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 Because of the spherical
symmetry of the charge distribution, the magnitude of the electric
field E(r) depends only on the radial distance, no matter what the
direction of r Its direction is along (or opposite to) the radius vector r
from the origin to the point P The obvious Gaussian surface is a
spherical surface centred at the nucleus