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!

Homework Help: 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


    User Avatar
    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


    User Avatar
    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
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook