#### 2.3.1 A sphere with the charge on the surface

We consider in the following simulation a sphere made up of 200 charges with value 10000 u each, and a single charge of value -10 u that, when "animation" is clicked, moves across the sphere. The force acting upon his negative charge is presented, and we can observe how the strength of the force becomes higher when the charge approaches the sphere from the outside, immediately vanishing when the charge enters within the sphere.

The force due to the same sphere of charge acting upon some test charges (1 u) is shown in the following. It may be verified that the field within the sphere vanishes, and that the field outside equals that of a point charge located at the centre and whose magnitude is the total charge on the sphere. This last property holds in general for any spherical distribution of charge (it also holds for the gravitational field, and it allows to calculate forces between extended, approximately spherical, bodies as if they were point masses).

See experiment