Electrostatics Grade 11 notes

Electrostatics

The branch of physics which deals with the electrical charges at rest is called electrostatics or static electricity.

Current electricity:

The branch of electricity that deals with the charge in motion is called current electricity

Electric Charge:

The charge is the inherent property of some fundamental particles because of which matter experiences the force in the electromagnetic field.

In electrostatics, electric charges are produced by friction between two bodies due to the transfer of electrons from one body to another without their actual movement. So, an object can be changed due to friction.

When a glass rod is rubbed with silk and brought near small paper pieces it attracts them. The property developed

 

in glass rod on rubbing with silk is supposed to be positive charge and that developed in silk is supposed to be the negative charge.

When a plastic comb is rubbed with hair, a plastic comb

becomes electrically charged Which is conformed when is

brought near To the small piece of papers It attracts the piece of paper.

Types of Electric charge:

Electric charges are basically of two types

1. Positive charge: Positive charge is fundamentally carried by protons.

2. Negative charge: Negative charge is fundamentally carried by electrons.

The body which losses electrons become

positively charged and the body which gains

electrons become negatively charged.

A particle or an object which has net-zero charges is referred to as neutral. Neutrons are neutral.

Thus, electric charge is the fundamental property of every matter carried by subatomic particles such as

electrons and protons.

Unit and Dimension of Electric Charge

The unit of electric charge is the coulomb (C).

The charge is the product of two fundamental quantities: Electric current (I) and time (T)

The electric Charge(q) = Current(I) x Time(t)

Thus,

Electric charge is the scalar quantity and is usually denoted by “q”. Its SI unit is Coulomb(C) or Ampere second.

Note:

Remember that a positive charge experiences force along the direction of the electric field and a negative charge experiences force in the opposite direction.

The CGS unit of Charge is stat−coulomb.

1C=3×10⁹ Statcoulomb

1nC=10⁻⁹C

1μC=10⁻⁶C

Dimension

Charge (q) = 𝑐𝑢𝑟𝑟𝑒𝑛𝑡𝐼×𝑡𝑖𝑚𝑒𝑇

Or,

𝑞=[𝐼𝑇]or [𝐴𝑇]

The modern theory of electrification

The process of developing a charge in a body is called electrification.

According to this theory, a body can be charged by the transfer of electrons. Every material is composed of atoms. In an atom, the number of positively charged protons must be equal to the number of negatively charged electrons.

So, it is electrically neutral.

By any process, if some of the electrons are removed, the body becomes positively charged because several protons are more than electrons.

On the other hand, the material that receives electrons acquires a negative charge because of extra electrons added.

Properties of electric charge:

1. Only two types of electric charges exist in nature. They are positive and negative.

2. Like charges repel and unlike charges attract each other.

3. It is a scalar Quantity, So the total charge on the body is the algebraic sum of all charges distributed over different parts of it.

4. Electric Charge is quantized.

The charge on a body is always an integral multiple of the basic unit of charge. This basic unit is

taken as the charge on an electron or proton which is equal to 1.6x10 19 C.

i.e. Total Charge on a body

(Q) = ±𝐍e

This is called the quantization of charge.

Where, e = 1.6x10 19 C is charge of an electron and N= 0,1,2,3,4,……...

5. Electric charge is conserved, i.e. charge can neither be created nor be

6. The electric charge on a body does not depend on its speed.

Conductor:

Materials through which electricity (charge) can pass easily are called conductors. Generally, the metals like copper, silver, iron, acidic water, the human body, earth, etc. are the conductors of electricity.

Conductors can be charged by friction and the charges they gain spread over the whole surface.

Conductors like metals possess free electrons.

Metals like copper, silver, gold, and aluminum are very good conductors of electricity.

Insulator:

Materials which do not allow the electric charges to pass through them are called the insulators. Glass, wood, dry paper, rubber, ebonite, air, etc. are the insulators or non-conductors of electricity.

Insulators can gain charges by friction but the charges can’t move to other parts and remain localized.

Insulators do not possess free electrons.

Fused quartz is a very good example of an insulator. 

Charging of a Body:

The charging of a body is the process in which a body gains or loses charged particles (electron). There are three method for the charging of a body.

•    Friction.

•    Conduction.

•    Induction.

Charging by Friction:

Charge particles can be transferred by rubbing a body to another.

While rubbing, the electrons on a body move to another body. One which losses the electrons, gets positively charged and the other, which gains the electrons, gets negatively charged.

Nature of charge in rubbing pair materials
S.N.	Pair materials	Positively charged	Negatively charged
1	Glass rod, Silk Cloth	Glass Rod	Silk cloth
2	Fur, ebonite rod	Fur	Ebonite rod
3	Woolen cloth, Plastic pen	Woolen Cloth	Plastic Pen
4	Woolen carpet, rubber	Woolen Carpet	Rubber
5	Dry hair, plastic comb	Dry Hair	Plastic Comb

Charging a body by conduction

When a neutral body is brought into contact with a charged body some charges flow into it and become charged. The charge acquired by the neutral body is similar to that of the charged body.

Fig. charging of a metallic sphere by conduction

 

Charging by Induction: The temporary charging of a body when it is brought nearer to a charged body without touching it is known as induction. The charge on an originally charged body is called inducing charge and the charge in a conductor which is influenced by the charge on another body is known as induced charge. The charge developed at near ends is called induced bound charge and the charge developed at the far end is called induced  free charge.

(a) Charging a body negatively  by induction

Step (I):

If a positively charged glass rod is brought near a body PQ  (Kept on an insulating stand), the end P of the body acquires bound negative charges and the far end Q acquires free positive charges due to induction as shown in figure 1(a).

Step (II):

The body PQ is earthed with the help of the metal wire as shown in figure 1(b). In this case, the free positive charges flow to the earth. However, the negative charges being the bound charges do not move into the earth.

Step (III): The earthed is removed keeping the glass rod still in its position. The bound negative charges remain at end P as shown in figure 1 (c).

FFig1: Charging a body negatively by induction Step (IV):

Finally, the positively charged rod is removed away from the body PQ. The bound negative charge spread over the whole surface of the body PQ as shown in figure 1(d). Therefore, the body's PQ becomes negatively charged by induction.

Ebonite rod

•       Step (I): An ebonite rod is charged negatively by rubbing it with fur. The negatively charged ebonite rod is brought close to the body PQ to be charged as in figure (2) a. In doing so, the end P of the body acquires positive charges, and the end Q acquires negatives charges.

•       Step (II):

The conductor is earthed as shown in figure (2) b. The negative charges in the body PQ flows into the earth since they are free of charge. However, the positive bound charges remain in the body.

•       Step (III):

The earthing is removed from the body PQ keeping the ebonite rod as it were as in figure (2) c.

•       Step (IV):

The negatively charged ebonite rod is removed. In doing so, the positive charges on the body PQ became free and get distributed     Fig 2: Charging a body positively by induction uniformly throughout the body as in figure (2) d. 

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