1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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.
    http://www.astronomynotes.com/light/emanim.gif
    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
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Why do the E-field and B-field have different units?
  1. Why is the e field 0? (Replies: 5)

Loading...