NCERT QA For Class 12 Physics

Numerical problems from NCERT textbooks play an important role in developing conceptual understanding as well as problem-solving abilities for students. This article discusses solutions to selected numerical problems from NCERT Class 12 Physics textbook. The problems have been solved with step-by-step explanations to assist students in learning the concepts and approaches required to solve similar types of problems.

Chapter 1 – Electric Charges and Fields

Q1. What is an electric charge?

An electric charge is a fundamental property possessed by all subatomic particles, which causes an interaction between charged bodies through the electric force of attraction or repulsion. Electric charge comes in two types – positive and negative.

Q2. What is the basic interaction between electric charges?

According to the law of electrostatics:

• Like charges repel and unlike charges attract.
• The magnitude of the electrostatic force is directly proportional to the product of the charges and inversely proportional to the square of the distance between them. This is given by Coulomb’s law.

Q3. What are the units of electric charge?

The SI unit of electric charge is the coulomb (C). A single elementary charge is 1.60×10−19 C. Protons and electrons have charges of +1.60×10−19 C and -1.60×10−19 C respectively.

And so on. This format of QA helps consolidate all important concepts chapter-wise for thorough exam preparation.

Chapter 2 – Electrostatic Potential and Capacitance

Q1. What is meant by electric potential?

Electric potential refers to the potential energy per unit charge of an electric field. It specifies the amount of work done (or energy expended/gained) per unit charge to move a charged body against or with the electric field.

Q2. What are equipotential surfaces?

Surfaces where electric potential remains constant are known as equipotential surfaces. All points on an equipotential surface have equal potential and no work is required to move a charge from one point to another on the same equipotential surface.

Q3. What factors affect the capacitance of a capacitor?

The factors affecting capacitance are:

• Area of plates (larger area increases capacitance)
• Distance between plates (shorter distance increases capacitance)
• Dielectric material and its permittivity between the plates (higher εr increases capacitance)

Chapter 3 – Current Electricity

Q1. What is meant by electric current?

Electric current refers to the flow of charge in a conductor. It is defined as the rate of flow of charge through a surface placed within the conductor. The SI unit of current is Ampere (A).

Q2. Differentiate between electric current and electric current density.

• Electric current is the total charge flowing through a surface per unit time.
• Current density is the current per unit cross-sectional area of the conductor and its SI unit is Ampere/meter2 (A/m2).

Q3. State Ohm’s law and mention its limitations.

Ohm’s law states that the current (I) flowing through a conductor is directly proportional to the potential difference (V) across its ends, i.e. I = V/R. Where R is the resistance of the conductor.

It is valid only when temperature and other physical conditions of the conductor remain unchanged. It does not apply to imperfect conductors, circuits with an electromotive force source etc.

Chapter 4 – Moving Charges and Magnetism

Q1. How does a current carrying conductor generate a magnetic field?

Magnetic field is generated by the moving electric charges inside the conductor. According to Ampere’s law, a closed circuit gets encircled by a closed magnetic field whose direction is governed by the right-hand thumb rule.

Q2. State Fleming’s left-hand rule.

Fleming’s left-hand rule gives the direction of force experienced by a current carrying conductor placed in an external magnetic field. The thumb, forefinger and middle finger of the left hand are stretched at right angles to each other. Thumb points in the direction of magnetic field, forefinger in direction of current and the normal to these two gives the direction of force.

Q3. Define magnetic flux and state its SI unit.

Magnetic flux (ΦB) is defined as the total magnetic field passing through any surface enclosed by a closed loop. It is a scalar quantity and is given by ΦB =∫B.da, where B is the magnetic field intensity and da is an element of area. Its SI unit is Weber (Wb) or Tesla-meter squared (Tm2).

Chapter 5 – Magnetism and Matter

Q1. Define magnetic moment. What does it depend upon?

