# Rearange equations

1. Apr 14, 2010

### helpwiysciene

F = A Q1Q2/r2

simple steps to make A the subject and the SI units i should use?

2. Apr 14, 2010

### thebigstar25

3. Apr 14, 2010

### helpwiysciene

yes sorry i need simple steps to make a the subject of this equation

4. Apr 14, 2010

### helpwiysciene

this is coulombs law and i als need to use the correct SI unts to represent A

F = force
Q1 = Charge 1
Q2 = Charge 2
A is the constant
and r2 is the sepeation

5. Apr 14, 2010

### irycio

Still not sure, but, well, if i guess correctly:

0. F=A Q1Q2/r2 (i would expect to be r^2, but the steps are the same ;P) // multiply both sides by r2

1. F*r2=A Q1Q2 // Divide both sides by Q1Q2 (or first by Q1, and then by Q2)
2.F r2/Q1Q1=A
3.A=Fr2/Q1Q2

life can be that simple :P
But i still believe it's r^2

E: Units A=[N*m^2/C^2]=[kg*(m/s^2)*m^2/(As)^2]=[kg*m^3/(A^2*s^4)]... I hope it's fine XD

6. Apr 14, 2010

### thebigstar25

well, im not sure if i understood your question ..

anyway .. you know that:

F : is measured in newton (N)
r : is measured in meter (m)
Q1, Q2 : in columb (c) ..

so the constant A should have a unit of ( N * m^2 / c^2) , which usually is taken as 9x10^9

7. Apr 15, 2010

### helpwiysciene

great thankyou very much, how do you work out Nm^2 / c^2 are the SI units and what does Nm^2 actualy stand for, i am struggling to understand what a constant actualy is and how it is measured?

8. Apr 15, 2010

### irycio

Newtons and Coulombs are not SI units. I mean.. SI consists of just a couple of basic units:
kg
m
s
A
cd
K
mol

All other units derive from those 7. Hence if you want "pure" SI units, take a peek at my previous post.