# Homework Help: 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.

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