Thursday, 4 April 2013

Coulomb's Law

It states that the force of attraction or repulsion between two point charges, q1 and q2 at rest placed in "free space" separated by a distance r, is directly proportional to product of the charges and is inversely proportional to the square of distance between them i.e.
Here  is a constant, its value is 9 x 109 N-m2/C2
eo is called permittivity of free space and is equal to 8.85 x 10-12 C2/N-m2.

Permittivity is electrical property of a medium or that of free space.

Note that the term "free space" means that there is no medium between the charges, not even air.

When the space around the two interacting charges is filled by a medium, the force experienced by each of them changes.

If the charges are placed in a medium, the force experienced by each charge is given by
Note that the above expression, " e" has been replaced by " e ". It is the absolute permittivity of medium.

Relative Permittivity or Dielectric Constant (er or K)

It represents the ratio of absolute permittivity of a medium and absolute permittivity of free space.

The range of dielectric constant K is between '1' and '¥'.

Using Coulomb's Law:

You have just read an expression for the force of interaction between the charges. Before you start using the above expression in different problems, you must learn the following points about the force. Carefully study each of them.

(i) The force between the two charged particles is a central force. Following two figures describe the difference between a central and non central force.
(ii) The force is attractive , when the charges have opposite nature, i.e, one of the charge is positive while the other is negative. See the following figure

The positive charge is attracted to right and negative charge is attracted to left.

(iii) The force is repulsive , when the charges have same nature, i.e.; either both are positive or both are negative.
Vector Form of Coulomb's Law

A charge q1 is placed at A whose position vector is 

Another charge q2 is placed at B whose position vector is , such that | AB | = r

The magnitude of force is given by,
Force on q2 due to q1, in vector form is given by
     where  ia a unit vector along line joining A and B, pointing from A to B.

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