Physics is a vital part of the CBSE Class 12 syllabus, demanding a blend of conceptual clarity and problem-solving skills. Known for its challenging numerical problems and theoretical depth, Physics often intimidates students. However, with a structured approach and focused preparation, it can become one of the most rewarding subjects.
In this article, you’ll find an exhaustive list of important questions and topics for Class 12 Physics, along with practical strategies to master the subject. Whether your goal is to excel in the board exams or develop a profound understanding of the principles governing the physical world, this guide will serve as your roadmap.
Importance of Emphasis on Key Questions
Class 12 Physics encompasses a wide range of topics, including Mechanics, Electrodynamics, Modern Physics, and Optics, which may seem daunting at first glance. However, by focusing on important questions, the preparation process becomes streamlined and manageable. Concentrating on key questions allows you to target frequently asked problems, helping you become familiar with the exam pattern and enhancing your understanding of crucial concepts.
These questions prepare you for various types of questioning, including numerical problems, conceptual explanations, derivations, and application-based higher-order thinking skills (HOTS) questions. This focused approach not only saves time but also boosts your confidence for the exam.
Most Expected Questions
1. A paisa coin is made up of AI-Mg alloy and weighs 0.75g. It has a square shape, and its diagonal measures 17 mm. It is electrically neutral and contains equal amounts of positive and negative charges. Treating the paisa coins made up of only Al, find the magnitude of the equal number of positive and negative charges. What conclusion do you draw from this magnitude?
2. Consider a scenario of a coin. It is naturally electrically neutral and contains equal amounts of the negative and positive charge of magnitude 34.8 kC. Suppose that these equal charges were concentrated in two point charges separated by (i) 1 cm (∼ 1⁄2 diagonal of the one paisa coin), (ii) 100 m (∼ length of along building), and (iii) 106 m (radius of the Earth). Find the force on each such point charge in each of the three cases. What do you conclude from these results?
3. Two charges -3q and q are placed fixed on the x-axis separated by distance ‘d’. Where should a third charge 2q be placed so that it will not experience any force?
4. Two fixed, identical conducting plates (α & β), each of surface area S, are charged to -Q and q, respectively, where Q > q > 0. A third identical plate (γ), free to move, is located on the other side of the plate with charge q at a distance d (diagram). The third plate is released and collides with plate β. Assume the collision is elastic and the time of collision is sufficient to redistribute charge g amongst β & γ.
5. Two charges -q each is fixed and separated by distance 2d. A third charge q of mass m placed at the mid-point is displaced slightly by x (x<<d) perpendicular to the line joining the two fixed charges, as represented in the figure. Show that q will perform simple harmonic -q oscillation of time period.
6. Can there be a potential difference between two adjacent conductors carrying the same charge?
7. Prove that a closed equipotential surface with no charge within itself must enclose an equipotential volume.
8. Find the equation of the equipotentials for an infinite cylinder of radius r0, carrying charge of linear density λ.
9. A capacitor is made of two circular plates of radius R each, separated by a distance d << R. The capacitor is connected to a constant voltage. A thin conducting disc of radius r << R and thickness t << r is placed at the centre of the bottom plate. Find the minimum voltage required to lift the disc if the mass of the disc is m.
10. Two metal spheres, one of radius R and the other of radius 2R, both have the same surface charge density σ. They are brought in contact and separated. What will be the new surface charge densities on them?
11. What are the advantages of the null-point method in a Wheatstone bridge? What additional measurements would be required to calculate Runknown using any other method?
12. Power P is to be delivered to a device via transmission cables having resistance Rc. If V is the voltage across R and I is the current through it; find the power wasted and how it can be reduced.
13. Two conductors are made of the same material and have the same length. Conductor A is a solid wire with a diameter of 1mm. Conductor B is a hollow tube with an outer diameter of 2mm and an inner diameter of 1mm. Find the ratio of resistance RA to RB.
