Electromagnetic Wave Question

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2 total messages Started by Roger Briggs Fri, 13 May 1994 17:57

#1466

Author: Roger Briggs
Date: Fri, 13 May 1994 17:57

6 lines
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I am a high school Physics teacher, and have had this question for some time:
In an electromagnetic wave are the two fields in phase (both sine
function) or out of phase (one sine and one cosine)?  Every book I have
consulted shows E and B in phase, but this doesn't make sense if the
derivative of the electric field gives the magnetic field (Faraday's Law).
What's the deal?

Re: Electromagnetic Wave Question

#1475

Author: George Spagna
Date: Wed, 18 May 1994 14:57

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In article <Pine.3.89.9405131120.B18178-0100000@bvsd.k12.co.us> Roger Briggs <rbriggs@BVSD.K12.CO.US> writes:
>Date: Fri, 13 May 1994 11:57:19 -0600
>From: Roger Briggs <rbriggs@BVSD.K12.CO.US>
>Subject: Electromagnetic Wave Question
>I am a high school Physics teacher, and have had this question for some time:
>In an electromagnetic wave are the two fields in phase (both sine
>function) or out of phase (one sine and one cosine)?  Every book I have
>consulted shows E and B in phase, but this doesn't make sense if the
>derivative of the electric field gives the magnetic field (Faraday's Law).
>What's the deal?

They are in phase (assuming a plane wave for now).  I think your confusion 
is between Maxwell's equations (directly relating "source" terms  -  source 
of E is -dB/dt, source of B is dE/dt) and the "wave equation" which involves 
second derivatives.

Check the derivation in Halliday and Resnick, Ohanian, Tipler, etc.

******************************
*       George Spagna        *      "Sanctified Ignorance
*       Physics Dept.        *       is still Ignorance"
*  Randolph-Macon College    *       --the Rt. Rev. John Shelby Spong
******************************

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