Magnetic moment (m) is a quantitative property of a magnet or any system that generates a magnetic field. It is defined as the torque experienced by the magnet/object when placed in an external magnetic field. Magnetic moment depends on the:

• Amount of current (I)
• Area (A) enclosed by the current loop
• Number of loops (n)

Q2. What is diamagnetism? Give examples.

Diamagnetism is a form of magnetism whereby the applied magnetic field is expelled from the material rather than getting attracted to it. All materials exhibit diamagnetism but it is a very weak effect. Examples of diamagnetic substances are copper, water, wood, plastics.

Q3. What are ferromagnetic materials? Give examples and state one of their properties.

Ferromagnetic materials strongly attract an external magnetic field. Their magnetic moments align parallel even in the absence of field due to exchange interaction. Common ferromagnetic materials are iron, nickel, cobalt and their alloys. They retain their magnetism even after removal of an external field, known as residual magnetism.

Chapter 6 – Electromagnetic Induction

Q1. Define electromagnetic induction. What is an induced current?

Electromagnetic induction refers to the process of producing an induced electromotive force (emf) and current in a conductor due to its relative motion in a changing magnetic field. Induced current refers to the flow of charges in a conductor due to electromagnetic induction effect.

Q2. State Faraday’s laws of electromagnetic induction.

• Faraday’s first law: Whenever there is a change in magnetic flux linked with a closed circuit, an emf will be induced in that circuit.
• Faraday’s second law: The magnitude of induced emf is proportional to the rate of change of magnetic flux through the circuit.

Q3. Draw a neat labeled diagram of the transformer and state its working principle.

A transformer consists of two coils – primary and secondary wound around an iron core. An ac supply is connected to the primary. Changing current in the primary produces changing magnetic flux which induces emf in the secondary through mutual induction. The ratio of primary and secondary turns determines the voltage transformation ratio.

Chapter 7 – Alternating Current

Q1. What are the characteristics of alternating current?

• It reverses its direction periodically at regular time intervals following a sine wave.
• Its peak value, frequency and time period can be measured.
• It has zero average value over one cycle but flows continuously in a circuit in alternating directions.

Q2. Define the terms: voltage, current, phase difference, frequency, period and their relations for ac.

Voltage (V) and current (I) vary sinusoidally with time and a phase difference occurs between them. Frequency (f) is the number of cycles/sec. Period (T) is time for one cycle. Relations: V= Vmsin(ωt), I= Imsin(ωt+φ), f = 1/T , ω = 2πf

Q3. Give practical applications of ac mains over dc.

• Generation is easier using alternators.
• Transmission over long distances at high voltages has less power loss.
• Domestic use like electric motors, heaters, induction motors require a power supply.
• Electronic devices after rectification.

Related reading: How Do RC Electric Motors Work?

Chapter 8 – Electromagnetic Waves

Q1. What are electromagnetic waves? Mention their basic characteristics.

EM waves are a self-propagating wave of electric and magnetic fields oscillating in phase perpendicular to each other and the direction of energy propagation. They travel with the speed of light. Their characteristics are – broadband spectrum, transverse nature, polarization, superposition and diffraction.

Q2. What are the components of an electromagnetic wave?

The six components of an EM wave are:
Electric field intensity (E), Magnetic field intensity (B), Wavelength (λ), Amplitude (A), Velocity (v) and Frequency (f). They are related as v = fλ.

Q3. Name some EM wave applications with their wavelength ranges.

• Radio waves – 100m to 1 mm
• Microwaves – 1mm to 1cm
• Infrared – 700 nm to 400 nm
• Visible light – 400nm to 700nm
• UV light – 10 nm to 400 nm
• X-rays – 0.01nm to 10nm
• Gamma rays – less than 0.01nm

Final Thoughts

This post discussed solutions to some selected essential topics from NCERT Class 12 Physics textbook. Solving rigorous problems is important for developing conceptual clarity as well as problem-solving skills. Students are advised to practice more questions to develop proficiency in applying concepts to different quantitative problems. You can always consider opting for Physics tuition in Dubai. It will help them succeed in board exams as well as engineering entrance exams.