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Chapt. 26 Capacitors and Capacitance 203
026 qfull 01000 1 3 0 easy math: classical electron radius
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Uses electric Feld energy, and so is capacitor lecture problem.
4. People once tried to devise a semiclassical model of an electron in which the rest mass energy
given by the Einstein equation
E
=
mc
2
(
E
for energy here) was entirely stored as electric
Feld energy. Remember this energy can be easily calculated from the potential energy of the
assembled charge distribution provided you don’t have to calculate the potential energy of a
point charge.
a) What they did was assume that all the electron charge was uniformly spread on the surface
of a sphere of radius
R
. (Note this distribution avoids any point charges). What is the
potential energy of a sphere of radius
R
with uniform surface density
Q
?
b) What formula and value for
R
did those longago folks arrive at from their model?
c) As a matter of fact, the proton has a measurable radius of about 0
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This note was uploaded on 11/16/2011 for the course PHY 2053 taught by Professor Buchler during the Fall '06 term at University of Florida.
 Fall '06
 Buchler
 Physics, Capacitance, Energy

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