Electrostatics Capacitors Electric Flux Notes With JEE NEET MCQS Physics Class 12 CBSE Study Material Full Chapter Download pdf-Anand Classes

What is Electrostatics?

Electrostatics is a branch of physics that deals with the phenomena and properties of stationary or slow-moving electric charges. Electrostatic phenomena arise from the forces that electric charges exert on each other and are described by Coulomb’s law. Even though electrostatically induced forces seem to be relatively weak.

Conductors, Insulators, and Semiconductors

• A body in which electric charge can easily flow through is called a conductor (For example, metals).
• A body in which electric charge cannot flow is called an insulator or dielectric. (For example, glass, wool, rubber, plastic, etc.)
• Substances which are intermediate between conductors and insulators are called semiconductors. (For example, silicon, germanium, etc.)

What Is an Electric Charge?

Atoms are the building blocks of the universe. Whatever you see around you can be divided into smaller and smaller parts until you finally reach a part you cannot divide further. This building block is what we call an Atom. Inside an atom are protons, electrons and neutrons. Out of the three, electrons and protons fit the definition of an electric charge. The protons are positively charged, the electrons are negatively charged, and the neutrons are neutral. A majority of the mass of the atom is concentrated in a very tiny space in the centre called the nucleus and the electrons revolve around this heavy nucleus.

This means that electrons are held very loosely compared to protons. Therefore, the movement of charges here will be restricted to the movement of electrons. Since the atoms are made up of protons and electrons, we can safely conclude that all things are made up of electric charges. The charge of one proton is equal in strength to the charge of one electron. When the number of protons in an atom equals the number of electrons, the atom itself has no overall charge, it is neutral.

Static Electricity

Static electricity refers to an imbalance between the electric charges in a body, specifically the imbalance between the negative and the positive charges on a body. The imbalance in the charge is introduced by physical means. One of the most common causes of static electricity is contact between solid objects. It was mentioned earlier that the movement of protons is not possible and the only movement of electric charge seen in static electricity is electrons.

Electrons in materials are held extremely loosely meaning that they can be exchanged through simple contact like rubbing. The image below is an example of rubbing a glass rod with silk which causes static electricity. When two objects are rubbed together to create static electricity, one object gives up electrons and becomes more positively charged while the other material collects electrons and becomes more negatively charged. We should keep in mind that the rules such as like charges repel and unlike charges attract is applicable here.

Electric Lines of Force

The line of force is the path along which a unit +ve charge accelerates in the electric field. The tangent at any point to the line of force gives the direction of the field at that point.

Properties of Electric Lines of Force

• Two lines of force never intersect.
• The number of lines of force passing normally through a unit area around a point is numerically equal to E, the strength of the field at the point.
• Lines of force always leave or end normally on a charged conductor.
• Electric lines of force can never be closed loops.
• Lines of force have a tendency to contract longitudinally and exert a force of repulsion on one another laterally.
• If in a region of space, there is no electric field, there will be no lines of force. Inside a conductor, there cannot be any line of force.

The number of lines of force passing normally through a unit area around a point is numerically equal to E.

Difference between electric lines of force and magnetic lines of force

• Electric lines of force never form closed loops, while magnetic lines are always closed loops.
• Electric lines of force do not exist inside a conductor, but magnetic lines of force may exist inside a magnetic material.

Electric field

Electric field lines help visualize the electric field. Field lines begin on a positive charge and terminate on a negative charge. Electric field lines are parallel to the direction of the electric field, and the density of these field lines is a measure of the magnitude of the electric field at any given point.

We show charge with “q” or “Q,” and the smallest unit charge is 1.6021 x 10^-19 Coulomb (C). One electron and a proton have the same amount of charge.

Dielectric Strength: It is the minimum field intensity that should be applied to break down the insulating property of the insulator.

• Dielectric strength of air = 3 x 10⁶ V/m
• Dielectric strength of Teflon = 60 × 10⁶ Vm^–1

The maximum charge a sphere can hold depends on the size and dielectric strength of the medium in which the sphere is placed.

1. The maximum charge density of a sphere of radius ‘R’ in terms of electric intensity E at a distance in free space is Eε₀(R/r)².
2. When the electric field in the air exceeds its dielectric strength, air molecules become ionised and are accelerated by fields and the air becomes conducting.

Surface Charge Density σ

The charge per unit area of a conductor is defined as surface charge density.

Its unit is coulomb/metre, and its dimensions are ATL^–2. It is used in the formula for the charged disc, charged conductor, an infinite sheet of charge etc.  The surface charge density depends on the shape of the conductor and the presence of other conductors and insulators in the vicinity of the conductor.

1. σ is maximum at pointed surfaces, and for plane surfaces, it is minimum.
2. Surface charge density is maximum at the corners of rectangular laminas and at the vertex of the conical conductor.

Electric Charges and Fields Class 12 Notes CBSE Physics

Electric Flux

The number of electric lines of force crossing a surface normal to the area gives electric flux Φ_E.

he electric flux through an elementary area ds is defined as the scalar product of area and field.

dΦ_E = Eds cos θ

• Electric Flux will be maximum when the electric field is normal to the area (dΦ = Eds)
• Electric Flux will be minimum when the field is parallel to the area (dΦ = 0)
• For a closed surface, outward flux is positive and inward flux is negative

CBSE Class 12 Physics Electrostatics Exam Notes

Electric Potential (V)

The electric potential at a point in a field is the amount of work done in bringing a unit +ve charge from infinity to the point. It is equal to the electric potential energy of unit + ve charge at that point.

• It is a scalar quantity.
• The SI unit is volts.

A positive charge in a field moves from high potential to low potential, whereas an electron moves from low potential to high potential when left free. Work done in moving a charge q through a potential difference V is W = q V joule

Electrostatics cbse notes study material class 12

Equipotential Surface

A surface on which all points are at the same potential is called an equipotential surface.

1. The electric field is perpendicular to the equipotential surface.
2. Work done in moving a charge on the equipotential surface is zero.

Electron Volt

This is the unit of energy in particle physics and is represented as eV.

• 1 eV = 1.602 x 10^‑19 J.

Charged Particles in Electric Field

When a positive test charge is fired in the direction of an electric field,

• It accelerates
• Its kinetic energy increases
• Its potential energy decreases

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Electric Dipole

Two equal and opposite charges separated by a constant distance is called an electric dipole.

Dipole Moment

It is the product of one of the charges and the distance between the charges. It is a vector directed from the negative charge towards the positive charge along the line joining the two charges.

When two unlike equal charges, +Q and –Q, are separated by a distance

• The net electric potential is zero on the perpendicular bisector of the line joining the charges.
• The bisector is an equipotential and zero potential line.
• Work done in moving a charge on this line is zero.
• Electric intensity at any point on the bisector is perpendicular to the bisector.
• Electric intensity at any point on the bisector parallel to the bisector is zero.

Combined Field Due to Two Point Charges

Due to two similar charges

If charges q₁ and q₂ are separated by a distance ‘r’, a null point (where the resulting field intensity is zero) is formed on the line joining these two charges.

1. The null point is formed within the charges.
2. The null point is located nearer to the weak charge.

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