It can be seen that the strain is proportional to the difference of
the square of the fields at the edge of the sphere and at the
considered point. Consequently, the electric field inside the material
has to be calculated. The applied electric field E0 is considered to
be directed along the uz direction. Using boundaries conditions and
considering only the Z-component (1D), the electric field created
out (z a) of the dielectric sphere, can be expressed as: