Class-12 Ch-1 Electrostatics

Electrostatics notes class 12 CBSE

Ch-1 Electric Charges and Fields (Electrostatics)

Topic Covered

–Latest CBSE syllabus 2024-2025

-Introduction, Charges, Properties of Charges

-Additivity of Electric charges, conservation and Quantization of electric charges

-Coulomb’s Law, Superposition of Force

-Electric Field Intensity, Electric field Lines

-Electric Dipole, Electric fields due to a dipole at Axial and Equitorial Positions

-Torque, Potential Energy and work due to Electric Dipole

-Electric Flux

-Gauss Theorem, Application of Gauss theorem.

In Electrostatics Notes Class 12 The study of stationary electric charges is known as electrostatics. A plastic or glass rod that has been touched with fur or silk may draw tiny bits of paper and is considered electrically charged. When plastic is touched with fur, the charge is characterised as negative; when glass is rubbed with silk, the charge is classified as positive.

electrical power

Several significant properties are present in electrically charged objects:

Positive charges repel other positive charges while negative charges repel other negative charges.

In contrast, charges are attracted to one another—positive attracts negative.

Additivity of Charges Charges are scalars and they add up like real numbers. It means if a system consists of n charges q₁, q₂, q₃ , … ,q_n, then total charge of the system will be q₁ +q₂ + … +q_n.

Conservation of Charge The total charge of an isolated system is always conserved, i.e. initial and final charge of the system will be same.

Quantisation of Charge Charge exists in discrete amount rather than continuous value and hence, quantized.
Mathematically, charge on an object, q=±ne
where, n is an integer and e is electronic charge. When any physical quantity exists in discrete packets rather than in continuous amount, the quantity is said to be quantized. Hence, charge is quantised.

There is conservation of charge. There is no net charge on a neutral object. A negative charge is transferred from the fur to the plastic rod when the fur charges it, assuming the fur and rod are originally neutral. The net positive charge on the fur and the net negative charge on the rod are the same.

The Superposition of Electrostatic Forces Principle According to this concept, the net electric force that a particular charge particle, q0, experiences from a system of charged particles is equal to the vector sum of the forces that all of the other charged particles in the system impose on it.

Electric Dipole Two point charges of same magnitude and opposite nature separated by a small distance altogether form an electric dipole.
Electric Dipole Moment The strength of an electric dipole is measured by a vector quantity known as electric dipole moment (p) which is the product of the charge (q) and separation between the charges (2l).

Electroscope with Gold Leaf (Electrostatics Notes Class 12)

A tool for measuring or identifying the existence of electrostatic charges is an electroscope. The Coulomb Force Law is the only foundation for the device’s functionality. This is a very basic tool that is solely used in teaching. Using modern devices built on solid state or vacuum tube technology, minuscule charge levels can be measured.

Inside a metal container D are two gold leaves (A) suspended from a metal rod (B). A conducting disc C (or sphere) is supported at the top of the rod. A highly insulating stopper (E) supports the rod.

The gold leaves resist one another once they become charged because every leaf has the same electrical potential.

The gold leaves are shielded from outside effects by the metal case. The case has a window that allows you to see the leaves.

 

in Electrostatics Notes Class 12 the electroscope is a highly helpful tool for demonstrating the two processes of conduction and induction of electrical charges. The principles are illustrated in the two figures below.

 

The electroscope is earthed first. When a negatively charged object is brought close to the electroscope system, the positive charges go in the direction of the charged objects, and the negative charges go in the direction of the gold leaves, causing the charges to separate. Negative charges go to earth if the electroscope is grounded without moving the charged objects. Positive charges spread throughout the electroscope system if the earth and the charged objects are removed.

 

A charged object comes into physical contact with the electroscope and transfers charge. This indicates to us that the item is charged.

