What is Coulomb’s Law?

What is Coulomb’s Law?Coulomb law was firstly published by Charles  Augustin de Coulomb in 1785.The  apparatus used in finding coulomb law is known as Torsion Balance .By the use of Torsion Balance we can accurately measure the  force and distance between the two charges.

It state that;

The Force ( attraction or repulsion) between charges which  is directly proportional to the product of magnitude of charges and inversely proportional to the square of distance between them.

Explanation of Coulomb’s Law

Consider two charge bodies of charge q1   and q2 are separated by distance r . Then the force is represented by

                             F q1 x q2 …….. (1)

                                       F  1/r  ……………(2)

BY combining   1 and 2 we get

                          F q1 x q2 /r                        

 For changing the proportionality in equation we should add Constant

F = K x q1 x q2 /r

Where K is constant of proportionality known  as coulomb constant .

Factor Coulomb constant depand

The value of K is depend on following factors ;

1. SI unit of international
2. Medium between charges

For Coulomb law the two charges must be point charge mean that the distance the charge is smaller then the size of charges .The force of interaction between charges is attractive if the charges have opposite signs and repulsion if the charges have same charges.

Vector form

Let r12 unit vector show the direction of force   F1 by  charge q1 toward q2 .

F1= K x q1 x q2 /r (r12)

And r21 unit vector show the direction of force F2  by charge q2 toward q1.

F2= K x q1 x q2 /r (r21)

If the both charges have same sign then the product q1q2 is positive and the direction of force is given by vector r21 . If the charges have different   sign then the product q1q2 is negative and the direction of force is given by vector r12.

Limitation

1. Charge must be point
2. Charges must be stationary
3. Charges must has spherically symmetric distribution

Value of Coulomb constant  

             K=

      Where the value of permittivity of free ɛ• is = 8.85✕ 10 -12 Faraday/meter

So the value of k will be  8.9875517×10⁹ N·m²/C^{2    .}

Value of coulomb constant of some substance

Materials	coulomb constant	Materials	coulomb constant
Air dry	1.00059	Polyviniyl chloride	3.4
Bakelite	4.9	procrlain	6
Fused quertz	3.78	Pyrex glass	5.6
Neopene rubber	6.7	Silicon coil	2.5
Nylon	3.4	Strontium titanate	233
paper	3.7	Vacuum	1.00
polystyrene	25.6	water	80

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