14. A room has AC run for 5 hours a day at a voltage of 220V. The wiring of the room consists of Cu of 1 mm radius and a length of 10m, and power consumption per day is ten commercial units. What fraction of it goes in the joule heating in wires? What would happen if the wiring is made of the same dimensions?
15. Two cells of voltage 10V and 2V and internal resistances 10O and 5O, respectively, are connected in parallel with the positive end of the 10V battery connected to the negative pole of the 2V battery. Find the effective voltage and effective resistance of the combination.
16. A current carrying loop consists of 3 identical quarter circles of radius R, lying in the positive quadrants of the x-y, y-z and z-x planes with their centres at the origin, joined together. Find the direction and magnitude of B at the origin.
17. A charged particle of charge e and mass m is moving in an electric field E, and magnetic field B. Construct dimensionless quantities and quantities of dimension [T]-1.
18. Do magnetic forces obey Newton’s third law? Verify for two current elements dl1 – dl i (unit vector) located at the origin and dl2 = dl j (unit vector) located at (O, R, O). Both carry current I.
19. A long straight wire carrying a current of 25A rests on a table. Another wire PQ of length 1m, mass 2.5g, carries the same current but in the opposite direction. The wire PQ is free to slide up and down. To what height will PQ rise?
20. A rectangular conducting loop consists of two wires on two opposite sides of length ‘I’ joined together by a rod of length ‘d’. The wires are each of the same material but with cross-sections differing by a factor of 2. The thicker wire has a resistance E, and the rods are of low resistance, which in turn are connected to a constant voltage source V0. The loop is placed in uniform magnetic field B at 45o to its plane. Find τ, the torque exerted by the magnetic field on the loop about an axis through the centres of rods.
21. A permanent magnet in the shape of a thin cylinder of length 10 cm has M = 106A/m. Calculate the magnetisation current IM.
22. Verify Gauss’s law of the magnetic field of a point dipole of dipole moment at the origin for the surface, which is a sphere of radius R.
23. Suppose we want to verify the analogy between electrostatic and magnetostatic by an explicit experiment. Consider the motion of (i) electric dipole p in an electrostatic field E and (ii) magnetic dipole m in a magnetic field B. Write down a set of conditions on E, B, p, m so that two motions are verified to be identical. (Assume identical initial conditions)
24. A bar magnet of magnetic moment m and moment of inertia I (about the centre, perpendicular to length) is cut into two equal pieces perpendicular to the length. Let T be the period of oscillations of the original magnet about an axis through the midpoint, perpendicular to the length, in a magnetic field B. What would be the similar period ‘T’ for each piece?
25. What are the dimensions of χ, the magnetic susceptibility? Consider an H-atom. Guess an expression for χ, up to a constant, by constructing several dimensions of χ out of parameters of the atom: e, m, v, R, and µ0. Here, m is the electronic mass, v is electronic velocity, and R is Bohr radius. Estimate the number so obtained and compare it with the value of |χ| ~ 10-5 for many solid materials.
26. A magnetic field in a certain region is given by B = B, cos(wt) ^k and a coil of radius a with resistance R is placed in the x-y plane with its centre at the origin in the magnetic field. Find the magnitude and the direction of the current at (a, 0, 0) at t = n/2w, t = n/w and t = 3π/2ω.
27. Consider a closed loop C in a magnetic field. The flux passing through the loop is defined by choosing a surface whose edge coincides with the loop and using the formula B1.dA1+B2.dA2+… Now if we chose two different surfaces S₁ and S2 having Cas on their edge, would we get the same answer for flux? Justify your answer.
28. A magnetic field B = Bosin(ot)^k covers a large region where a wire AB slides smoothly over two parallel conductors separated by a distance d. The wires are in the x-y plane. The wire AB (of length d) has resistance R and the parallel wires have negligible resistance. If AB is moving with velocity v, what is the current in the circuit? What is the force needed to keep the wire moving at constant velocity?
