Find the electric field strength vector if the potentail of this field has the form `varphi = ar`, where is a constatn vector, and `r` is the radius vector of point of the field.

Explanation

In accordance with the problem `varphi = vec(a). vec(r )`.
Thus from the equation : `vec(E) = vec(grad) varphi`
`vec(E) = -[(del)/(del x) (a_(x) x) vec(i) + (del)/(del_(y)) vec(j) + (del)/(del_(z)) (a_(z) z) vec(k)] = -[a_(x) vec(i) + a_(y) vec(j) + a_(z) vec(k)] = -vec(a)`