Re: Force on a Capacitor Plate

[Bob Sciamanda <trebor@VELOCITY.NET>]

The "bound" surface charge is "fictitious".  The electric field due to the
volume of polarized dielectric is equivalently (to dipole order)
calculable as the field of a surface layer of "bound" charge.  Note: if
the polarization is not spatially uniform within the dielectric and has a
non-zero divergence, a volume contribution of "bound " charge must also be
included in the field calculation.

Bob Sciamanda
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor
----- Original Message -----
From: "Doug Craigen" <dcc@ESCAPE.CA>
To: <PHYS-L@lists.nau.edu>
Sent: Saturday, February 17, 2001 02:17 PM
Subject: Force on a Capacitor Plate


> Suppose you measure the force on the plate of an air-filled capacitor
> held at a fixed potential difference - call that Fo.  Now fill all
> relevant surrounding space with a dielectric liquid of permittivity e_r
> .  The capacitance increases by a factor of e_r, hence the charge on the
> plates increases by the same, but the E field is unaffected - so the
> standard textbook answer is that the new force on the plate is e_r*Fo.
>
> I am curious as the whether this is physically what would be observed.
> Has anybody here tried the measurement?  A couple of alternative
> possibilities spring to mind, depending on what happens at the
> interface.  Noteably, at the interface you have a free surface charge
> density of e_r*Qo and a bound surface charge density of
> (1-e_r)*Qo.  So the total charge on the interface is still Qo, hence the
> total force on the interface is still Fo.  It seems to me that this
> means the force on the plate would be e_r*Fo but it would have the
> liquid pushing against it with a force of (1-e_r)*Fo, so the force you
> would actually measure would still be Fo.
>
> Comments???
>
> \_/^\_/^\_/^\_/^\_/^\_/^\_/^\_/^\_/^\_/^\_/^\_/^\_/^\
>
> Doug Craigen
>           http://www.dctech.com/physics/about_dc.html