Teaching Physics with the Physics Suite

Edward F. Redish

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Comparing electric and magnetic forces: 1

1.(From an exam) In this problem, we will consider situations corresponding to three different long thin lines of matter containing charges:

  1. a copper wire carrying an electric current,
  2. a long amber rod that has been rubbed with fur and has a uniform excess of negative charge, and
  3. a beam of electrons passing through a vacuum inside a cathode ray tube. They are represented schematically in the figure below. The direction of the electric current and of the electron flow are from left to right. A location marked x is indicated on the diagram and a set of directions with labels are shown at the right.

    For each of the three cases, indicate in what direction the electric and magnetic fields at the location x would point. If any of the fields are zero, write 0.

    CaseDirection of Electric Field at xDirection of Magnetic Field at x
    current carrying wire  
    rubbed amber rod  
    electron beam   

    Now consider placing a positive charge at the location x. In one case it is stationary, while in a second case it is moving in the direction C. Indicate the direction you believe nearest to the total force the charge would feel. (Ignore gravity and air resistance.)

    CaseDirection of force
    on stationary + charge at x
    Direction of force
    on + charge moving in C direction at x
    current carrying wire  
    rubbed amber rod  
    electron beam   

    Note to the instructor: This presents students a nice challenge. Many are confused about what a current is and think it carries charge. This problem clearly reveals that difficulty. Note that the last entry in the table -- the moving charge near the electron beam -- the answer is ambiguous: it depends on the charge density and the energy (speed) of the charges, since the electric and magnetic forces point in opposite directions. This is OK for a homework, but perhaps a little much to expect students to be able to handle under the pressure of an exam. (I give it, but accept A, E, or 0 as an answer.)

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    Page last modified October 19, 2002: MG03