RKMHSSCH17XIIB PHYSICS, Lesson 1 : Electrostatics

1. Electrostatics  

Austin, Science Teacher

Electrostatics is the branch of Physics, which deals with static electric charges or charges at rest. In this chapter, we shall study the basic phenomena about static electric charges. The charges in an electrostatic field are analogous to masses in a gravitational field. These charges have forces acting on them and hence possess potential energy. The ideas are widely used in many branches of electricity and in the theory of



From: Rajesh Subramaniam, XII B                                   12th August 2017,    8.12 p.m.

                       Sir,
                              What is the theory of atom?
                                                               
    Reply      (From : Austin, Science Teacher)                   12th August 2017,   8.45 p.m.
                   
                            Rajesh,  I think you haven't watched the Video 1. It contains the explanation for atom                                    theory. Anyway, i will explain this one in tomorrow's session.

                                                                   
     From : Rajesh Subramaniam, XII B                                12th August 2017,    9.10 p.m.

                              Thanks Sir. I watched the video and I got it.
     

                                                                                        

VŹ

                                                 Video 1 : Electrostatics                                                 

1.1  Electrostatics - frictional electricity

In 600 B.C., Thales, a Greek Philosopher observed that, when a piece of amber is rubbed with fur, it acquires the property of attracting light objects like bits of paper. In the 17th century, William Gilbert discovered that, glass, ebonite etc, also exhibit this property, when rubbed with suitable materials.

           

   

                                                                                       Thales, Greek Philosopher

The substances which acquire charges on rubbing are said to be 'electrified' or charged. These terms are derived from the Greek word elektron, meaning amber. The electricity produced by friction is called frictional electricity. If the charges in a body do not move, then, the frictional electricity is also known as Static Electricity.                                
                                 




                                                    Frictional Electricity Video from YouTube

                     

1.1.1    Two kinds of charges

(i)    If a glass rod is rubbed with a silk cloth, it acquires positive charge while the silk cloth acquires an equal amount of negative charge.

(ii)    If an ebonite rod is rubbed with fur, it becomes negatively charged, while the fur acquires equal amount of positive charge. This classification of positive and negative charges were termed by American scientist, Benjamin Franklin.

                                                                      Benjamin Franklin


Thus, charging a rod by rubbing does not create electricity, but simply transfers or redistributes the charges in a material.




           
1.1.2    Like charges repel and unlike charges attract each other

Experimental verification.

A charged glass rod is suspended by a silk thread, such that it swings horizontally. Now another charged glass rod is brought near the end of the suspended glass rod. It is found that the ends of the two rods repel each other (Fig 1.1). However, if a charged ebonite rod is brought near the end of the suspended rod, the two rods attract each other (Fig 1.2). The above experiment shows that like charges repel and unlike charges attract each other.

                                                Picture: Ebonite Rob Rubbed with Fur


                                               Hide Doubts (2)

I have some doubt in the following statement quoted in our textbook " If a glass rod is rubbed with a silk cloth, it acquires positive charge while the silk cloth acquires an equal amount of negative charge." Is there any video or any kind of published documents available for this statement. Because still I haven't got the idea.                              
S. Dinakaran, XII  B                   0th Feb 2017              19.47                          

                                                                                                                          

If a glass rod is rubbed with ordinary cotton cloth what will happen? Likewise, what will be the outcome of glass rob with other materials like polyester, wool etc? I need some clarification.

K. Srividhya, XII B                   20th Feb 2017                18.35

The property of attraction and repulsion between charged bodies have many applications such as electrostatic paint spraying, powder coating, fly ash collection in chimneys, inkjet printing and photostat copying (Xerox) etc.





1.1.3 Conductors and Insulators

According to the electrostatic behaviour, materials are divided into two categories : conductors and insulators (dielectrics). Bodies which allow the charges to pass through are called conductors. e.g. metals, human body, Earth etc. Bodies which do not allow the charges to pass through are called insulators. e.g. glass, mica, ebonite, plastic etc.

1.1.4 Basic properties of electric charge

(i)    Quantisation of electric charge

The  fundamental  unit  of  electric  charge  (e)  is  the  charge

carried by the electron and its unit is coulomb. e has the magnitude 1.6 × 1019 C.

In nature, the electric charge of any system is always an integral multiple of the least amount of charge. It means that the quantity can take only one of the discrete set of values. The charge, q = ne where n is an integer.

Conductors and Insulators PDF file to download

The remaining part of the lesson with attached learning materials will be sent by tomorrow.