Re: Electric Field Question

[Rick Tarara <rtarara@SAINTMARYS.EDU>]

If q(a) and q(b) are equal in magnitude and opposite in sign--then yes,
outside the spheres the field is zero.

To see this:

a)  Use Gauss' law with your surface outside both spheres.  The NET enclosed
charge is zero hence the field is zero.

b)  Look at the field lines that must start at + charges (or infinity) and
end at negative charges (or infinity).  With an equal number of charges, you
have one line per set of + and - charge.  All lines start and end between
the inner surface of the outer shell and the outer surface of the inner
shell.  No field lines left over--so no external field.

If the magnitude of the charges are not the same, then (a) can calculate the
field, and (b) can help visualize the field.

Rick

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Saint Mary's College
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rtarara@saintmarys.edu

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----- Original Message -----
From: "Tina Fanetti" <FanettT@QUEST.WITCC.CC.IA.US>
To: <PHYS-L@lists.nau.edu>
Sent: Tuesday, February 05, 2002 11:58 AM
Subject: Electric Field Question


> Okay, my students have stumped me.
>
> Say you have a hollow metallic spherical shell of radius a with charge
q(a).  Concentric to that is a hollow metallic spherical shell of radius b
with charge q(b).
> Now we get what happens when the charges are the same sign.
>
> Now what through me off is what if q(a) and q(b) have opposite signs.
What would the electric field be outside both spheres.  I realize it won't
change the answer inside sphere a or inbetween the spheres.  But will the
electric field outside the spheres be zero?  I can't imagine they would
cancel out?
> Thanks
> Tina
>
> Tina Fanetti
> Physics Instructor
> Western Iowa Technical Community College
> 4647 Stone Ave
> Sioux City IA 51102
> 712-274-8733 ext 1429