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Re: E field and capacitance question



Good evening everyone,

My fellow physics teacher and I ran into a problem that has us perplexed
so I thought I'd run it by any of you who would be interested. My own
background in electricity is rather weak so I wasn't able to offer any
insight. Here's the problem stated as clearly as possible in the absence
of a diagram.

Two capacitors, A and B, are connected in parallel with a 45 V voltage
source. They are identical in construction except that A uses air as a
dielectric and B uses mica as a dielectric (K=6.0). If the electric
field for A is Ea and the electric field for B is Eb, then

(a) Ea = Eb
(b) Ea > Eb
(c) Eb > Ea

Your best answer and an explanation would be greatly appreciated. I
can't recall my colleague's reasoning exactly, but he disagrees with the
textbook's stock answer. (I can't imagine that the textbook could
contain an incorrect answer. Can you? Yeah, right...)

(a) is the correct answer. It would even be the correct answer if the
capacitor were compared a resistor made by filling the space between
the plates with carbon gunk. The E-field is related to the potential
difference between the plates by geometry alone.

The meaning of potential difference between two points (or plates) is
this: it is the work that must done per unit charge to move a charge
from one point to the other in the electric field. As the potential
difference is the same in each case so must be the electric field.

Leigh