The question is to calculate the speed of a proton that is accelerated from rest through a potential difference of 120V. Qp(charge of proton) ~ 1.60E-19C...with this, DeltaU = Qp*DeltaV...in which DeltaU is positive. It makes sense since the proton is heading towards a higher electric potential. DeltaU = -DeltaK and since the proton accelerates from rest, DeltaU = -(1/2)*m*Vf^2 (Vf as final velocity). The problem I get here is that since DeltaU is positive, if I solve for Vf, I'll be square-rooting a negative number. I get the right answer if I ignore the negative sign, but otherwise, I get an imaginary number...what am I doing wrong? Is it because of the DeltaU = -DeltaK...does that only hold valid for conservative forces? In this case, the rise in potential energy implies that the work is done by an external force.