1 29 The total negative charge in the uniform spherical
charge distribution of radius R must be –Z e, since the atom (nucleus
of charge Z e + negative charge) is neutral This immediately gives us
the negative charge density r, since we must have
3
4
0–
3
R
Ze
ρ
π
=
or
3
3
4
Ze
R
ρ = −
π
To find the electric field E(r) at a point P which is a distance r away
from the nucleus, we use Gauss’s law