 2

Module 4—Magnetic and Electric Fields in Nature and Technology

 

Lesson 2—Moving Charges in Magnetic Fields

 

Get Focused

 

A photograph shows a cathode ray inside a cathode ray tube.

The Cathode Ray Tube Site

http://members.chello.nl/h.dijkstra19

The cathode ray tube (CRT) consists of a glass tube from which air has been removed, with an electrode inserted at each end. When a very large voltage is applied to the electrodes a cathode ray is produced. In this photo the cathode ray is contacting a fluorescent material that coats the metal sheet inside the evacuated tube. With this kind of technology and the aid of a magnetic field, J.J. Thompson was able to discover the electron in 1897.

 

Today, the most commonly used cathode ray tube is an old-fashioned television set. Glass-tube televisions are no longer produced in vast quantities as newer plasma, LCD, and other display technologies have replaced most of the market share with compact, higher-resolution systems. The cathode ray tube, however, remains a very important stepping-stone on the way to developing theories about electromagnetism.


 

A set of three photographs illustrates cathode rays in different positions.

The Cathode Ray Tube Site

http://members.chello.nl/h.dijkstra19


It was initially explored using magnetic fields, which deflected the ray in various directions, depending on the orientation of the beam and the magnet. In the photograph on the left there is no magnetic field. In the middle photograph the beam is deflected by a magnetic field perpendicular to the beam. The photograph on the right is the beam when the magnetic field has been reversed. The deflection of the beam when it is in a magnetic field indirectly indicates that the beam is made up of negative charges. How could this conclusion be reached?

 

Recall from Module 3: Lesson 4 that at sufficient voltages, the air separating two electrodes will ionize, temporarily allowing the passage of charge (electrons) in the form of a spark. This is what you would experience if you rubbed sock feet on a carpet and then touched a doorknob. In a cathode ray tube, most of the air has been removed and the residual gas becomes ionized by the electron beam. The spark mentioned in Module 3: Lesson 4 becomes a “current” of moving charges for as long as the voltage is maintained. The glow results from the interaction of the moving charge with fluorescent coatings in the tube.

 

Recall from Lesson 1 in this module that a moving charge will produce a circular magnetic field that surrounds the charge and is perpendicular to its motion. Therefore, if the cathode ray is indeed a moving charge, a circular magnetic field will surround it. Now imagine what happens when a second, external, magnetic field is present. Like two magnets repelling one another, the two fields interact to produce a magnetic force on the moving charge. This force repels the charges and creates a deflection of their movement. (See “Figure 12.16” on page 594 of the textbook.) In addition, the direction of this force is indicative of the type of charge. The cathode ray, it turns out, is a beam of moving electrons, negatively charged according to the deflection caused when it encounters an external magnetic field. You can read more about this in “The Motor Effect” on pages 593−595 in your textbook.

 

In this lesson you will answer the following essential questions:

  • How is moving charge affected by a magnetic field?

  • What variables determine the magnitude of the magnetic force?

  • How is the direction of the magnetic force determined?

Module 4: Lesson 2 Assignment

 

Your teacher-marked Module 4: Lesson 2 Assignment requires you to submit a response to the following questions:

  • Assignment—A 1, A 2, A 3, A 4, A 5, and A 6
  • Discuss—D 3

The other questions in this lesson are not marked by the teacher; however, you should still answer these questions. The Self-Check and Try This questions are placed in this lesson to help you review important information and build key concepts that may be applied in future lessons.

 

After a discussion with your teacher, you must decide what to do with the questions that are not part of your assignment. You should record the answers to all the questions in this lesson and place those answers in your course folder.