Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-l] Poynting Vector



Right. As John Denker said, it's really the divergence of the Poynting vector which will point to a net flux of energy through a control surface. Note that Poynting's theorem says

(u)t + div(S) = -J . E

where u is the energy density at a point in space, and where I've taken the t "subscript" to represent a partial of u w.r.t t, and the period to represent the dot-product of the quantities J---the current density---and E---the electric field.
If J is zero (as would be the case in the static capacitor plates problem), then the only electromagnetic means of getting energy into or out of the control volume bounded by our surface is via the _divergence_ of the Poynting vector. If S is divergenceless (fun with vector identities, and recognizing that (B)t = 0 --> curl(E) = 0), then u is constant in any given control volume.



/************************************
Down with categorical imperative!
flutzpah@yahoo.com
************************************/



----- Original Message ----
From: chuck britton <britton@ncssm.edu>
To: Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu>
Sent: Saturday, June 28, 2008 3:53:30 PM
Subject: Re: [Phys-l] Poynting Vector

Where ever there is an E and a B (not parallel), there is a Poynting
Vector





On Jun 28, 2008, at Jun 28(Sat) 5:37 , Jack Uretsky wrote:

As Curt suggests, this sounds like a static situation. If so,
there is no
EM wave, and, consequently, no Poynting vector.
Regards,
Jack

_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l