# Homework Help: Quantum physics problem

1. Jun 15, 2009

### Brendanphys

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

This is a problem I did during a test, and I am looking it over to see what I did wrong..

1. An Electron is accelerated by a potential difference of 75V. What is the de Broglie wavelength

2. Relevant equations

P = h/λ
Ek = 1/2mv^2
eV = 1.6E-19

3. The attempt at a solution

P = h/λ
λ = h/p (1)

Ek = 1/2mv^2
(2Ek/m)^1/2 (2)

Sub (1) into (2)
λ = h/p
λ = h/m((2Ek/m)^1/2)

I figured 75V = 75*eV = 75*(1.6E-19) = 1.2E-17J
So 1.2E-17J = Ek

I Plugged in all the known values into
λ = h/m((2Ek/m)^1/2)
and got the answer as: 1.4E21

Any clues as to what I did wrong? I assume it has something to do with my volt conversion, any help would be greatly appreciated.

2. Jun 15, 2009

### glueball8

I think its just a math error. The energy is fine.

Mass of a electron is 9.10938188*10^-31.

Try putting it into your calculator again.

3. Jun 15, 2009

### ideasrule

There's nothing wrong with your equations. You probably just calculated the numbers incorrectly. Try again, and you should get 1.4e-10 m.