CBSE Class 12 Physics — Real Previous-Year Papers

In a series LCR circuit, the voltage across the resistor, capacitor and inductor is 10V each. If the capacitor is short circuited, the voltage across the inductor will be (A) 10V (B) 5√(2)V (C) dfrac5√(2)V (D) 10√(2)V

Electromagnetic waves used in a diagnostic tool in medicine have a wavelength range (A) 1nm to 10⁻³nm (B) 400nm to 1nm (C) 1mm to 700nm (D) 0.1m to 1mm

The 'distance of closest approach' of an alpha-particle is 'd' when it moves with a velocity v head-on towards the target nucleus. If the velocity of alpha particle is halved, the new 'distance of closest approach' will be (A) (d)/(2) (B) 2d (C) (d)/(4) (D) 4d

A concave lens of focal length 10cm is cut into two identical plano-concave lenses. The focal length of each lens will be (A) 20cm (B) 30cm (C) 40cm (D) 5cm

Four independent waves are expressed as (i) y₁ = A₁ sinω t (ii) y₂ = A₂ sin 2ω t (iii) y₃ = A₃ cosω t (iv) y₄ = A₄ sin(ω t + (π)/(3)) The interference between two of these waves is possible in (A) (i) and (iii) only (B) (iii) and (iv) only (C) (i), (iii) and (iv) only (D) All of them

Two heaters rated as (P₁, V) and (P₂, V) are connected in series across a dc source of (V)/(2) volt. The power consumed by the combination will be (A) (P₁ + P₂) (B) (P₁ + P₂)/(2) (C) (P₁ P₂)/(2(P₁ + P₂)) (D) (P₁ P₂)/(4(P₁ + P₂))

In an unbiased p-n junction, at equilibrium, which of the following statements is true? (A) Diffusion current is zero but drift current exists. (B) Diffusion current exists but drift current is zero. (C) Diffusion and drift currents are equal and opposite. (D) Both the diffusion and drift currents exist but are unequal.

For questions 13 to 16, two statements are given – one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer from the codes (A), (B), (C) and (D) below: (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Both Assertion (A) and Reason (R) are false. Assertion (A) : All atoms have a net magnetic moment. Reason (R) : A current loop does not always behave as a magnetic dipole.

For questions 13 to 16, two statements are given – one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer from the codes (A), (B), (C) and (D) below: (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Both Assertion (A) and Reason (R) are false. Assertion (A) : If accelerated electrons are passed through a narrow slit, a diffraction pattern is observed. Reason (R) : Electrons behave as both particles and waves.

For questions 13 to 16, two statements are given – one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer from the codes (A), (B), (C) and (D) below: (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Both Assertion (A) and Reason (R) are false. Assertion (A) : The mass of a nucleus is less than the sum of the masses of the constituent nucleons. Reason (R) : Energy is absorbed when the nucleons are bound together to form a nucleus.

For questions 13 to 16, two statements are given – one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer from the codes (A), (B), (C) and (D) below: (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Both Assertion (A) and Reason (R) are false. Assertion (A) : In Bohr model of hydrogen atom, the energy levels are discrete and quantised. Reason (R) : In a hydrogen atom, the electrostatic force on the electron provides the necessary centripetal force to it to revolve around the nucleus.

In a photoelectric experiment, the emitter plate is irradiated with radiation of 200nm. The photocurrent becomes zero when the collector plate potential is -0.80V. Calculate the work function (in eV) of the emitter.

(a) A beam of light consisting of two wavelengths 400nm and 600nm is used to illuminate a single slit of width 1mm. Find the least distance of the point from the central maximum where the dark fringes due to both wavelengths coincide on the screen placed 1.5m from the slit. OR (b) In a Young's double-slit experimental set-up with slit separation 0.6mm a beam of light consisting of two wavelengths 440nm and 660nm is used to obtain interference pattern on a screen kept 1.5m in front of the slits. Find the least distance of the point from the central maximum where the bright fringes due to both the wavelengths coincide.

A wire of length L is bent round into (i) a square coil having N turns and (ii) a circular coil having N turns. The coil in both cases is free to turn about a vertical axis coinciding with the plane of the coil, in a uniform, horizontal magnetic field and carry the same currents. Find the ratio of the maximum value of the torque acting on the square coil to that on the circular coil.

