Re: Electric Force and Shells

[Bernard Cleyet <anngeorg@PACBELL.NET>]

People!

Below is a msg. I was to send to Tina, when I discovered a contradiction
between Harnwell and F, L, and S.

F. et al. states (p. 5-9 V. II '77) "The same arguments can be used to
show that no static distribution of charges inside a closed conductor can
produce any fields outside.  Shielding works both ways!"

Harnwell (and Bernard) (p 18 2nd Ed. '49)  "If there is a charge, say q,
inside the cavity an equal and opposite charge -q must be induced on the
inner walls in order that surface integral E dot ds may vanish  over a
Gaussian surface in the conductor enclosing the cavity."  Continuing in
answer to Tina:  "In consequence, if the conductor has no net charge, a
charge q must appear over the outer surface.  This gives rise to an
external electric field and thus it is seen that the conductor does not
shield the external region from the effects from an internal charge."
Furthermore, "However, it will be shown later that the external effects
are independent of the position of the charge with in the cavity."  Sort
of like a charged black hole.

bc who expects correction

Tina!

Do you have a copy of Feynman (Leighton and Sands)?

I think it's the best text for these kinds of questions and will be
quicker than the list.  However, I agree these problems are difficult.
Witness my difficulty with use of G's law and a parallel cap.

bc  Remember this:  the squeaky person gets the cleyet.

P.s.
1.  no diff. if not in center (Q)  i.e. any where inside works the same
and doesn't need to be spherical either.
2.  unless grounded, an enclosing conductor does not shield internal q's
3.  Q never experiences a force due to q. (this fact proves that the
force law for q is inverse square [one part per billion - another
thread.]), also doesn't require sphericity.
4.  q does.  first with no Q.   the outside charge q polarizes the sphere
(in order, anthropomorphically, to leave a resultant zero field in the
sphere.  The force is a tidal one due to the attraction of the near
oppositely induced charge on the sphere and the lesser force of repulsion
due to the induced like charge on the opposite side of the sphere
(further away).  These charges and equivalent positions are found by the
method of images.  I'm too lazy to give you the answer here.  If F, L,
and S. is not readily available I'll scan and send. Second add the force
due to Q see 1 and 2 above.  The additional force (repulsion or
attraction depending ..) due to Q is the same as if it were in the center
with out the sphere.

bc



Tina Fanetti wrote:

> I never understood this problem...
>
> Okay you have a point charge Q at the center in a spherical conductin=
> g shell.  There is a point charge q outside the shell. =20
>
> Does q experience a force?  Does Q experience a force? =20
>
> I want to say yes they experience the same force but then there is th=
> at shell.  I don't know what to do with it...
>
> Would the shell pickup the charge Q and then the force would between =
> the shell and q?  that doesn't make sense though.
>
> Maybe they don't experience a force?  But why wouldnt they? =20
>
> COnfused
> Tina
>
> Tina Fanetti
> Physics Instructor
> Western Iowa Technical Community College
> 4647 Stone Ave
> Sioux City IA 51102
> 712-274-8733 ext 1429