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capacitors



Hi,

I'm new to the list as of 5/26/99. Hope this question hasn't come up recently.

A common Physics 1 text problem (i.e.,. Halliday and Resnick 27.61 and 27.62) :

A parallel plate capacitor is "charged" by a voltage source and then
disconnected from that source. A conducting slab is introduced between
the plates. It can easily be shown by energy considerations that U(final)<
U(initial). Therefore the slab must be "sucked into" the capacitor, the
capacitor doing positive work on the slab. It can also easily be shown
that a net electric force (due to the fringing of the field) acts on the
("polarized") slab drawing it into the capacitor.

Now to my problem. If the capacitor remains connected to the voltage
source, the slab must now be pushed in because U(final)>U(initial). Can
someone suggest an electrostatic interaction between slab and capacitor's
field that will result in an electric force on the slab directed away from
the capacitor?

Many thanks.

JS




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