Phy--Electric Fields

無．話可說

係個sphere以外佢就behave一個point charge
but係佢入面 i.e.個sphere入面
因為冇electric field, so個electric potential係同sphere表面一樣
for instance, let a be the radius of the sphere
Vmax = Von the sphere surface = Q/4piE0a
where Q is the total charge on the sphere surface
if r > a, it is the same as a point charge as follow
however, if there is a point charge
when r < a, V = Q/4piE0r
which is larger than Q/4piE0a, i.e. Vmax of the sphere
As a result, sphere behaves like a point charge if and only if r > a