Calculate the p.d. between two points

[bsmm11]

Homework Statement

The work done by an external force to move a -8.0μC charge from point a to point b is 8.0 × 10^-3J. If the charge initially at rest had 4.0 × 10^-3J of kinetic energy ab point b, calculate the potential difference between a and b.

Homework Equations

V = W / q

The Attempt at a Solution

Work done per unit charge to move a positive charge from a to b is
V = W / q = 8.0 × 10^-3 / -8.0 × 10^-6 = -1.0 × 10³V

At a E_k is 0J and at b it is 4.0 × 10^-3J, so
ΔE_k = 4.0 × 10^-3J.
So work done for each electron to give E_k is
V = W / q = 4.0 × 10^-3 / -8.0 × 10^-6 = -0.5 × 10³V

Having E_k means that energy is not dissipated. P.d. is the power dissipated per unit current so
(-1.0 × 10³) - (-0.5 × 10³) = -0.5 × 10³V
Is it right way to do it?
Do I need to get rid of negative sign when I calculate the p.d.?

 

[ehild]

The result is correct, but look after how is potential difference between a and b defined. Is it the potential at "a" minus potential at "b" or the opposite? The best thing is to make it clear, giving Vb-Va, or saying which point is positive with respect to the other point. So which one?

ehild