Coulomb's law

COULOBM’S LAW

“The force of (attraction and repulsion) between two point charges is directly proportional to the product of the magnitude of charges and inversely proportional to the square of the distance between them”

The first attempt to measure the force between electric charges was made by a French military engineer Charles coulomb in 1784. He deduced a law known as coulomb’s law.

Explanation

Consider two point charges “q1” and “q2” placed at a distance “r” from each other. Then magnitude of force according to coulomb’s law is given by:

Where k is constant of proportional called electrostatics constant.

Dependence of k

Its value depends upon

The system of units

Nature of medium between the charges

For free space

If the medium between two points charges is free space then

Where ε is an electrical constant known as permittivity of free space in S.I. units its value is

Thus coulombs force for free space is:

Mutual force

F is the magnitude of mutual force that act on each of two points charges

The force F always acts along the line joining the two points charges.

As coulomb is mutual force thus charges q1 and q2 exert equal and opposite forces on each other.

Vector form

If the force exerted on q1 by q2 is denoted by F21and that on charge q1 due to q2 as F12 then,

The magnitude of these two forces is same but negative sign show that they point in opposite direction.If r21 is the unit vector directed from q1 to q2 and r12is the unit vector directed from q2 to q1 then forces are given as

The sign of charges represents either the forces are attractive or repulsive

This also impulse that coulomb law is in accordance of newton third law.

Effect of medium on coulombs force:

If an insulating medium called as dielectric is placed between the two charges then it is experimentally observed that the force is reduced as compared to free space.

The factor by which the force is reduced is called relative permittivity and is denoted by ε_r

Thus the force between charges in the presence of dielectric is given by.

The value of relative permittivity is different for different dielectrics.

The value of ε_r is 1.0006. (table 12.1)

The value of ε_r is infinity for all metals

It has no unit and hence is a dimensionless quantity.

Permittivity

The ability of a medium to allow the electric force to pass through it is determined by a factor known as permittivity

Relative permittivity

It is ratio between permittivity of medium to the permittivity of free space

Point charge:

A charge is said to be a point charge if its size is very small as compared to the distance from any other charge.

View more on psalmfresh.blogspot.com for more educating and tricks article Thank you