Calculating values of electric and amgnetic fields of laser beam


by krhisjun
Tags: electric field, electrodynamics, magnetic field, maxwells equations
krhisjun
krhisjun is offline
#1
Nov19-10, 11:39 AM
P: 22
1. The problem statement, all variables and given/known data
A continuous wave laser beam in free space carries a power of 15w and has a circular cross section with diameter 1mm. Calculate peak values of the oscillatory electric and magnetic fields Eo and Ho repectively.


2. Relevant equations

Eox = ([tex]\mu[/tex]/[tex]\epsilon[/tex])^1/2 Hoy
[tex]\pi[/tex]r^2
Energy flow = 1/2 (HE + EH)
energy flow = | E x H |
E = Eo cos ([tex]\omega[/tex] t )


3. The attempt at a solution

Okay so i have the energy flow as 19098.593 Kj / s / m^2
I know energy flow = 1/2 \HE + EH ) = EH = |E x H| this energy flow is in direction of wave..

But i cant work out how to relate this to get the mag of electric field or magnetic field
Phys.Org News Partner Science news on Phys.org
Simplicity is key to co-operative robots
Chemical vapor deposition used to grow atomic layer materials on top of each other
Earliest ancestor of land herbivores discovered
Matterwave
Matterwave is offline
#2
Nov19-10, 01:14 PM
P: 2,043
Since you are in vacuum, I don't think there's any reason to complicate things. E=cB, and B=μ0H
krhisjun
krhisjun is offline
#3
Nov19-10, 01:16 PM
P: 22
Quote Quote by Matterwave View Post
Since you are in vacuum, I don't think there's any reason to complicate things. E=cB, and B=μ0H
Bu then HHow do i calculate H?

Matterwave
Matterwave is offline
#4
Nov19-10, 02:11 PM
P: 2,043

Calculating values of electric and amgnetic fields of laser beam


Just invert the second equation for H in terms of B.
krhisjun
krhisjun is offline
#5
Nov19-10, 02:48 PM
P: 22
Sorry, i mean to say i dont know how i would get H B or E.

i can see easily how with any of the variables allows the other for calculation but im at a loss to get any.
Matterwave
Matterwave is offline
#6
Nov19-10, 04:02 PM
P: 2,043
[tex]\bar{S}=\frac{E^2}{2\mu_0c}[/tex]
krhisjun
krhisjun is offline
#7
Nov19-10, 04:20 PM
P: 22
for fear of asking the obvious, S being?
Matterwave
Matterwave is offline
#8
Nov19-10, 09:00 PM
P: 2,043
S bar is the average of the magnitude of the Poynting Vector, it is the flux (or intensity) of the laser measured in Watts per meter squared.
krhisjun
krhisjun is offline
#9
Nov20-10, 08:23 AM
P: 22
Ok, didnt realise that equation,
so using those values E = [tex]\sqrt{2S\mu c}[/tex] ?
Therefore E = 119959.9933 V m^-1 ?

H = E/c[tex]\mu[/tex] ?
Therefore H = 318.4160428 A m^-1 ?

I tried to confirm the equations using dimensional analysis:
E = V m^-1
mu= kgms−2A−2
C = m S^-1
S = J s^-1 m^-2

I cant get that to equal but i think i may be rearranging wrong.

Thankyou for yor help so far


Register to reply

Related Discussions
Calculation of an Electron Beam path when subjected to magnetic and electric fields Classical Physics 17
Calculating Multiple Integrals on Mathematica (Electric fields) Advanced Physics Homework 1
Calculating Ka and Kb values from estimated pH values Biology, Chemistry & Other Homework 4
Calculating electric fields through integration General Physics 3
Calculating electric fields Introductory Physics Homework 1