# Homework Help: Force of repulsion of alpha particle

1. Aug 18, 2011

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

An alpha particle has a mass m = 6.64x10^-27 kg and a charge q = +2e.
Compare the force of electric repulsion between two alpha
particles and the force of gravitational attraction between them. Explain briefly
why the gravitational force is ever significant, given its seeming insignificance
here.

2. Relevant equations
F=qE, F=kqq/r, F=ma

3. The attempt at a solution
coloumbs law cant be applied here since im not given r
and i cant think of a way to obtain the E_field or a in f=ma when i equate qE=ma
(maybe Electric flux can be found using gauss's, but i dont think thats the right way to go)

Some guidance would be much appreciated

thanks

2. Aug 18, 2011

### fzero

Both the magnitude of the electromagnetic force and that of the gravitational force depend on the distance between sources. Can you come up with a mathematical combination of the two that is independent of the distance?

3. Aug 18, 2011

im not sure how eliminate the distance

F_e=kqq/r
F_g=Gmm/r^2

equating those 2 will still leave me with distance
I cant think of any other formula for F_e that doesnt require finding the electric field or using the distance

4. Aug 18, 2011

### fzero

First, go back to your text or notes and find the correct expression for the Coulomb force (what you have is the potential energy). Second, you don't really want to equate the EM and gravitational forces, but the correct expression should be clearer once you can compare the correct dependences on the separation.

5. Aug 18, 2011