# Homework Help: Electric Potential Energy of electron orbits

1. Sep 23, 2008

### nckaytee

An electron and a proton are initially very far apart (effectively an infinite distance apart). They are then brought together to form a hydrogen atom, in which the electron orbits the proton at an average distance of 5.43 * 10-11 m. What is EPEfinal - EPEinitial, which is the change in the electric potential energy?

I have no idea where to start

2. Sep 23, 2008

### LowlyPion

Isn't the potential energy given by PE = kq1*q2/r

If EPE at infinity is 0, then ... The change is ...

3. Sep 23, 2008

### nckaytee

In my notes I have $$\Delta E.P.E$$ of electron = q(Vb-Va) = (-e)(Vb-Va)

I am really not getting this

Im sorry, I might have something here... One minute..

4. Sep 23, 2008

### nckaytee

Okay I am confused

5. Sep 23, 2008

### LowlyPion

What is the charge of an electron and a proton?

Last edited: Sep 23, 2008
6. Sep 23, 2008

### nckaytee

So, would it be...

(-1.6*10^19)(0 - 5.43 *10^-11)

7. Sep 23, 2008

### LowlyPion

It's the product of both charges.

PE = kq1*q2/r

And check the exponent of the charges. And don't forget k.

8. Sep 23, 2008

### nckaytee

Okay, so I did PE = [(9*10^9)(-1.6*10^-19)(1.6*10^-19)] / 5.43*10^-11

My answer is right!!! -4.24 * 10^18

Thank you

9. Sep 23, 2008

### LowlyPion

You're welcome.

And don't act so surprised. You had it nailed from kqq/r.

Good luck.