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 Ë†**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 – Ë†**r**_{21}(= Ë†**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

R u shure this answer is right

DeleteSay

Delete1. 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.

ReplyDeleteðŸ‘ŒðŸ‘ŒðŸ‘Œ

ReplyDeleteVisit this blog for cool gizmos:

ReplyDeleteSi-Figadgets.blogspot.in

This comment has been removed by the author.

ReplyDeletewhy we take r21 for the direction from q1 to q2. In some books for the direction of q1 to q2 they simply took r12 as a unit vector. Please tell me what should we take as an unit vector

ReplyDelete