Electric charges and fields

Electric Charge and Field Class 12 Topics(NCERT Syllabus)

Electric Charge and Field Class 12 covers the basic concepts of electrostatics. It explains properties of charges, Coulomb’s law, electric field, electric flux, and Gauss’s law. These topics form the foundation for solving numerical and theoretical problems in electricity and modern physics.

1. Introduction

We often see sparks when removing synthetic clothes, feel shocks when touching metal after sliding, or observe lightning in the sky. These happen due to the discharge of accumulated charges, called static electricity. Electrostatics is the study of forces, fields, and potentials arising from such static charges.

2. Electric Charge

Electric charge is a fundamental property of matter responsible for electric forces and interactions. It can be positive or negative and like charges repel while unlike charges attract each other. Charge is quantised, conserved and its SI unit is the coulomb (C).

• Electric Charge

3. Conductors and Insulators

Conductors are materials that allow electric charges to flow easily through them (e.g., copper, aluminium). Insulators are materials that do not allow charges to move freely (e.g., rubber, wood, plastic).

• Conductors and Insulators

4. Basic Properties of Electric Charge

• Quantisation of Charge - Electric charge exists in discrete packets, always in multiples of the elementary charge $e=1.6 \times 10^{-19} C$.
• Conservation of Charge - Charge can neither be created nor destroyed; it is always conserved in an isolated system.
• Additivity of Charge - Total charge on a body is the algebraic sum of all individual charges present.
• Like Charges Repel, Unlike Charges Attract - Charges exert force on each other; similar charges repel, and opposite charges attract.

5. Coulomb’s Law

Coulomb’s law describes the force between two point charges at rest. According to this law, the electrostatic force is directly proportional to the product of the two charges and inversely proportional to the square of the distance between them. The force acts along the line joining the charges and may be attractive or repulsive depending on the nature of charges.

$$
F=k \frac{q_1 q_2}{r^2}
$$

where $q_1$ and $q_2$ are the charges, $r$ is the distance between them, and $k=\frac{1}{4 \pi \epsilon_0}$ is the electrostatic constant.

• Electrical Force

6. Forces Between Multiple Charges

When several charges are present, the net force on any charge is obtained using the principle of superposition. It states that each pair of charges exerts a force independently, and the total force is the vector sum of all these forces.

$$
\vec{F}=\vec{F}_1+\vec{F}_2+\vec{F}_3+\ldots
$$

7. Electric Field

The region around a charged body where it exerts a force on another charge is called the electric field. It is a vector quantity, defined as the force experienced per unit positive test charge placed at that point.

$$
\vec{E}=\frac{\vec{F}}{q}
$$

• Electric Field

8. Electric Field Lines

Electric field lines are imaginary lines drawn to represent the direction and strength of an electric field. They start from positive charges and end on negative charges. The tangent at any point on a field line gives the direction of the electric field at that point.

• Electric Field Lines

9. Electric Flux

Electric flux is a measure of how many electric field lines pass through a given surface. It helps us understand the flow of the electric field across an area.

$$\phi_E=\vec{E} \cdot \vec{A}=E A \cos \theta$$

• Electric Flux

10. Electric Dipole

An electric dipole is a system of two equal and opposite charges separated by a small distance. The strength of a dipole is measured by its dipole moment (p), given as:

$$
p=q \times 2 a
$$

• Electric Dipole

11. Dipole in a Uniform External Field

When an electric dipole is placed in a uniform external electric field, each charge experiences a force. Since the forces are equal and opposite, they cancel out in terms of net force but create a torque that tends to align the dipole with the field.

$$
\tau=p E \sin \theta
$$

• Torque On An Electric Dipole In A Uniform Electric Field

12. Continuous Charge Distribution

In many practical situations, charges are spread over a length, surface, or volume instead of being concentrated at a point. Such an arrangement is called a continuous charge distribution. Instead of dealing with individual charges, we use charge density to describe how charge is distributed:

1. Linear charge density ( $\lambda$ ): charge per unit length, $\lambda=\frac{q}{L}$
2. Surface charge density ( $\sigma$ ): charge per unit area, $\sigma=\frac{q}{A}$
3. Volume charge density ( $\rho$ ): charge per unit volume, $\rho=\frac{q}{V}$

• Electric Field Intensity: Continuous Charge Distribution

13. Gauss's Law

Gauss's law is a fundamental law of electrostatics that relates the electric flux through a closed surface to the charge enclosed within it. It states:

$$
\Phi_E=\oint \vec{E} \cdot d \vec{A}=\frac{q_{\text {enclosed }}}{\varepsilon_0}
$$

Here, $\Phi_E$ is the total electric flux, $\vec{E}$ is the electric field, $d \vec{A}$ is a small area vector on the closed surface, $q_{\text {enclosed }}$ is the total charge inside, and $\varepsilon_0$ is the permittivity of free space.

• Gauss Law And It's Application

14. Applications of Gauss's Law

Gauss's law is widely used to calculate electric fields for symmetric charge distributions. Common applications include:

1. Spherical Charge Distribution - Electric field outside a uniformly charged sphere behaves as if all charge is concentrated at the center.
2. Infinite Line of Charge - Electric field around a long charged wire can be easily calculated using cylindrical symmetry.
3. Infinite Plane Sheet of Charge - Electric field near a uniformly charged plane is constant and perpendicular to the surface.
4. Conductors - Explains why electric field inside a conductor is zero and charges reside on the surface.

Electric Charge and Field Real Life Examples

• Lightning: Discharge of charges in clouds.
• Static Electricity: Sparks from clothes or metal objects.
• Van de Graaff Generator: Demonstrates high voltage in labs.
• Photocopiers & Printers: Use charges to move toner.
• Touchscreens: Work by detecting changes in electric fields.

Exam - Wise Weitage of Electric Field and Charge

Related Topics,

NCERT Notes Subject wise link:

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• NCERT Notes for class 12 Chemistry.
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• NCERT Notes for class 12 Biology.

Importance of Electric charges and fields class 12:

Class 12 Physics Chapter 1 is taken under consideration to be the foremost important part for school students aiming to clear the NEET exam. In CBSE Class 12 Physics chapter 1, several important derivations and formulas are presented to the students which are crucial to forming the essential skills required for a medical & engineering career. NCERT Class 12 syllabus has various important topics, diagrams and definitions that students require to be thorough with to be able to score well within the category 12 board exam. To secure good marks in class 12, students are required to arrange thoroughly and practice with NCERT Class 12 Chapter one Electric Charges & Fields Physics Marks Wise Question.

Approach to Solve Electric Charge and Field Questions

To solve questions on electric charge and field, first identify the type of problem-whether it is about charge, force, electric field, or flux. Draw a clear diagram showing charges, distances, and directions. Use Coulomb's law for forces between point charges and the principle of superposition for multiple charges. For electric fields, calculate $E=F / q$ and apply vector addition if needed. For symmetrical charge distributions, use Gauss's law to find the electric field, and apply its common applications like spherical, cylindrical, or planar charge systems. Always check units and directions to ensure the answer is correct.

• NCERT solutions for class 12 physics chapter 1 Electric Charges and Fields.
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NCERT Solutions Subject wise link:

• NCERT solutions for class 12 Physics.
• NCERT solutions for class 12 Chemistry.
• NCERT solutions for class 12 Mathematics.
• NCERT solutions for class 12 Biology.

NCERT Exemplar Solutions Subject wise link:

• NCERT exemplar solutions for class 12 Physics.
• NCERT exemplar solutions for class 12 Chemistry.
• NCERT exemplar solutions for class 12 Mathematics.
• NCERT exemplar solutions for class 12 Biology.