## Coulombâ€™s Law â€“ Force between two point charges

**Note**:- This law is strictly applied to a stationary point charges.

*between the charges.*

__forceÂ__*attractiveÂ*orÂ

*repulsiveÂ*in nature. If both the charges are of same nature that isÂ

*both are positive or both are negative then repulsive force act.*

*If one charge is positive and other is negative then attractive force act between them.*

__, interaction (force) acting between two point charges isÂ__

*Coulombâ€™s law*(i) directly proportional to product of magnitude of charges

[latex]F \propto q_{1} \times q_{2} \text{ ……………(i)} [/latex]

and (ii) inversely proportional to square of distance (i.e separation )Â between the charges.

[latex]F \propto \frac{1}{r^{2}} \text{……………(ii)} [/latex]

Â

combining both equations

[latex]F \propto \frac{q_{1} \times q_{2}}{r^{2}}Â [/latex]

[latex]F = k \frac{q_{1} q_{2}}{r^{2}}Â [/latex]

Here k is a constant of proportionality and also known as electrostatic force constant for coulomb law.

[latex]k = \frac{1}{4\pi \varepsilon _{0}} \text{…..(This is for Air / Vacuum / free space)}Â [/latex]

The value of k is different for different type of medium.

[latex]k = \frac{1}{4\pi \varepsilon } \text{ …..(This is for medium only)}Â [/latex]

[latex]where \hspace{1cm} \varepsilon = \varepsilon_{0} \times \varepsilon_{r}Â [/latex]

Here â„‡ <sub>0</sub> is known as absolute electrical permittivity of air/vacuum/free space between the two charges.

Here â„‡ is known as absolute electrical permittivity of the dielectric medium between the two charges.

Here â„‡r is known as relative electrical permittivity.