# Homework Help: Rearranging coulombs law equation

1. Apr 15, 2010

### Eranu

I have just started the physics part of a science foundation course and im just really struggling how to rearrange equations

I have been given a quesiton to rearrange the coulombs law equation as below, to make K the subject and i have been sitting here for hours and i am no further forward. Can anyone walk me through it please and you will be saving my sanity.

F=K*Q1*Q2/r2

Regards

2. Apr 15, 2010

### bobbajobb

Right. You need to get K on one side by itself, with nothing else on that side of the equals sign.

F=K*Q1*Q2/r2 (I don't know the Law, i'm assuming Q2/R2 should be in brackets)

Whatever you do to one side, you have to do to the other side as well, so its equal.

Divide by Q1 on both sides

F / Q1 = K * Q2/R2

See how there is less on K's side of the equals sign now?

Times by R2 on both sides

(F / Q1)*R2 = K * Q2

Divide by Q2 on both sides

(F / Q1)*R2*Q2 = K

See how for each step we remove one thing that is on the wrong side of the equation?

3. Apr 15, 2010

### snshusat161

I can't understand what you mean to say but I guess this would be your answer:

K = F* r^2/( Q1 * Q2)

4. Apr 15, 2010

### bobbajobb

Oh....I thought the R2 referred to something you put into the equation. Not R^2. I suppose I shouldn't really comment when i don't know the Law. :( Sorry.

5. Apr 15, 2010

### snshusat161

OP didn't ask us for the law. He asked to rearrange and anybody can try to explain him. No fault of Yours.

Edit: And your way of explanation is really amazing.

6. Apr 15, 2010

### bobbajobb

Thanks. :)

7. Apr 15, 2010

### Eranu

Thanks for the really quick replies - im sorry i wasnt clear enugh still getting the hang of trying to type equations on the board.

I tried to follow what u said bobbajob

i got something along the lines of K=(sqr root)of F/Q1Q2 but im guessing im miles off
The equation in its true form is at http://en.wikipedia.org/wiki/Coulomb's_law and is the first one on the page if it makes it clearer.

Thanks again

8. Apr 15, 2010