29. A conducting wire XY of mass m and negligible resistance slides smoothly on two parallel conducting wires. The closed circuit has a resistance R due to AC. AB and CD are perfect conductors. There is a magnetic field B = B(t)k^.
30. A rod of mass m and resistance R slides smoothly over two parallel perfectly conducting wires kept sloping at an angle & to the horizontal. The circuit is closed through a perfect conductor at the top. There is a constant magnetic field B along the vertical direction. If the rod is initially at rest, find the velocity of the rod as a function of time.
31. A device ‘X’ is connected to an a.c source. The variation of voltage, current and power in one complete cycle. (a) Which curve shows power consumption over a full cycle? (b) What is the average power consumption over a cycle? (c) Identify the device ‘X’.
32. Both alternating current and direct current are measured in amperes. But how is the ampere defined for an alternating current?
33. A coil of 0.01 Henry inductance and 1 ohm resistance is connected to a 200-volt, 50 Hz AC supply. Find the impedance of the circuit and time lag between max. Alternating voltage and current.
34. 1 MW power is to be delivered from a power station to a town 10 km away. One uses a pair of Cu wires of radius 0.5 cm for this purpose. Calculate the fraction of ohmic losses to the power transmitted if (i) power is transmitted at 220V. Comment on the feasibility of doing this. (ii) a step-up transformer is used to boost the voltage to 11000 V, power is transmitted, then a step-down transformer is used to bring the voltage to 220 V. (p<sub<Cu1.7 x 10-8 SI unit)
35. If an LC circuit is considered analogous to a harmonically oscillating spring block system, which energy of the LC circuit would be analogous to potential energy and which one is analogous to kinetic energy?
36. Show that the radiation pressure exerted by an EM wave of intensity I on a surface kept in a vacuum is I/c.
37. What happens to the intensity of light from a bulb if the distance from the bulb is doubled? As a laser beam travels across the length of a room, its intensity essentially remains constant. What geometrical characteristic of the LASER beam is responsible for the constant intensity which is missing in the case of light from the bulb?
38. . Even though an electric field E exerts a force qE on a charged particle the electric field of an EM wave does not contribute to the radiation pressure (but transfers energy). Explain.
39. Sea water at frequency v = 4 x 10° Hz has permittivity & ≈ 80 €, permeability µ µo and resistivity p = 0.25 2-m. Imagine a parallel plate capacitor immersed in seawater and driven by an alternating voltage source V(t) = Vo sin (2n vt). What fraction of the conduction current density is the displacement current density?
40. . Why does a microwave oven heat a food item containing water molecules most efficiently?
41. For the same objective, find the ratio of the least separation between two points to be distinguished by a microscope for light of 5000 Å and electrons accelerated through 100V used as the illuminating substance.
42. The optical properties of a medium are governed by the relative permittivity (s) and relative permeability (pr). The refractive index is defined as Vurer. For ordinary material ɛ, > 0 and µ. > 0 and the positive sign is taken for the square root. In 1964, a Russian scientist V. Veselago postulated the existence of material with ε, < 0 and µ. < 0. Since then such ‘metamaterials’ have been produced in laboratories and their optical properties studied. For such materials n = με. As light enters a medium of such refractive index the phases travel away from the direction of propagation. (i) According to the description above shows that if rays of light enter such a medium from air (refractive index =1) at an angle & in the 2nd quadrant, then the refracted beam is in the 3rd quadrant. (ii) Prove that Snell’s law holds for such a medium.
43. To ensure almost 100 per cent transmissivity, photographic lenses are often coated with a thin layer of dielectric material. The refractive index of this material is intermediated between that of air and glass (which makes the optical element of the lens). A typically used dielectric film is MgF2 (n = 1.38). What should the thickness of the film be so that at the centre of the visible spectrum (5500 Å) there is a maximum transmission?
44. What is the shape of the wavefront on Earth for sunlight?
45. A polaroid (I) is placed in front of a monochromatic source. Another polaroid (II) is placed in front of this polaroid (I) and rotated till no light passes. A third polaroid (III) is now placed in between (I) and (II). In this case, will light emerge from (II). Explain.
