Question

How much work does the electric field do in moving a proton from a point with a potential of +125 v to a point where it is -55 v? express your answer both in joules and electron volts?

Answer 1

The work W done by the electric field in moving the proton is equal to the difference in electric potential energy of the proton between its initial location and its final location, therefore:
$W= qV_i - qV_f$
where q is the charge of the proton, $q=1 e = 1.6\cdot 10^{-19}C$, with $e$ being the elementary charge, and $V_i = +125 V$ and $V_f = -55 V$ are the initial and final voltage.

Substituting, we get (in electronvolts):
$W=e(125 V-(-55 V))=180 eV$
and in Joule:
$W=(1.6 \cdot 10^{-19})(125 V-(-55V))=2.88 \cdot 10^{-17}J$