# Re: electric field outside infinitely long cyclinder

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 question-asker 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).

--
E-prints: http://eprint.uq.edu.au/view/person/Nieminen,_Timo_A..html
Shrine to Spirits: http://www.users.bigpond.com/timo_nieminen/spirits.html

.

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