46. The optical properties of a medium are governed by the relative permittivity (c) and relative permeability (µ.). The refractive index is defined as Vμερ. For ordinary material ɛ, > 0 and µ. > 0 and the positive sign is taken for the square root. In 1964, a Russian scientist V. Veselago postulated the existence of material with ε, < 0 and µ. < 0. Since then such ‘metamaterials’ have been produced in laboratories and their optical properties studied. For such materials n = με. As light enters a medium of such refractive index the phases travel away from the direction of propagation. (i) According to the description above shows that if rays of light enter such a medium from air (refractive index =1) at an angle & in the 2nd quadrant, then the refracted beam is in the 3rd quadrant. (ii) Prove that Snell’s law holds for such a medium.
47. To ensure almost 100 per cent transmissivity, photographic lenses are often coated with a thin layer of dielectric material. The refractive index of this material is intermediated between that of air and glass (which makes the optical element of the lens). A typically used dielectric film is MgF2 (n = 1.38). What should the thickness of the film be so that at the centre of the visible spectrum (5500 Å) there is a maximum transmission?
48. Can reflection result in plane polarised light if the light is incident on the interface from the side with a higher refractive index?
49. Can reflection result in plane polarised light if the light is incident on the interface from the side with a higher refractive index?
50. A polaroid (I) is placed in front of a monochromatic source. Another polaroid (II) is placed in front of this polaroid (I) and rotated till no light passes. A third polaroid (III) is now placed in between (I) and (II). In this case, will light emerge from (II). Explain.
51. Consider a metal exposed to light of wavelength 600 nm. The maximum energy of the electron doubles when light of wavelength 400 nm is used. Find the work function in eV.
52. Assuming an electron is confined to a 1nm wide region, find the uncertainty in momentum using the Heisenberg Uncertainty principle (Ref Eq 11.12 of NCERT Textbook). You can assume the uncertainty in position Ax as 1nm. Assuming p Ap, find the energy of the electron in electron volts.
53. Two particles A and B of de Broglie wavelengths 1 and 2 combine to form a particle C. The process conserves momentum. Find the de Broglie wavelength of the particle C. (The motion is one-dimensional).
54. A particle A with a mass ma is moving with a velocity v and hits a particle B (mass me) at rest (one-dimensional motion). Find the change in the de Broglie wavelength of the particle A. Treat the collision as elastic.
55. A proton and an a-particle are accelerated, using the same potential difference. How are the de Broglie wavelengths Ap and a related to each other?
56. The mass of a H-atom is less than the sum of the masses of a proton and electron. Why is this?
57. When an electron falls from a higher energy to a lower energy level, the difference in the energies appears in the form of electromagnetic radiation. Why cannot it be emitted as other forms of energy?
58. Positronium is just like a H-atom with the proton replaced by the positively charged anti-particle of the electron (called the positron which is as massive as the electron). What would be the ground state energy of positronium?
59. Using the Bohr model, calculate the electric current created by the electron when the H-atom is in the ground state.
60. In the Auger process an atom makes a transition to a lower state without emitting a photon. The excess energy is transferred to an outer electron which may be ejected by the atom. (This is called an Auger electron). Assuming the nucleus to be massive, calculate the kinetic energy of an n = 4 Auger electron emitted by Chromium by absorbing the energy from a n = 2 to n = 1 transition.
61. Are the nucleons fundamental particles, or do they consist of still smaller parts? One way to find out is to probe a nucleon just as Rutherford probed an atom. What should be the kinetic energy of an electron for it to be able to probe a nucleon? Assume the diameter of a nucleon to be approximately 10-15 m.
62. . Consider a radioactive nucleus A which decays to a stable nucleus C through the following sequence: A-B-C Here B is an intermediate nucleus which is also radioactive. Considering that there are no atoms of A initially, plot the graph showing the variation of number of atoms of A and B versus time.
