Energy flux of an EM wave


by guillefix
Tags: electromagntic, energy flux, radiation
guillefix
guillefix is offline
#1
Apr22-13, 07:41 AM
P: 77
Hello,

The energy density of an electromagnetic wave is [itex]ε_{0}E^{2}[/itex]. To calculate the energy flux, at least in the derivation's I've seen, people just multiply by the speed of the wave, i.e., c. But doesn't this assume that the energy density is constant at all points?; but E changes periodically! Why isn't it then the integral of the energy density in the corresponding volume, so it would give something close to a half of the usual answer i see!?

Thanks in advance
Phys.Org News Partner Physics news on Phys.org
The hemihelix: Scientists discover a new shape using rubber bands (w/ video)
Mapping the road to quantum gravity
Chameleon crystals could enable active camouflage (w/ video)
guillefix
guillefix is offline
#2
Apr22-13, 07:58 AM
P: 77
Just read in here: http://hep.ph.liv.ac.uk/~hutchcroft/...CN6EMWaves.pdf that I was right yeah. Walter Lewin's lecture was a bit missleading.
jtbell
jtbell is offline
#3
Apr22-13, 08:52 AM
Mentor
jtbell's Avatar
P: 11,255
One has to be very careful with terminology and keep in mind exactly which quantity is being discussed. The instantaneous power density (W/m2) passing through a surface oriented perpendicular to the wave propagation direction is indeed ε0cE2.

However, the magnitude of E oscillates rapidly between 0 and the amplitude Emax (often called E0). Finding the time-average over a whole number of cycles gives half of the maximum power density, so the time-averaged power density (which is what we can actually measure in practice) is (1/2)ε0cEmax2 which is often written as (1/2)ε0cE02.

I haven't watched Levin's lecture so I don't know which terminology he's using.


Register to reply

Related Discussions
what is flux...?? is it a scalar or a vector and difference bet flux and flux density Classical Physics 5
speed of mass-energy flux, derived from stress-energy? Special & General Relativity 7
energy flux density and and energy fluence question! work shown please help!!!! Advanced Physics Homework 0
Energy spectrum from flux? High Energy, Nuclear, Particle Physics 1
Wave energy ~ wave height squ. or wave amplitude squ.? General Physics 2