# Homework Help: Electricity~ Coulomb's Law

1. Aug 16, 2008

### dummie90

1. The problem statement, all variables and given/known data
By what factor must you change the distance between two point charges to change the force between them by a factor of ten?

2. Relevant equations
F1=kq1/r^2
F2=kq2/r^2

3. The attempt at a solution
F1:F2= 10
r1/r2=sq.root(F2/F1)= 1/sqroot10

I do not understand how to get to r1/r2=sq.root(F2/F1)= 1/sqroot10
I do understand that you square root F2/F1 to get rid of the square of r. but why is F2 on top of the fraction instead of F1 (i.e. sqroot F1/F2) and how does that become 1/sqroot 10? And how did that equation even come about?

Sorry I'm very confused I've tried to make sense of this worked example many times,
Thank you so much for the help
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Aug 16, 2008

### tiny-tim

Welcome to PF!

Hi dummie90! Welcome to PF!

(have a square-root: √ )

You're very confused because your equations are wrong.

They should be:

F1=kqQ/r1^2
F2=kqQ/r2^2

now can you see how it comes out as √(F2/F1)?

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