CBSE Class 12 Physics (2026–27)
Chapter 1: Electric Charges and Fields
20 Important Questions and Answers
Chapter 1 covers electric charge, Coulomb’s law, electric field, field lines, electric flux, electric dipoles, and Gauss’s law with applications. These are the key topics prescribed in the CBSE syllabus.
Q1. What are the basic properties of electric charge?
Answer:
Electric charge possesses three important properties: additivity, conservation, and quantization. According to the additive nature, the total charge on a system is the algebraic sum of individual charges. The law of conservation of charge states that charge can neither be created nor destroyed; it can only be transferred from one body to another. Quantization of charge means that charge exists in discrete packets and is always an integral multiple of the elementary charge (e = 1.6 × 10⁻¹⁹ C). These properties help explain various electrostatic phenomena and form the basis for understanding electric interactions in matter.
Q2. State Coulomb’s law and explain its significance.
Answer:
Coulomb’s law states that the electrostatic force between two stationary point charges is directly proportional to the product of their charges and inversely proportional to the square of the distance between them. The force acts along the line joining the charges. Mathematically,
genui{“math_block_widget_always_prefetch_v2”:{“content”:”F=k\frac{q_1q_2}{r^2}”}}
This law is significant because it provides the fundamental relationship governing electric forces. It helps calculate interactions between charged particles and forms the foundation of electrostatics. The force is attractive for unlike charges and repulsive for like charges. Coulomb’s law is analogous to Newton’s law of gravitation but applies to electric charges.
Q3. What is the principle of superposition of charges?
Answer:
The principle of superposition states that when several charges act on a test charge, the net electrostatic force experienced by the test charge is the vector sum of the individual forces due to each charge separately. This principle is useful in calculating forces in systems containing multiple charges. Instead of considering all charges simultaneously, the effect of each charge is determined independently and then added vectorially. The principle simplifies complex electrostatic problems and is widely used in determining electric fields and forces due to continuous charge distributions. It remains valid because electrostatic forces obey vector addition rules.
Q4. Define electric field intensity.
Answer:
Electric field intensity at a point is defined as the electrostatic force experienced by a unit positive test charge placed at that point. It indicates both the strength and direction of the electric field. Mathematically,
E=\frac{F}{q}
Its SI unit is newton per coulomb (N/C). The direction of the electric field is the direction of the force acting on a positive test charge. Electric field intensity helps describe how charged bodies influence the space around them. A stronger electric field exerts a greater force on a charge placed within it.
Q5. What is an electric field? Why is it important?
Answer:
An electric field is the region around a charged body where another charge experiences an electrostatic force. It is produced by electric charges and extends throughout space. The concept of an electric field is important because it explains how charges interact without physical contact. Instead of direct action at a distance, the source charge creates a field that exerts force on other charges entering that region. Electric fields are represented by field lines and are used extensively in electrostatics, electronics, and electrical engineering. Understanding electric fields is essential for analyzing charge distributions and electrostatic forces.
Q6. What are electric field lines? Mention any four properties.
Answer:
Electric field lines are imaginary curves that represent the direction and strength of an electric field. The tangent at any point on a field line gives the direction of the electric field. Important properties are:
- Field lines originate from positive charges and terminate on negative charges.
- No two field lines intersect each other.
- The density of field lines indicates field strength.
- Field lines are perpendicular to the surface of a conductor in electrostatic equilibrium.
These properties help visualize electric fields and understand the behavior of charges in different electrostatic situations.
Q7. Define electric flux.
Answer:
Electric flux is the measure of the total electric field passing through a given surface. It represents the number of electric field lines crossing the surface. Mathematically, electric flux is the dot product of the electric field and the area vector. Its SI unit is N m²/C. Electric flux depends on the strength of the electric field, the area of the surface, and the angle between the electric field and the normal to the surface. The concept of electric flux is fundamental in understanding Gauss’s law and solving problems involving symmetrical charge distributions.
Q8. State Gauss’s law.
Answer:
Gauss’s law states that the total electric flux through any closed surface is equal to the net charge enclosed within the surface divided by the permittivity of free space. It is expressed as:
\Phi=\frac{q_{enclosed}}{\varepsilon_0}
This law is one of the fundamental laws of electrostatics. It is especially useful for calculating electric fields produced by highly symmetrical charge distributions such as spherical, cylindrical, and planar systems. Gauss’s law simplifies calculations that would otherwise require complex integration using Coulomb’s law.
Q9. What is an electric dipole?
Answer:
An electric dipole consists of two equal and opposite charges separated by a small distance. The product of either charge and the separation distance is called the dipole moment. The dipole moment is a vector quantity directed from the negative charge to the positive charge. Electric dipoles are commonly found in molecules and play a significant role in electrostatics. The electric field due to a dipole varies with position and is stronger near the charges. Understanding dipoles helps explain molecular polarity, dielectric behavior, and interactions between charged particles.
Q10. Define dipole moment and mention its SI unit.
Answer:
Dipole moment is the measure of the strength of an electric dipole. It is defined as the product of the magnitude of either charge and the separation distance between the charges. Mathematically,
p=q\times 2a
The SI unit of dipole moment is coulomb-meter (C m). Dipole moment is a vector quantity directed from the negative charge toward the positive charge. A larger dipole moment indicates a stronger dipole. It plays an important role in determining the behavior of molecules in external electric fields and understanding molecular polarity.
