$3.$ Refer to the Figure in the section heading. Calculate the work required to move a charge of $2\mathrm{.}78$ pC from d to k in units of nJ$.$

HINT: Recall that the mechanical work required to move a mass from a lower height to a greater height is equal to the difference in gravitational potential energy. One can similarly reason that the work required to move a positively charged particle from a lower potential to a higher potential is equal to the difference in electric potential energy.

$4.$ Calculate the magnitude of the electric field at $"$g$"$ in units of V$/$m$.$ Use the given $5$ cm scale to an appropriate accuracy.