What is the order of magnitude of drift velocity of electrons in a conductor? Deduce the relation between the current flowing through a conductor and drift velocity of electrons in it.

Draw the plot of potential energy of a pair of nucleons as a function of their separation. Write two important conclusions that can be drawn from this plot.

(a) Using Gauss's law, deduce an expression for electric field at a point due to a uniformly charged infinite plane thin sheet. (b) Two large thin plane sheets, each having surface charge density σ are held close and parallel to each other in air. What is the net electric field at a point (i) inside and (ii) outside, the sheets? OR (a) Obtain the condition of balance of a Wheatstone bridge. (b) Find net resistance of the network of resistors connected between A and B, as shown in figure.

A parallel plate capacitor of capacitance C has a dielectric slab between its plates. It is charged to a potential difference V by connecting it across a battery. The battery is then disconnected. If the dielectric slab is now withdrawn from the capacitor, how will the following be affected? (a) Capacitance of the capacitor, (b) Energy stored in the capacitor, and (c) The potential difference between the plates of the capacitor. Justify your answer in each case.

Figure shows a narrow beam of electrons entering with a velocity of 3×10⁷m/s, symmetrically through the space between two parallel horizontal plates P₁P₁' and P₂P₂' kept 2cm apart. If each plate is 3cm long, calculate the potential difference V applied between the plates so that the beam just strikes the end P₂'.

An ac voltage Vᵢ = 12sin(100π t)V is applied between points A and B in a network of two ideal diodes and three resistors as shown in figure. During the positive half-cycle of the input voltage Vᵢ supplied to the network: (a) Identify which of the two diodes will conduct and why? (b) Redraw an equivalent circuit diagram to show the flow of current. (c) Calculate the output voltage drops V₀ across the three resistors when the input voltage attains its peak value.

Briefly explain the two important processes that occur during the formation of a p-n junction.

(a) Draw the ray diagram to show the image formation by a refracting telescope and write the expression for angular magnification for the telescope in normal adjustment. (b) Give two reasons to explain why a reflecting telescope is preferred over a refracting telescope.

(a) State the two conditions under which total internal reflection occurs. (b) A transparent container contains layers of three immiscible transparent liquids A, B and C of refractive indices n, (3)/(4)n and (2)/(3)n, respectively. A laser beam is incident at the interface between A and B at an angle θ as shown in figure. Prove that the beam does not enter region C at all for sinθ ≥ (2)/(3).

A galvanometer is used to detect or/and measure small currents in an electrical circuit. It essentially works on the fact that a current-carrying coil experiences a deflecting torque when placed in a magnetic field. This deflection in the coil can be measured and it is related to the current flowing in the coil, the number of turns in the coil, area of the coil and the magnetic field. A hair spring attached to the coil provides a counter torque and helps in measuring the deflection. A galvanometer can be converted to an ammeter or a voltmeter of desired range by using suitable resistances. (I) The torque on the coil remains constant irrespective of the coil's orientation during rotation due to (A) use of soft iron core which increases the magnetic field. (B) radial magnetic field (C) hair spring which provides the counter torque (D) eddy current in the iron core which causes damping. (II) The best way to increase current sensitivity of a galvanometer is by (A) increasing number of turns of the coil (B) increasing area of coil and magnetic field strength (C) decreasing area of coil and magnetic field strength (D) increasing torsional constant of the hair spring (III) A moving coil galvanometer has a coil with area of cross-section 4.0×10⁻³m² and number of turns 50. The coil is rotating in a magnetic field of 0.25T. The torque acting on the coil when a current of 5A passes through it is (A) 1.0N m (B) 2.0N m (C) 0.50N m (D) 0.25N m OR A galvanometer coil has a resistance of 15Ω and the meter shows full scale deflection for a current of 3mA. The value of resistance required to convert it into a voltmeter of range (0-12V) is (A) 4015Ω (B) 3985Ω (C) 415Ω (D) 385Ω (IV) A galvanometer with coil of resistance 20Ω shows full scale deflection for a current of 5mA. To convert it into an ammeter of range (0-10A), a resistance of (A) 0.05Ω should be connected in series with it. (B) 0.05Ω should be connected in parallel with it. (C) 0.01Ω should be connected in parallel with it. (D) 0.01Ω should be connected in series with it.

