Calculating Average Intensity of a laser blackboard pointer

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AI Thread Summary
To calculate the average intensity of a laser blackboard pointer with 0.10 mW power and a 0.90 mm diameter beam, the correct formula is average intensity = power / (π * r^2), where r is the radius of the beam. The user initially struggled with the formula and values but confirmed that using a radius of 0.45 mm led to an average intensity of 160 W/m². This intensity is crucial for determining the peak electric and magnetic fields. The user successfully calculated part b but encountered difficulties with part c, needing clarification on the relationship between average intensity and the magnetic field calculation. The discussion emphasizes the importance of using the correct area for intensity calculations in laser physics.
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Homework Statement



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

Homework Equations



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



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 can't do anythin bout it because i need the average intensity
in order to find the elecric and magnetic fields.

somebody plez help me asap...thanx!
 
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Isn't the power measured in the area of the beam?

Avg power = .1*10-3w / (3.14*(.45*10-3m)2) = ... w/m2
 
LowlyPion said:
Isn't the power measured in the area of the beam?

Avg power = .1*10-3w / (3.14*(.45*10-3m)2) = ... w/m2


is there 4 in the bottoom becuz isn't the
formula is ave s= p/4*pie*r^2 ?
 
patelpalak said:
is there 4 in the bottoom becuz isn't the
formula is ave s= p/4*pie*r^2 ?

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.
 
ok i got 160 W/m^2
thanx for the help!
 
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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