Any substance that permits the easy passage of electric charges is called a conductor. A material that allows electric charges to travel very slowly or not at all is called an insulator. A basic tool for showing whether there is charge is an electroscope. The electroscope is comprised of a conducting knob and lightweight conducting leaves, which are usually constructed of aluminium or gold foil, as illustrated in Figure 1. Similar charges repel and push the leaves apart when a charged object comes into contact with the knob. The electroscope will show whether a charge is there, but it won’t tell you if it’s positive or negative.

Figure -1

The law of Coulomb (Electrostatics Notes Class 12)

According to Coulomb’s law, the electrostatic force (F) between two charges is as follows:

where k is the proportionality constant, r is the distance between the charges, and q ₁ and q ₂ are the charges. The coulomb is the SI unit of charge. The force in newtons can be calculated using the approximate value of k given the charge in coulombs and the separation in metres. You have k = 9.0 × 10 ⁹ N · m ²/C ² . The signatures of the charges determine which way the electrostatic force is directed. When two charges are dissimilar, they attract each other this is important for Electrostatics Notes Class 12.

Static electricity is the ability of all bodies to hold an electrical charge. The force between the charges is used to measure the charge on a body.

The following is the Coulomb force law, which only holds true for charged points.

Between two charged points, the force of attraction or repulsion is inversely proportional to the square of their distance apart and directly relates to the charges.

Lines of force and electric fields (Electrostatics Notes Class 12)

A force is exerted away from a large positive charge towards a tiny positive test charge when it is brought close to it. According to Coulomb’s law, the electrostatic force produced by the test charge when it is distant from the large charge is less than when it is close. An electrostatic field is the direction and amount of an electrostatic force caused by a fixed charge or collection of fixed charges. The force per unit charge applied to a modest positive test charge (q ₀) at that location is the definition of the electric field. In terms of math,

Keep in mind that the electric field and force are vector quantities. In order to prevent the test charge’s field from influencing the field of the set charges under examination, the test charge must be tiny. The electric field is measured in newtons per coulomb (N/C), the SI unit and it is important for Electrostatics Notes Class 12.

The electric fields surrounding a positive charge and a negative charge are depicted graphically in Figure. These are referred to as force lines or field lines.

For every static setup of charges, the guidelines for sketching electric field lines are

Positive charges start the lines, and negative charges finish them.

The amount of lines drawn that emerge from or end on a charge is directly correlated with its size.

No two field lines ever intersect in an area free of charges. (One line can only be at each location since the direction of the resulting force is represented by the tangent to the field line.)

The line is perpendicular to the conducting surface as it approaches and here Electrostatics Notes Class 12 play an important role.

Electric Flux (Electrostatics Notes Class 12)

The number of field lines that flow through a specific surface is known as the electric flux. Lines of electric flux that emerge from a point charge in Figure traverse a hypothetical spherical surface that has the charge at its centre.

The following is an expression for this definition: The formula is Φ = ∑E · A, where E is the electric field, A is the area perpendicular to the field lines, and (the Greek symbol phi) represents the electric flux. Electric flux is a scalar quantity that is quantified in N · m ² / C ². The expression is Φ = ∑ EA cos θ if the surface being considered is not perpendicular to the field lines.

For Electrostatics Notes Class 12 Broadly speaking, flux can be defined as the closed integral of the dot product between the vector A and the electric field vector. The outward drawn normal to the imaginary surface is the direction of ΔA. Strictly speaking, Φ = ΦE · dA. Flux lines are generally understood to be positive when they leave a surface and negative when they enter one.

Gauss’s Law (Electrostatics Notes Class 12)

Although Gauss’s law gives one a way to calculate any electric field, its use is limited to fields with extremely symmetric distributions of fixed charges. According to the law, the net electric flux passing through any closed surface, real or imaginary, is equal to the net electric charge contained within divided by ε. Therefore, there are the same number of flux lines entering and exiting a given closed surface if there is no charge present. The gaussian surface is the imaginary surface that must exist in order to implement Gauss’s law which play an important role Electrostatics Notes Class 12.