Grade-11 Electric Charges (Electrostatics)

Chapter1.

Electric Charges(Static Electricity)

What is Electricity

The branch of physics which deals with the study of charges (either at rest or in motion) is called the electricity.

What is Electrostatics

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

What is Current electricity:

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

What is Electric Charge:Charge is the inherent property of some fundamental particles because of which matter experience 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 charged due to friction.

When a glass rod is rubbed with silk and brought near small paper pieces it attracts them. The property that is 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, Plastic comb becomes electrically charged Which is conformed when is brought near To the small piece of papers It attracts the piece of paper.

What is Types of Electric charge:

Electric charges arebasically of two types,

1.Positive charge: fundamentally carried by protons

2.Negative charge: fundamentally carried by electrons

The body which losseselectrons becomes positivelycharged and the body which gainselectrons becomes negativelycharged.

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

Thus, electric charge is the fundamental property of every matter carried by sub-atomic particles such as electrons and protons.

What is Unit and Dimensions of Electric charge:

The unit of electric charge is 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 electric field and a negative charge experiences force in the opposite direction.

TheCGSunitofChargeisstat−coulomb.

1C=3×109Statcoulomb

1nC=10−9C

1μC=10−6C

Dimension

Charge (q) = 𝑐𝑢𝑟𝑟𝑒𝑛𝑡𝐼×𝑡𝑖𝑚𝑒𝑇

Or, 𝑞=[𝐼𝑇]or [𝐴𝑇]

What is Modern theory of electrification

The process of developing 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 positivity 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 of a number of protons are more than electrons.

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

Properties of electric charge:

1.Only two types of electric charges exists in nature.

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 charge distributed over different part of it.

4.Electric Charge is quantized.

The charge on a body is always an integral multiple of basic unit of charge. This basic unit is taken as the charge on an electron or proton which is equal to 1.6x10-19C.

i.e. Total Charge on a body (Q) = ±𝐍e

This is called quantization of charge.

Where, e = 1.6x10-19Cis 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 destroyed.

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

Conductors and Insulators:

What is Conductor:

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

What is 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 insulator.

What is Charging by Friction:

Charge particles can be transferred on 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.

Experiments with the different substances show that when any two substances rubbed together to produce electric charges the element occurring earlier on the following list becomes positively charged

1. Fur (rabbit’s) 2. Flannel 3. Glass 4. Mica 5. Cat skin

6. silk 7. Cotton 8. Wood 9. Human body 10. Amber

11. Resin 12. Metal (Ag, Cu, Ni) 13. Sulphur 14. Ebonite

15. Metal (Pt, An) 16. Celluloid

What is 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 a originally charged body is called inducing chargeand the charge in a conductor which is influenced by the charge on another body is known as induced charge.

Charging a body negatively by induction

Step (I):

If a positively charged glass rod is brought near a body PQ (Kept on a 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 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 in to the earth.

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

Fig1 : 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 whole surface of the body PQ as shown in figure 1(d). Therefore, the body PQ becomes negatively charged by induction.

The charge developed at near ends is called induced bound charge and the charge developed at far end is called induced free charge.

Charging a body positively by induction

•Step (I):

An ebonite rod is charge 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 flow into the earth since they are free charges. 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 uniformly through-out the body as in figure (2) d.

What is Surface charge density (σ):

Surface charge density of a charged conductor is defined as the charge per unit surface area of the conductor.

If charge Q is distributed uniformly over the surface area A of a conductor, then the surface density of charge is given by

σ = 𝑄𝐴

In SI unit, the unit of surface charge density is coulomb meter-2(C/m2).

What is Linear charge density (λ):

The linear charge density of charge conductor is defined as charge per unit length of the conductor.

If charge Q be distributed on a conductor of length

λ = 𝑄𝑙

In SI unit, the unit of linear charge density is coulomb meter-1(C/m).

What is Volume charge density (ρ):

The volume charge density is defined as the amount of charge per unit volume in conductor.