# Homework Help: Find Change in the Electric Potential

1. Sep 16, 2011

### fal01

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

A proton is released from rest in a uniform
electric field of magnitude 40000V/m di-
rected along the positive x axis. The proton
undergoes a displacement of 0.3 m in the di-
rection of the electric field

The mass of a proton is 1.672623×10−27 kg

A.Find the change in the electric potential
if the proton moves from the point A to B.

B.Find the change in potential energy of the
proton for this displacement. Answer in units
of J.

C.Apply the principle of energy conservation to
find the speed of the proton after it has moved
0.5 m, starting from rest. Answer in units of
m/s.
2. Relevant equations

V=Ed
U=qV
Kf-Ki+u=0
k=.5mv^2
Q

3. The attempt at a solution
A.) E=40000 V/m
d=.3m
mp=1.672623*10^-27kg
q=1.60218*10^-19

V=Ed
V=(40000)(.3)=12000V ---but that is wrong

B.)
U=qV

C.)
Kf-Ki+U=0
.5mv^2+U=0

2. Sep 16, 2011

### PeterO

(a) Electric potential is measured in volts. You were given V/m and metres. Just combine them to get volts [Joules per Coulomb; which you will need for part (b)]

3. Sep 16, 2011

### fal01

so what I started doing was right.... (40000V/m)(.3m) =12000 V

when I put in that answer it says I am wrong :(

4. Sep 16, 2011