Q11. What is charging by induction?
Answer:
Charging by induction is the process of charging a conductor without direct contact with a charged body. When a charged object is brought near a neutral conductor, charges inside the conductor redistribute due to electrostatic influence. If the conductor is then grounded and the ground connection is removed before removing the charged object, the conductor acquires a net charge opposite to that of the inducing charge. This method demonstrates the movement and redistribution of charges. Charging by induction is widely used in electrostatic applications because it allows objects to be charged without physical contact.
Q12. Differentiate between conductors and insulators.
Answer:
Conductors are materials in which electric charges can move freely due to the presence of free electrons. Examples include copper, silver, and aluminum. Insulators, on the other hand, do not allow charges to move freely because their electrons are tightly bound to atoms. Examples include rubber, glass, and plastic. In conductors, charge distributes itself over the surface, whereas in insulators charge remains localized. Conductors are used in electrical wiring and circuits, while insulators are used for protection and electrical isolation. Understanding this distinction is important for studying electrostatic phenomena and electrical devices.
Q13. Explain conservation of charge with an example.
Answer:
The law of conservation of charge states that the total electric charge in an isolated system remains constant. Charge cannot be created or destroyed; it can only be transferred from one body to another. For example, when a glass rod is rubbed with silk, electrons transfer from the glass rod to the silk cloth. As a result, the rod becomes positively charged and the silk becomes negatively charged. Although charges appear on both bodies, the total charge of the system remains unchanged. This principle is universally valid and forms one of the fundamental laws of physics.
Q14. What is quantization of charge?
Answer:
Quantization of charge means that electric charge exists only in discrete amounts and not in arbitrary values. The charge on any body is given by:
[q = ne]
where (n) is an integer and (e) is the elementary charge (1.6 × 10⁻¹⁹ C). This implies that charge can only be transferred in integral multiples of the elementary charge. Fractional charges are not observed on isolated bodies under normal conditions. The concept of quantization was established through experimental observations and plays an important role in atomic and subatomic physics. It confirms the particle nature of electric charge.
Q15. Why do electric field lines never intersect?
Answer:
Electric field lines never intersect because the electric field at any point has a unique direction. If two field lines were to intersect, the point of intersection would have two different directions of the electric field simultaneously. This is physically impossible because a test charge placed at that point can experience force in only one direction at a time. Therefore, field lines cannot cross each other. This property helps maintain the consistency of electric field representation and allows accurate visualization of electric fields around charged bodies.
Q16. What is the significance of permittivity of free space?
Answer:
Permittivity of free space, denoted by ε₀, is a physical constant that characterizes the ability of vacuum to permit electric field lines. Its value is 8.85 × 10⁻¹² C² N⁻¹ m⁻². It appears in Coulomb’s law and Gauss’s law and influences the magnitude of electrostatic forces between charges. A larger permittivity results in a weaker electric force. The concept is important in understanding electric fields, capacitors, and electromagnetic phenomena. It serves as a fundamental constant in electrostatics and plays a key role in many physical equations.
Q17. What is meant by a continuous charge distribution?
Answer:
A continuous charge distribution refers to a system in which charge is spread continuously over a length, surface, or volume rather than being concentrated at discrete points. Such distributions are described using linear, surface, or volume charge densities. Continuous charge distributions are commonly found in charged rods, spheres, and sheets. Instead of treating the entire charge as a point charge, infinitesimal charge elements are considered to calculate electric fields. This concept is important for understanding practical electrostatic systems and forms the basis for applying integration techniques and Gauss’s law in electrostatics.
Q18. What happens to a dipole placed in a uniform electric field?
Answer:
When an electric dipole is placed in a uniform electric field, the two charges experience equal and opposite forces. These forces form a couple that produces a torque on the dipole. The torque tends to align the dipole along the direction of the electric field. The magnitude of torque depends on the dipole moment, electric field strength, and angle between them. No net force acts on the dipole in a uniform field, but rotational motion occurs. This behavior is important in understanding molecular orientation, dielectric materials, and electrostatic applications involving dipoles.
Q19. Why is Gauss’s law useful in electrostatics?
Answer:
Gauss’s law is useful because it simplifies the calculation of electric fields for highly symmetrical charge distributions. Instead of applying Coulomb’s law to each charge element individually, the total electric flux through a closed surface is considered. This greatly reduces mathematical complexity. Gauss’s law is particularly effective for spherical shells, infinite plane sheets, and long cylindrical conductors. It provides quick and elegant solutions for electric field calculations and is one of Maxwell’s fundamental equations. The law also helps establish important properties of electric fields and charge distributions.
Q20. Explain the electric field due to a point charge.
Answer:
A point charge produces an electric field in the surrounding space. The magnitude of the electric field at a distance (r) from a point charge is directly proportional to the charge and inversely proportional to the square of the distance. The field is directed radially outward for a positive charge and radially inward for a negative charge. As the distance from the charge increases, the field strength decreases rapidly according to the inverse square law. The concept of the electric field due to a point charge serves as the foundation for understanding more complex electric field configurations and electrostatic interactions.