# Velocity of a proton question

## Homework Statement

Calculate the veolcity of a proton of energy 0.12 keV

proton mass = 1.7 x 10^-27

## The Attempt at a Solution

I thought...

0.12 keV = 120 eV

120/1.60 x 10^-19 = 7.5 x 10^20 J

E = 0.5mv^2

sqrt(2E/m) = v

but I'm getting something to x 10^23 so its obviously wrong.

its A-Level physics so shouldnt be a really complex answer...

Going wrong somewhere but cant work it out. Probably where I divided by the charge of an electron?

Any help appreciated.

James

What exactly is an electronVolt?

You divided instead of multiplying. Try using dimentional analysis to convert from eV to Joules. It takes a lot of guess work out of the problem.

What exactly is an electronVolt?

By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electrostatic potential difference of one volt. (from Wiki)

You divided instead of multiplying. Try using dimentional analysis to convert from eV to Joules. It takes a lot of guess work out of the problem.

Oh, my tutor told me to divide...

Is it because 1 Volt = 1 Joule/1 Coulomb

There for Joules = V * C ?

So eV multiplied but the charge of an electron give the Energy of the proton?

Also, I've not heard of Dinentional Analysis before, what exactly is it?

Thanks alot

james

So would the correct answer be...

120 * 1.6 x 10-19

= 1.92 x 10 -17

sqrt(2*1.92 x 10 -17/1.7 x 10-27) = v = 1.5 x 105 ms-1