# Average intensity

## 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 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!!

Related Introductory Physics Homework Help News on Phys.org
LowlyPion
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

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 isnt the
formula is ave s= p/4*pie*r^2 ?

LowlyPion
Homework Helper
is there 4 in the bottoom becuz isnt 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