CBSE Class 12 Physics 2020 — Previous-Year Question Paper

A biconvex lens of glass having refractive index 1·47 is immersed in a liquid. It becomes invisible and behaves as a plane glass plate. The refractive index of the liquid is (A) 1·47 (B) 1·62 (C) 1·33 (D) 1·51

For a glass prism, the angle of minimum deviation will be smallest for the light of (A) red colour. (B) blue colour. (C) yellow colour. (D) green colour.

Which of the following statements is not correct according to Rutherford model ? (A) Most of the space inside an atom is empty. (B) The electrons revolve around the nucleus under the influence of coulomb force acting on them. (C) Most part of the mass of the atom and its positive charge are concentrated at its centre. (D) The stability of atom was established by the model.

Photons of energies 1 eV and 2 eV are successively incident on a metallic surface of work function 0·5 eV. The ratio of kinetic energy of most energetic photoelectrons in the two cases will be (A) 1 : 2 (B) 1 : 1 (C) 1 : 3 (D) 1 : 4

The magnetic field and angle of dip at a place on the earth are 0·3 G and 30^°, respectively. The value of vertical component of the earth's magnetic field at the place is ______ .

The number of turns of a solenoid are doubled without changing its length and area of cross-section. The self-inductance of the solenoid will become ______ times.

According to Bohr's atomic model, the circumference of the electron orbit is always an ______ multiple of de Broglie wavelength. OR In β-decay, the parent and daughter nuclei have the same number of _______ .

A ray of light on passing through an equilateral glass prism, suffers a minimum deviation equal to the angle of the prism. The value of refractive index of the material of the prism is ______ .

Write the mathematical form of Ampere-Maxwell circuital law.

How does an increase in doping concentration affect the width of depletion layer of a p-n junction diode ?

The nuclear radius of ²⁷₁₃Al is 3·6 fermi. Find the nuclear radius of ⁶⁴₂₉Cu. OR A proton and an electron have equal speeds. Find the ratio of de Broglie wavelengths associated with them.

The variation of the stopping potential (V₀) with the frequency (nu) of the light incident on two different photosensitive surfaces M₁ and M₂ is shown in the figure. Identify the surface which has greater value of the work function.

The space between the plates of a parallel plate capacitor is completely filled in two ways. In the first case, it is filled with a slab of dielectric constant K. In the second case, it is filled with two slabs of equal thickness and dielectric constants K₁ and K₂ respectively as shown in the figure. The capacitance of the capacitor is same in the two cases. Obtain the relationship between K, K₁ and K₂.

Define wavefront of a travelling wave. Using Huygens principle, obtain the law of refraction at a plane interface when light passes from a denser to rarer medium. OR Using lens maker's formula, derive the thin lens formula (1)/(v) - (1)/(u) = (1)/(f) for a biconvex lens.

Two long straight parallel wires A and B separated by a distance d, carry equal current I flowing in same direction as shown in the figure. (a) Find the magnetic field at a point P situated between them at a distance x from one wire. (b) Show graphically the variation of the magnetic field with distance x for 0 < x < d.

Using Bohr's atomic model, derive the expression for the radius of nth orbit of the revolving electron in a hydrogen atom. OR (a) Write two main observations of photoelectric effect experiment which could only be explained by Einstein's photoelectric equation. (b) Draw a graph showing variation of photocurrent with the anode potential of a photocell.

Explain the terms 'depletion layer' and 'potential barrier' in a p-n junction diode. How are the (a) width of depletion layer, and (b) value of potential barrier affected when the p-n junction is forward biased ?

(a) Two cells of emf E₁ and E₂ have their internal resistances r₁ and r₂, respectively. Deduce an expression for the equivalent emf and internal resistance of their parallel combination when connected across an external resistance R. Assume that the two cells are supporting each other. (b) In case the two cells are identical, each of emf E = 5 V and internal resistance r = 2Ω, calculate the voltage across the external resistance R = 10Ω.