A researcher performs an experiment on photo-electric effect using two metals A and B with unknown work functions. She illuminates the surfaces of A and B with monochromatic radiation of various frequencies and records the value of corresponding stopping potentials (Vs). The graph shows the variation of stopping potential (Vs) with the frequency of incident radiation (nu) for metals A and B. Answer the following questions: (I) From the graph, the work functions of A and B are (h is Planck's constant and e value of charge on an electron) (A) nu₁ and nu₂ (B) V₁ and V₂ (C) hnu₁ and hnu₂ (D) (hnu₁)/(e) and (hnu₂)/(e) (II) For radiation of frequency nu > nu₂ incident on the surfaces of A and B, the maximum kinetic energy of ejected electron is (A) greater for metal A because it has a smaller work function. (B) greater for metal B because it has a larger work function. (C) greater for metal B because it has higher threshold frequency. (D) the same for both metal A and metal B because it is independent of work functions of metals. (III) If the intensity of the incident radiation for both metals A and B, is doubled keeping its frequency constant, then (A) the slope of the parallel lines will increase. (B) the slope of the parallel lines will decrease. (C) the threshold frequencies for both A and B will decrease. (D) the slope of the parallel lines will not change but more electrons will be emitted per second. (IV) The threshold frequency for a metal surface is nu₀. If the radiation of frequency 3nu₀ illuminates the surface, the maximum kinetic energy (KE) of photoelectrons is E₁. If the frequency were increased to 6nu₀, the maximum KE of the photoelectrons becomes E₂. Then (E₁)/(E₂) equals (A) (1)/(3) (B) (1)/(2) (C) (2)/(5) (D) (3)/(4) OR Let m be the slope of the graph line for metal B. If e is the value of electron charge, then Planck's constant 'h' is given by (A) me (B) (1)/(me) (C) (m)/(e) (D) (e)/(m)

(a) An electric dipole consists of two point charges q and -q separated by a distance 2a. Derive an expression for the electric field vecE due to this dipole at a point distant r from the centre of the dipole on the equatorial plane. Write the expression for the electric field at a far off point, i.e. r gg a. (b) A dipole is placed in x-y plane such that charges q and -q are located at x = a and x = b respectively. There exists an electric field vecE = 2hatidfracNC in the region. Calculate the force vecF and torque vecτ experienced by the dipole. OR (a) E₁ and E₂ are the emfs of two cells with internal resistances r₁ and r₂ respectively, connected in parallel. Deduce an expression for the equivalent emf and equivalent internal resistance of the combination. (b) A parallel combination, as stated in (a) above, of two cells of emfs E and 3E and internal resistances R each is connected across a resistance 2R. Find the current that flows through resistance 2R.

(a) Using the relation for refraction at a curved spherical surface, derive the expression for lens maker's formula. (b) Three lenses L₁, L₂ and L₃, each of focal length 40cm, are placed coaxially. The distance between L₁ and L₂ and between L₂ and L₃ are 120cm and 20cm respectively. An object is kept at a distance of 80cm to the left of lens L₁. Find the distance of the final image formed from the object. OR (a) Draw a ray diagram to show the image formation by a concave mirror when the object is kept between its focus and the centre of curvature. Using this diagram, derive the mirror formula. (b) A concave mirror produces a two times magnified virtual image of an object kept 10cm in front of it. Calculate the focal length of the mirror.

(a) State Faraday's law of electromagnetic induction. (b) Derive an expression for the self-inductance of an air-filled long solenoid of length l and cross-sectional area A having N turns. (c) A conducting rod of length 50cm, with one end pivoted, is rotated with angular speed of 60rpm in a uniform magnetic field of 4.0mT directed perpendicular to the plane of rotation of rod. Find the emf induced in the rod. OR (a) Draw a labelled diagram of a step-up transformer. State the principle on which it works and obtain the ratio of secondary voltage to primary voltage in terms of number of turns and currents in the two coils. (b) The ratio of the number of turns in the primary to the secondary of an ideal transformer is 1 : 5. If 5kW power at 200V is supplied to the primary, find (i) current in the primary, and (ii) output voltage.