# Homework Help: Electric Field problem

1. Oct 8, 2008

### Precursor

1. The problem statement, all variables and given/known data
When a charged particle is injected into an electric field:
a) velocity changes and kinetic energy changes.
b) velocity changes and kinetic energy stays the same.
c) velocity stays the same and kinetic energy changes.
d) velocity stays the same and kinetic energy stays the same.

2. Relevant equations
None are necessary.

3. The attempt at a solution
I think it's a) because when a charged particle enters an electric field velocity changes, and therefore kinetic energy. Am I right?

2. Oct 8, 2008

yup...

3. Oct 8, 2008

### gabbagabbahey

You're correct, however kinetic energy doesn't always change with a changing velocity. KE depends only on the magnitude of the velocity, not on the direction; so if a particles velocity changes direction but retains the same magnitude (such as in uniform circular motion), the KE won't change.

Electric fields exert a velocity independent force (ignoring relativistic effects) on a charged particle, and hence do work on the particle, so both the KE and the velocity will change.

Magnetic forces always act perpendicular to a particle's velocity and so they only change its direction, not its magnitude. They will change the velocity, but not the KE.