Page 10 - Electrostatics 11 class

Grade 11 © GC Shiba
Coulomb's law of electrostatics:

The force acting between two charges is

➢ directly proportional to the product of charges
➢ inversely proportional to the square of distance between the charge.

Q1 Q2

r
Let us consider two charges Q1 and Q2 are at a distance r. The force acting between
two charges is F.

According to Coulomb's law, the force acting between two charges is
i. directly proportional to the product of charges

i.e., ∝ ..................... ( i )
1 2
ii. inversely proportional to the square of distance between the charge.
1
i.e., ∝ ..................... (ii)
2

Combining equations (i) and (ii), We get

1 2
∝
2

= ; where k is proportionality constant.
1 2
2

The value of k depends upon unit chosen and medium on which charges are kept.

For free space in cgs unit, k = 1

1 2
∴ =
2

For free space in SI unit, k is replaced by new constant 1 Where ∈ is
0
4 ∈ 0
2
-1
-2
-1
permittivity of the free space having value 8.85 × 10 −12 C N m (Fm )
and the value of 1 = 9 × 10 Nm C .
9
2
-2
4 ∈ 0

1 2
∴ =
4 ∈ 2
0
For medium in SI unit, k is replaced by new constant 1 Where ∈ is
4 ∈
permittivity of the medium.
10 Electrostatics