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mintsnapple
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Homework Statement
a. Calculate the energy density of the electric field at a distance r from an electron (presumed to be a particle) at rest.
b. Assume now that the electron is not a point but a sphere of radius R over whose surface the electron charge is uniformly distributed. Determine the energy associated with the external electric field in vacuum of the electron as a function of R.
Homework Equations
$$ u_e = 1/2\epsilon_0E^2 $$
The Attempt at a Solution
a. The electric field of an electron can be assumed to be the same as a point charge, that is
$$ E = \frac{q}{4\pi\epsilon_0r^2} $$
Since
$$E^2 = \frac{q^2}{16\pi(\epsilon_0)^2r^4} $$,
$$u_e = \frac{q^2}{32\pi \epsilon_0 r^4} $$
b. We use Gauss's law to find the electric field of this sphere.
$$ EA = \frac{\sigma A}{\epsilon_0} $$
So that
$$ E = \frac{\sigma}{\epsilon_0}$$, where $$\sigma$$ is the charge per unit area.
So the energy density is
$$ u_e = \frac{1}{2}\epsilon_0\frac{\sigma}{(\epsilon_0)^2} = \frac{1}{2} \frac {\sigma^2}{\epsilon_0} $$
The total energy is therefore the energy density multiplied by the volume, so
$$ U = \frac{4\sigma^2\pi R^3}{6\epsilon_0} $$
Is this correct?