# Acceleration of a proton in an electric field

## Homework Statement

A proton (+e) accelerates from rest in a uniform electric field of 640 N/C. At some later time, the proton's speed is 1.20 x 10^6 m/s. Find the acceleration of the proton. The mass of a proton is 1.67x10^-27 kg. Charge on a proton is +1.6x10^-19C.

F=ma
F=qE

## The Attempt at a Solution

F=qE
F=ma
ma=qE
(1.6x10^-19)(640)=(1.67x10^-27)(a)
a=6.13x10^10m/s^2

I don't think this is correct. I ignored the (+e) because I didn't know what to do with it. Wouldn't an electron and a proton mean a net neutral charge? Also, the acceleration is just way too high. The particle would have to travel faster than the speed of light for a short time.

Doc Al
Mentor
I don't think this is correct.
Looks fine to me.
I ignored the (+e) because I didn't know what to do with it.
+e is just the proton's charge, which you used just fine. e is the elementary charge; the charge on an electron is -e, on a proton it's +e.

Wouldn't an electron and a proton mean a net neutral charge?
There's no electron in this problem.

Also, the acceleration is just way too high. The particle would have to travel faster than the speed of light for a short time.
The acceleration is high. Once the speed gets too high, Newton's laws must be modified using relativity. Note that the speed they quote is only a small fraction of light speed.

Okay. I thought that by +e the problem meant that the electron and the proton were a pair (as in a hydrogen atom). They just meant that the charge on a proton is the same as the charge on an electron, but positive. Thanks for clearing that up.