Let the position vectors of
charges

*q*_{1}and*q*_{2}be**r**_{1}and**r**_{2}respectively**Fig.**We denote force on*q*_{1}due to*q*_{2}by**F**12 and force on*q*_{2}due to*q*_{1}by**F**21. The two point charges*q*_{1}and*q*_{2}have been numbered 1 and 2 for convenience and the vector leading from 1 to 2 is denoted by**r**_{21}:**r**

_{21}=

**r**

_{2}–

**r**

_{1}. In the same way, the vector leading from 2 to 1 is denoted by

**r**

_{12}:

**r**

_{12}

_{ }=

**r**

_{1}

_{ }–

**r**

_{2}= –

**r**

_{21}

_{ }.The magnitude of the vectors

**r**

_{21}and

**r**

_{12}is denoted by

*r*

_{21}and

*r*

_{12}, respectively (

*r*

_{12}=

*r*

_{21}). The direction of a vector is specified by a unit vector along the vector. To denote the direction from 1 to 2 (or from 2 to 1), we define the unit vectors:

Coulomb’s force law between
two point charges

*q*_{1}and*q*_{2}located at**r**_{1}and**r**_{2}is then expressed as
If

*q*_{1}and*q*_{2}_{ }are of the same sign (either both positive and both negative),**F**_{21}is along ˆ**r**_{21}, which denotes repulsion, as it should be for like charges. If*q*_{1}and*q*_{2}are of opposite signs,**F**21 is along – ˆ**r**_{21}(= ˆ**r**_{12}), which denotes attraction, as expected for unlike charges.
Thus, we do not have to write
separate equations for the cases of like and unlike charges. The force

**F**12 on charge*q*_{1 }due to charge*q*_{2}, is obtained from force**F**21, by simply interchanging 1 and 2, i.e.
Thus, Coulomb’s law agrees
with the Newton’s third law.

___________________________________________________________________________________________

Q2: Does motion of a body affect its charge?

Q3: What is the origin of frictional forces?

**Some of these questions which may be asked in your Board Examination 2012-2013****Q1: When a plastic comb is passed through dry hair, what type of charge is acquire by comb?**

Q2: Does motion of a body affect its charge?

Q3: What is the origin of frictional forces?

**Answer these questions in comment box and help your friends**
the comb should accquire a negative charge because your hair has a natural tendency to give up electrons

ReplyDeleteAns2.no charge on a body does not change with motion of the body

1. positives (+ev) 2.yes, more motions more charges 3. From the surface of the rubbing body

ReplyDeletehow this r sq. inverse proportionality is proved theoritically

ReplyDeleteif charge depends on mass and mass changes with motion then why charge does not change with motion...

ReplyDeleteSince our body has a high no of protons and electrons its has a tendency to gain the electrons and the charge passes on to the earth and thus comb acquires a positive charge.and attracts neutral small objects.

ReplyDelete