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Average intensity

  1. Apr 25, 2009 #1
    1. The problem statement, all variables and given/known data

    A laser blackboard pointer delevers 0.10-mW average power in a beam 0.90 mm in diameter.

    2. Relevant equations

    a) Find the average intensity.
    b) Find the peak electric field.
    c) Find the peak magnetic field.



    3. The attempt at a solution

    ok for part a) i used the formula ave. s = p/4 pie r^2 (as p=.10 mW and r=0.45 mm)
    but i didnt get the right answer
    so am i using the wrong formula or the wrong value for r?


    for part (b) and (c) i cant do anythin bout it cuz i need the average intensity
    in order to find the elecric and magnetic fields.

    somebody plez help me asap....thanx!!
     
  2. jcsd
  3. Apr 25, 2009 #2

    LowlyPion

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    Homework Helper

    Isn't the power measured in the area of the beam?

    Avg power = .1*10-3w / (3.14*(.45*10-3m)2) = ... w/m2
     
  4. Apr 25, 2009 #3

    is there 4 in the bottoom becuz isnt the
    formula is ave s= p/4*pie*r^2 ?
     
  5. Apr 25, 2009 #4

    LowlyPion

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    Homework Helper

    That r is for distance isn't it?

    This is a laser beam and I'd say dispersion is minimal, so you only want the cross section of the beam.
     
  6. Apr 25, 2009 #5
    ok i got 160 W/m^2
    thanx for the help!!!!
     
  7. Apr 25, 2009 #6
    hey i got part b
    but have problem with part C

    is this right?

    B^2 = ave. S *2* 4*pie*10^-7/ (3*10^8) = T
     
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