# Coulomb's law

1. Oct 17, 2015

### Drizzy

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

A small ball has been loaded so that its charge is -2.6 nC. Another similar ball has the charge +1.2 nC. The balls are located at the distance 2.5 cm from each other . With how much force do the balls attracts each other?

2. Relevant equations

F = k * (Q1 * Q2)/r2

3. The attempt at a solution

k = 8,99 * 109
Q1 = -2.6 n
Q2 = +1.2 n
r2 = 0,00252

F = 8,99 * 109 (-2.6 * 1.2)/0,00252
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This is what my book has written:

F = 8,99 * 109 (-2.6 * 10-9 1.2 * 10-9)/0,00252

So my question is: From where did they get 10-9?

2. Oct 17, 2015

### ehild

What do you think nC means?

3. Oct 17, 2015

### Drizzy

It means how many electrons and atoms there is in the ball but how am I supposed to know how big the ball is and what material it is?

4. Oct 17, 2015

### Staff: Mentor

milli, micro, nano, pico, ... ring a bell?

You shouldn't care about the ball size or material. The problem says they are small so you can assume that their exact dimensions can be ignored and won't interfere with your calculations. In other words, assume that they won't touch or overlap when they are set 2.5 cm apart.

5. Oct 17, 2015

### Drizzy

oh yeah nano is 10-9 I thought n was the number of electrons in the object (lika a variable) but now I know what it is thank you for the help :)