1. Not finding help here? Sign up for a free 30min 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!

Question on the derivation of Intensity

  1. Jan 26, 2013 #1
    1. The problem statement, all variables and given/known data

    When deriving the intensity of an electromagnetic wave in my textbook, there was a part I didn't quite get.

    It's how I = Watts/m^2 becomes I= u c

    My textbook gave an example of a electromagnetic wave through a cylinder, with cross-section area A. In 1 second it would have traveled length c. Now that makes sense.

    It then proceed with changing the cross-section area with 1 and by that derives I= u c.. ? I don't get it.

    2. Relevant equations

    I = Watts/m^2
    I= u c

    3. The attempt at a solution
    So can anybody help me conceptualise this derivation?
     
  2. jcsd
  3. Jan 26, 2013 #2
    "u" is energy density, right? So, for a given volume ΔV you can find the energy content of it, using the relation ΔE = u ΔV. Ok, so far?

    If yes, then consider a given surface ΔA and try to find the energy passed through it for some time interval Δt. That amount of energy, after leaving the surface, wouldn't be accumulated in a volume ΔV = ΔA (c Δt)?

    If the above makes sence, then use the definition of intensity (I = ΔE/(ΔA Δt) to get the desired result.
     
  4. Jan 26, 2013 #3
    Great. Yeah, that was clear. I'm grateful ^^
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Question on the derivation of Intensity
  1. Intensity question (Replies: 1)

  2. Wave intensity question (Replies: 11)

  3. Intensity Question (Replies: 2)

Loading...