# Homework Help: Energy of a proton using de broglie formula

1. Aug 18, 2010

### curiouschris

I am trying to help my son in his year 12 physics. I know very little of this

find in eV (electron volts) the energy of a proton with a de broglie wavelength of 40 pm (picometres)

What would the correct formula be ?

A good chance for me to learn as well ;)

CC

2. Aug 18, 2010

### rl.bhat

Wave length λ = h/p or

p = h/λ.

And energy E = p^2/2m.

3. Aug 18, 2010

### curiouschris

Thanks but I am obviously still way off beam
(my Sons gone to bed but I am trying this on my own)

de brognie wave length = 4x10^-11
plancks constant = 6.63x10^-34
mass of proton = 1.673x10^-27

first part

p= h/λ
.: p = (6.63x10^-34)/(4 x 10^-11)
.: p = 1.6575x10^-23

2nd part
E = p^2/2m
.: E = (1.6575x10^-23)^2/(2*1.673x10^-27)
.: E = 2.747x10^-46/3.46x10^-27
.: E = 8.2x10^-20

The answer should be E =0.51eV

So what did I do wrong :(

CC

4. Aug 18, 2010

### rl.bhat

E = 8.2x10^-20 J.

To convert it into eV, divide it by 1.6x10^-19.

5. Aug 19, 2010

### curiouschris

So from this I gather E = Energy in Joules?

6. Aug 19, 2010

### rl.bhat

Energy in eV = 8.2x10^-20/1.6x10^-19