# Proportionality calculations (Coulombs Law)

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.

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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.
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

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

alphysicist
Homework Helper
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?