Question

the voltage difference 800v what is the electric field

Answer 1

The increase of voltage from point $x_1$ to point $x_2$ is (see https://en.wikipedia.org/wiki/Voltage)

$\Delta V = -\int\limits_{x_1}^{x_2}\vec{E}\,d\vec{l}.$ Therefore, to obtain the value of electric field we should know something about the distance between points $x_1$ and $x_2$ .

If we consider the uniform fields (see https://en.wikipedia.org/wiki/Electric_field#Uniform_fields), the formula will have a simpler form

$E = -\dfrac{\Delta V}{d}$ . So we can calculate the field as $E = - \dfrac{800\,\mathrm{V}}{d\,\mathrm{m}},$ so for distance $d=1\,\mathrm{m}$ the module of field will be 800 V/m, for distance $d=2\,\mathrm{m}$ the field will be 400 V/m, and so on. We also can see that the dependence of E on V is proportional to $\dfrac{1}{d}$ .