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Homework Help: Find the rms electric field strength of a given EM wave

  1. Feb 23, 2016 #1

    I was able to correctly calculate the answer to part 1 below but the answer to part 2 is incorrect
    and I cannot see the error of my ways... any help gratefully received.

    1. The problem statement, all variables and given/known data

    An electromagnetic wave has an intensity of 127 W/m2.

    1) Find its rms electric field strength.
    2) Find its rms magnetic field strength.

    2. Relevant equations

    Bo = Eo/c

    Brms = B/sqr(2)

    3. The attempt at a solution

    Answer to part 1 : Eo = 219 V/m

    Part 2

    Calculate Bo = 219/2.998 x 10^8 = 7.3 x 10^-7

    Brms = (7.3 x 10^-7)/sqr(2) = 5.17 x 10^-7 T

    This answer for part 2, doesn't match the expected answer.
  2. jcsd
  3. Feb 23, 2016 #2


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    Does the answer to part 1 give E0 or Erms? How do you find Brms from Erms?
  4. Feb 24, 2016 #3
    Thank you for replying and your question helped me understand my mistake.

    Part 1's answer was Erms, and I was using this answer for part 2 instead of using Eo.
    When I recalculated using the correct value I got the answer I expected.
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