How Can You Find the Energy Densities of an Electromagnetic Wave?

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Homework Help Overview

The discussion revolves around finding the energy densities of an electromagnetic wave, specifically focusing on the contributions from the electric and magnetic field components. Participants are exploring the underlying principles and relationships involved in this topic.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • The original poster attempts to understand the derivation process for energy densities and questions whether it relates to continuous charge distributions or uniform currents. Others raise questions about the ratio of energy densities and how it connects to the characteristic impedance of free space.

Discussion Status

Participants are actively engaging with the problem, sharing insights and clarifications. Some have offered guidance on using relationships between electric and magnetic fields, while others express uncertainty about the connections between different concepts. There is a mix of attempts to derive formulas and clarify definitions without reaching a consensus.

Contextual Notes

Participants are navigating through the complexities of electromagnetic theory, with some noting the need for specific relationships and identities from Maxwell's equations. There is an emphasis on understanding the derivation rather than simply obtaining results.

bon
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Homework Statement



Find the energy densities of an electromagnetic wave separately for the contribtions arising from the electric and magnetic field components

Homework Equations





The Attempt at a Solution



How does one do this?

Is it just the usual derivation for a continuous charge distribution and then the one for a uniform current etc?

Is there a good website with this on that anyone can point me in the direction of? Thanks!
 
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Another quick question:

SO it then asked me to work out the ratio of the energy denstities, which i did:

mu 0 times e0 times E^2/B^2

But then it says: find the characteristic impedence, defined as the ratio E/H of free space...

does this follow from what I've just done? If so, howwww?
 
anyone?
 
hi bon! :smile:

(have a mu: µ and an epsilon: ε and try using the X2 and X2 icons just above the Reply box :wink:)
bon said:
… the ratio of the energy denstities, which i did:

mu 0 times e0 times E^2/B^2

But then it says: find the characteristic impedence, defined as the ratio E/H of free space...

in free space, B = µ0H :wink:
 
Thanks but I am still not sure how to get to the answer:

the ratio of the energy denstities is µε E²/B²

so substituting B=µH I get

the ratio to be (ε/µ) E²/H²...but this still doens't allow me to get E/H -! This just tells me the ratio of the energy densities..
 
tiny-tim said:
hi bon! :smile:

(have a mu: µ and an epsilon: ε and try using the X2 and X2 icons just above the Reply box :wink:)


in free space, B = µ0H :wink:



Sorry I thought id quote you just so you see that I've replied!
 
you can also use E/B = c :wink:
bon said:
Sorry I thought id quote you just so you see that I've replied!


but i get an email notification anyway :confused:
 
tiny-tim said:
you can also use E/B = c :wink:


but i get an email notification anyway :confused:



Aha ok thanks!

And sorry - i didnt realize you get an email notification :)

Thanks again.
 
bon said:

Homework Statement



Find the energy densities of an electromagnetic wave separately for the contribtions arising from the electric and magnetic field components

Are you still having trouble with this? there's a very neat way to do it using conservation of energy.

Begin by writing down a relationship between du/dt , div N and Jf.E for some volume dV, if you're unsure of the origin of the J.E term consider the work done dW by fields in moving some charge ro dV by dl.

Once you have the relationship, start with J.E and manipulate it using maxwell's equations until you have a term that looks like div N and some other terms. Those other terms will be du/dt.

you will need to use the identity div (AxB)= A.curl B - B.curl A

u is energy density, Jf free current and ro is a charge density
 

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