# Calculate the p.d. between two points

1. Jan 9, 2012

### bsmm11

1. The problem statement, all variables and given/known data
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.

2. Relevant equations
V = W / q

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

Having Ek 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.?

2. Jan 9, 2012

### 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