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Homework Help: Why do the E-field and B-field have different units?

  1. Feb 10, 2010 #1
    Note this is more of a coursework theory question then a specific homework question.

    I am learning about E and B fields and electromagnetic waves. A common EM figure I find is the propagation of a photon where the E and B field propagate through space as seen in the following link.
    The confusing thing is that the E field has different units than the B field where E = cB. Is there a physical meaning to this or should I just accept the equation as it is? I am guessing it falls out of Maxwell's equations, but it feels odd that an E and B field have different units when things like the units of force and pressure don't change.

    1. The problem statement, all variables and given/known data
    Why do the E-field and B-field have different units?

    2. Relevant equations
    E = cB
    F = q(E + v x B)

    3. The attempt at a solution
  2. jcsd
  3. Feb 10, 2010 #2
    The Magnetic Force on a moving particle is proportional to its velocity. If you wish to use [tex]q \vec v \times \vec B[/tex] to describe the force, then obviously you can't have the same units for E and B.
    There are, however, different systems of measurement where E and B -do- have the same units. One such system is the CGS system. In that system, the Lorentz Force is: [tex]q(\vec E+\frac{\vec v}{c} \times \vec B)[/tex] and there E and B do have the same units.
  4. Feb 10, 2010 #3
    Thanks for your response. Can you explain how the CGS works to an introductory physics student? How does E remain the same, but B changes by a factor of distance/time in a different unit system? When I think of feet and meters, they scale by a dimensionless constant. Similarly pressure can be expressed in Pa, torr, and bar and the only difference is a dimensionless constant scaling factor. Is there some unit system where length and pressure has the same units? How does CGS work differently?

    Last edited: Feb 10, 2010
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