# Proportionality calculations (Coulombs Law)

1. Apr 25, 2008

### whitestrat

Two unlike charges attract each other with a force of 5 x 10^-5 when placed 12 cm apart.

1) What is the force between them when they are 4.0cm apart

2)At what distance apart will the force between them be 2.0 x 10^-4

now i have attempted these questions, but i cannot find a suitable method of solving these questions that is consistent with all types of these questions...
Can anyone be willing to show me a method of solving these types of questions.

2. Apr 25, 2008

### kamerling

you know Coulombs Law?

$$F = \frac { k q_1 q_2 }{r^2}$$

In this problem you have F = 5 x 10^-5 N if r = 5 cm. What would F be if r = 4.0 cm

3. Apr 26, 2008

### whitestrat

yes i know coulombs law, its just the question is worked out using proportionality from coulombs law

4. Apr 26, 2008

### alphysicist

Hi whitestrat,

Coulomb's law is an inverse square law, so, for example, if you double the distance between the charges the force decreases by a factor of four; if you halve the distance, the force increases by a factor of four.

Here the distance goes from 12cm to 4cm. So what happens to the force?