63. A nuclide 1 is said to be the mirror isobar of nuclide 2 if Z₁=N2 and Z2 = N1. (a) What nuclide is a mirror isobar of 2311 Na? (b) Which nuclide out of the two mirror isobars has greater binding energy and why?
64. Deuteron is a bound state of a neutron and a proton with a binding energy B = 2.2 MeV. A y-ray of energy E is aimed at a deuteron nucleus to try to break it into a (neutron + proton) such that the n and p move in the direction of the incident y-ray. If EB, show that this cannot happen. Hence calculate how much bigger than B must E be for such a process to happen.
65. Draw a graph showing the variation of decay rate with a number of active nuclei.
66. The amplifiers X, Y and Z are connected in series. If the voltage gains of X, Y and Z are 10, 20 and 30, respectively and the input signal is a 1 mV peak value, then what is the output signal voltage (peak value) (i) if the dc supply voltage is 10V? (ii) if the dc supply voltage is 5V?
67. Can the potential barrier across a p-n junction be measured by simply connecting a voltmeter across the junction?
68. Two car garages have a common gate which needs to open automatically when a car enters either of the garages or cars enter both. Devise a circuit that resembles this situation using diodes for this situation.
69. Why are elemental dopants for Silicon or Germanium usually chosen from group XIII or group XV?
70. In a CE transistor amplifier there is a current and voltage gain associated with the circuit. In other words, there is a power gain. Considering power a measure of energy, does the circuit violate the conservation of energy?
71. A TV transmission tower antenna is at a height of 20 m. How much service area can it cover if the receiving antenna is (i) at ground level, and (ii) at a height of 25 m? Calculate the percentage increase in area covered in case (ii) relative to case (i)
72. The maximum frequency for reflection of sky waves from a certain layer of the ionosphere is found to be fmax = 9(Nmax) 1/2, where Nmax is the maximum electron density at that layer of the ionosphere. On a certain day, it is observed that signals of frequencies higher than 5MHz are not received by reflection from the F₁ layer of the ionosphere while signals of frequencies higher than 8MHz are not received by reflection from the F₁ layer of the ionosphere. Estimate the maximum electron densities of the F₁ and F2 layers on that day.
73. (i) Draw the plot of amplitude versus ‘w’ for an amplitude-modulated wave whose carrier wave (omega_{c}) is carrying two modulating signals, omega_{1} and omega_{2} (omega_{2} > omega_{1}) [Hint: Follow derivation from Eq 15.6 of NCERT Textbook of XII] (ii) Is the plot symmetrical about omega_{c}? Comment especially about the plot in region omega < omega_{c}. (iii) Extrapolate and predict the problems one can expect if more waves are to be modulated. (iv) Suggest solutions to the above problem. In the process can one understand another advantage of modulation in terms of bandwidth?
74. An audio signal is modulated by a carrier wave of 20MHz such that the bandwidth required for modulation is 3kHz. Could this wave be demodulated by a diode detector which has the values of R and Cas (i) R = 1k*Omega C = 0.01mu*F (ii) R = 10k*Omega C = 0.01mu*F (iii) R = 10 ΚΩ, C = 0.1mu*F
75. Would sky waves be suitable for transmission of TV signals of 60 MHz frequency?
Benefits of Solving Most Expected Questions Class 12 Physics
Solving the most expected questions for Class 12 Physics is one of the most effective strategies to prepare for your exam. These questions are carefully selected to cover key concepts and frequently asked topics, ensuring that you focus on what truly matters. Practicing them helps you strengthen your understanding of fundamental principles like mechanics, electromagnetism, and modern physics, while also improving your speed and accuracy in solving numerical and theoretical problems.
Additionally, these questions are often aligned with the CBSE exam pattern, enabling you to familiarize yourself with the structure and style of the actual paper. This targeted preparation not only optimizes your revision time but also boosts your confidence, reducing anxiety on exam day. Start practising today and experience the benefits of a strategic approach to mastering Physics!
