What is Coulombs Law?

Where does charge come from?

Electric field lines between a positive and negative charge


While there really isn't an actual answer to where charge comes from -we have defined charged to be the thing that produces and electric field. Static electrons are charged and their charged is quantized meaning an integral number. We say that the charge on an electron is -e.

From the previous section on electric charges and electric fields, we know that two unlike charges will attract each. The force of attraction and electric field will increase as the two charges approach each other. In the case of two similar charges there will be a force of repulsion as the charges get closer and the force will decrease when the charges are moved further apart. This is because electric charges make an electric field. If two fields exist in the same space at the same time, then the two fields exert a force on each other. The force they make on each other is called Coulomb's force or electrostatic force ( electrostatic means electric charges without any motion).

Coulomb's law explains how big the force will be.

coulomb law formula


Coulombs law explains that the Force F is relative to the ratio of q1, q2, 1/r2.

q1 and q2 are the scales of each charge and r is the distance between the two electric charges. Kc is a constant value that does not change. Kc is called Coulomb's Force Constant or Electrostatic Force Constant.

Why is Coulombs Law related to the Inverse Square Law?

The relation between the force of push or pull (F) and the distance between the particles (r) follows the inverse square law in a similar manner as that shown for gravity, magnetism and light intensity. The inverse square law means the as distance increase the force (F) will decrease by the ratio of 1/r2. For more about the inverse square law see of activity labs on Mathematical Relationships in Science.

Why is that Force is inversely proportional to square of the distance between them?

inverse square law

All of these inverse square laws are related to the surface area of a sphere. For example take a beam of light from a source S (shown above)

Note: the Surface area of sphere - A = 4*pi*r2.

Assume the light source center (S) produces the light. Surround this with a sphere. As the sphere grows the amount of light falling on its surface is reduced by the r 2.

Test your Understanding:

1. Which statement is true about charge
a) unlike charges will repel each other
b) charge is quantized
c) the charge of an electron can vary
d) only moving electrons have a charge

2. Coulombs law is an example of which mathematical relationship
a) linear relationship
b) )inverse relationship
c) inverse square relationship
d) exponential relationship

3. The inverse square law means
a) the distance between charges increases the force will decrease in a linear fashion
b) The inverse square law means the as distance increase the force (F) will decrease by the ratio of 1/r2.
c) as the distance decreases the force will increase by the ratio of 1/r

4. A force is defined as
a) the ability to do work
b) the weight of an object
c) a push or pull
d) F= mass x velocity

5. The surface area of a sphere can be found using the formula
a) A = 4*pi*r^2
b) A = 4/3*pi*r^3
c) 2 pi. r