Coulomb’s law

Coulomb’s law –

In this topic we will discuss about the Coulomb’s law of electrostatic force between two charged particle separated by some distance .

Coulomb’s law –

According to Coulomb’s law , the force of attraction or repulsion between  two charged body is directly proportional to product of magnitude of their charges and inversely proportional to square of distance between them .

If Q1 and Q2 are two charges placed at distance ‘r’ from each other , then magnitude of force will be given as,

F α Q1.Q2 ………. (I) and

F α 1/ r2  …………(ii)

After combining these two we get

F α Q1.Q/r2

Or     F =  K Q1.Q/r2 where K is a constant given as

K= 1/4∏ϵ0 = 9 x 109 here ϵ0 is called permittivity in vacuum or air .

ϵ0 =8.85 x 10-12 C2‑N-1m-2 

Electric permittivity  of a medium is a measure of how the molecules of substance align or polarize under an electric field . The substance which has higher electric permittivity then its molecules polarization is maximum , and the more that substance resist the external electric field . It is denoted by  ϵ . its unit is C2‑N-1m-2 

For air its value is   given as

ϵ0 =8.85 x 10-12 C2‑N-1m-2 

To derive the Coulomb’s law using vector method click here-

To view the derivation video click here –

 

 

Dielectric constant or relative permittivity –     Dielectric constant of a given medium is the ratio of permittivity of the medium to the permittivity of free space (vacuum /air )

Or

Dielectric constant of a given medium is the ratio of force of interaction between two charges separated by a certain distance in vacuum/air to the force of interaction between two given charges at the same distance in that medium  .

Dielectric constant  K  ( ϵr )  =  ϵm / ϵo  = F0 /Fm  where ;

F0 =  force of interaction between two charges separated by a certain distance in vacuum/air

F0 =  force of interaction between two charges separated by a certain distance in the  given medium

*Dielectric constant is unit less .

To watch the video of relative permittivity and charge density click on the link given below –

 

Questions for the students after studying  click here- Some numericals for practice on coulomb’s law and properties of charges  

 

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