Coulombs Law

You should already be familiar with the fact that two charged objects would either attract or repel each other with a force.

There is an equation to calculate the size of this force.
This equation is called Coulombs Law

Call one charged object Q1 and the other Q2.
Of course, the size of this force depends on how far apart they are.
Call this distance between them r.

Put all this lot together, and we get an equation called Coulombs Law.

F   : the magnitude of the force
k   : Coulombs constant of value 9 x 109
Q1 : the magnitude of one of the charges
Q2 : the magnitude of the other charge
r   : the distance between the centers of the charges
Coulombs Law in Words
The magnitude of the force between two charges is:
• directly proportional to the product of the charges
• and inversely proportional to the square of the distance between their centers.

Essentially what we want to do is to find out how much force these two charged objects are applying on each other. Remember that even though the two CHARGES may be of DIFFERENT sizes, the FORCES they exert on each other is always the SAME SIZE. (Newton's Third Law).
Example 1
A small object has a charge of 5nC and is placed 0,04m away from another of charge -6nC.
1. Calculate the magnitude of electrostatic force between them.
2. Compare the size of force that each object exerts on the other.
3. State the nature of the forces.


1. Note that the -charge of -6nC was substituted as a positive. This is because we are working with magnitudes only.

2. The forces would be of the same size.
Although the equation shows only ONE force, there are actually TWO FORCES, one for each object.

3. attractive forces
The charges are unlike, and therefore attractive.

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