(a) Write an expression of magnetic moment associated with a current (I) carrying circular coil of radius r having N turns. (b) Consider the above mentioned coil placed in YZ plane with its centre at the origin. Derive expression for the value of magnetic field due to it at point (x, 0, 0). OR (a) Define current sensitivity of a galvanometer. Write its expression. (b) A galvanometer has resistance G and shows full scale deflection for current Ig. (i) How can it be converted into an ammeter to measure current up to I₀ (I₀ > Ig) ? (ii) What is the effective resistance of this ammeter ?

A resistance R and a capacitor C are connected in series to a source V = V₀ sin ω t. Find : (a) The peak value of the voltage across the (i) resistance and (ii) capacitor. (b) The phase difference between the applied voltage and current. Which of them is ahead ?

What is the effect on the interference fringes in Young's double slit experiment due to each of the following operations ? Justify your answers. (a) The screen is moved away from the plane of the slits. (b) The separation between slits is increased. (c) The source slit is moved closer to the plane of double slit.

(a) Write the expression for the speed of light in a material medium of relative permittivity varepsilonr and relative magnetic permeability μr. (b) Write the wavelength range and name of the electromagnetic waves which are used in (i) radar systems for aircraft navigation, and (ii) Earth satellites to observe the growth of the crops.

The nucleus ²³⁵₉₂Y, initially at rest, decays into ²³¹₉₀X by emitting an α-particle ²³⁵₉₂Y longrightarrow²³¹₉₀X +⁴₂He + energy. The binding energies per nucleon of the parent nucleus, the daughter nucleus and α-particle are 7·8 MeV, 7·835 MeV and 7·07 MeV, respectively. Assuming the daughter nucleus to be formed in the unexcited state and neglecting its share in the energy of the reaction, find the speed of the emitted α-particle. (Mass of α-particle = 6·68 × 10⁻²⁷ kg)

(a) Using Gauss law, derive expression for electric field due to a spherical shell of uniform charge distribution σ and radius R at a point lying at a distance x from the centre of shell, such that (i) 0 < x < R, and (ii) x > R. (b) An electric field is uniform and acts along +x direction in the region of positive x. It is also uniform with the same magnitude but acts in -x direction in the region of negative x. The value of the field is E = 200 N/C for x > 0 and E = -200 N/C for x < 0. A right circular cylinder of length 20 cm and radius 5 cm has its centre at the origin and its axis along the x-axis so that one flat face is at x = +10 cm and the other is at x = -10 cm. Find : (i) The net outward flux through the cylinder. (ii) The net charge present inside the cylinder. OR (a) Find the expression for the potential energy of a system of two point charges q₁ and q₂ located at vecr₁ and vecr₂, respectively in an external electric field vecE. (b) Draw equipotential surfaces due to an isolated point charge (-q) and depict the electric field lines. (c) Three point charges +1muC, -1muC and +2muC are initially infinite distance apart. Calculate the work done in assembling these charges at the vertices of an equilateral triangle of side 10 cm.

(a) Derive the expression for the torque acting on the rectangular current carrying coil of a galvanometer. Why is the magnetic field made radial ? (b) An α-particle is accelerated through a potential difference of 10 kV and moves along x-axis. It enters in a region of uniform magnetic field B = 2 × 10⁻³ T acting along y-axis. Find the radius of its path. (Take mass of α-particle = 6·4 × 10⁻²⁷ kg) OR (a) With the help of a labelled diagram, explain the working of a step-up transformer. Give reasons to explain the following : (i) The core of the transformer is laminated. (ii) Thick copper wire is used in windings. (b) A conducting rod PQ of length 20 cm and resistance 0·1Ω rests on two smooth parallel rails of negligible resistance AA' and CC'. It can slide on the rails and the arrangement is positioned between the poles of a permanent magnet producing uniform magnetic field B = 0·4 T. The rails, the rod and the magnetic field are in three mutually perpendicular directions as shown in the figure. If the ends A and C of the rails are short circuited, find the (i) external force required to move the rod with uniform velocity v = 10 cm/s, and (ii) power required to do so.