Page 7 - Electrostatics 11

Grade 11 © GC Shiba

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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

= 1 2 ; where k is proportionality constant.
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
-1
-2
permittivity of the free space having value 8.85 × 10 −12 C N m (Fm )

9
-2
2
and the value of 1 = 9 × 10 Nm C .
4 ∈ 0
7 Electrostatics