Calculating change in energy of a particle

1. Jun 19, 2013

x86

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

An electron moves from an initial location between parallel plates where the electric potential is Vi = 30 V and Vf = 150 V

a) Determine the change in the electrons potential energy

2. Equations
J/C = V = N/C * m

3. The attempt at a solution
a) The electron is naturally moving to a location of higher energy, therefore its potential energy decreases as its kinetic energy increases.

Potential energy change = Ep2 - Ep1 = Vfq - Viq = q(Vf-Vi) = q(120 V) = -1.92 * 10^-17 J.

The sign makes sense to me, since the electron has more potential energy at Vi than Vf.

However, the book says the answer is +1.92*10^-17 J. Can someone help me out with why I am is wrong? Thanks

2. Jun 19, 2013

Fightfish

Well your book looks like its wrong. Your qualitative reasoning seems correct to me.

3. Jun 19, 2013

rude man

Are you sure they didn't ask for the change in kinetic energy? Because otherwise I have to agree with your answer as being negative.

4. Jun 19, 2013

x86

Yes I'm sure they asked for potential energy. I guess the book is wrong. Thanks for confirming this.