Re: electric field outside infinitely long cyclinder
 From: "Timo A. Nieminen" <timo@xxxxxxxxxxxxxxxxx>
 Date: Sun, 12 Oct 2008 16:22:11 +1000
On Sat, 11 Oct 2008, tinesan@xxxxxxxxx wrote:
Say we have an infinitely long cylinder of radius r and uniform
surface charge density s (say positive).
What is the electric field at a point who's perpendicular distance to
the axis of the cyclinder is R?
Considering a Gaussian cylinder of radius R and of length L coaxial to
the infinitely long cyclinder, we get:
Qenclosed = s*2pi*r*L
Area Gaussian surface = 2pi*R*L
Thus by Gauss' law:
EA = Q/epislon => E = Q/(A*epislon) = (s*r)/(epsilon*R)
Correct?
Looks fine. Note that this is an unusual version of this question; the usual version is for a linear charge density along the cylinder (or thin line of charge, thin wire, or whatever).
The questionasker might be a bit pedantic, and want you to specify that the above expression you found is only for R>r (and, of course, state what the field is for R<r).

Timo Nieminen  Home page: http://www.physics.uq.edu.au/people/nieminen/
Eprints: http://eprint.uq.edu.au/view/person/Nieminen,_Timo_A..html
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 References:
 electric field outside infinitely long cyclinder
 From: tinesan
 electric field outside infinitely long cyclinder
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