CBSE Class 12 Physics 2020 — Set 55/2/1

At equilibrium, in a p-n junction diode the net current is (A) due to diffusion of majority charge carriers. (B) due to drift of minority charge carriers. (C) zero as diffusion and drift currents are equal and opposite. (D) zero as no charge carriers cross the junction.

In an n-type semiconductor, the donor energy level lies (A) at the centre of the energy gap. (B) just below the conduction band. (C) just above the valance band. (D) in the conduction band.

When two nuclei (A ≤ 10) fuse together to form a heavier nucleus, the (A) binding energy per nucleon increases. (B) binding energy per nucleon decreases. (C) binding energy per nucleon does not change. (D) total binding energy decreases.

If the electric flux entering and leaving a closed surface in air are φ₁ and φ₂ respectively, the net electric charge enclosed within the surface is ______ .

In Young's double slit experiment, the path difference between two interfering waves at a point on the screen is (5λ)/(2), λ being wavelength of the light used. The ______ dark fringe will lie at this point. OR If one of the slits in Young's double slit experiment is fully closed, the new pattern has _____ central maximum in angular size.

What is the impedance of a capacitor of capacitance C in an ac circuit using source of frequency n Hz ? OR What is the value of impedance of a resonant series LCR circuit ?

A conducting rod of length l is kept parallel to a uniform magnetic field vecB. It is moved along the magnetic field with a velocity vecv. What is the value of emf induced in the conductor ?

Draw the graph showing variation of the value of the induced emf as a function of rate of change of current flowing through an ideal inductor.

What is the wavelength of a photon of energy 3·3 × 10⁻¹⁹ J ?

Define the term 'threshold frequency' in photoelectric emission.

Define the term 'mobility' of charge carriers in a current carrying conductor. Obtain the relation for mobility in terms of relaxation time. OR Define the term 'drift velocity' of electrons in a current carrying conductor. Obtain the relationship between the current density and the drift velocity of electrons.

An ammeter of resistance 0·8Ω can measure a current up to 1·0 A. Find the value of shunt resistance required to convert this ammeter to measure a current up to 5·0 A.

(a) Explain the term 'sharpness of resonance' in ac circuit. (b) In a series LCR circuit, VL = VC ≠ VR. What is the value of power factor for this circuit ? OR An ac source of emf V = V₀ sin ω t is connected to a capacitor of capacitance C. Deduce the expression for the current (I) flowing in it. Plot the graph of (i) V vs. ω t, and (ii) I vs. ω t.

Which of the following electromagnetic waves has (a) minimum wavelength, and (b) minimum frequency ? Write one use of each of these two waves. Infrared waves, Microwaves, γ-rays and X-rays

An object is kept 20 cm in front of a concave mirror of radius of curvature 60 cm. Find the nature and position of the image formed.

In Geiger-Marsden scattering experiment, the trajectory of α-particles in Coulomb's field of a heavy nucleus is shown in the figure. (a) What do 'b' and 'θ' represent in the figure ? (b) What will be the value of 'b' for (i) θ = 0, and (ii) θ = 180° ?

Draw V-I characteristics of a p-n junction diode. Explain, why the current under reverse bias is almost independent of the applied voltage up to the critical voltage.

Two small identical electric dipoles AB and CD, each of dipole moment vecp, are kept at an angle of 120° to each other in an external electric field vecE pointing along the x-axis as shown in the figure. Find the (a) dipole moment of the arrangement, and (b) magnitude and direction of the net torque acting on it. OR In the figure given below, find the (a) equivalent capacitance of the network between points A and B. Given : C₁ = C₅ = 8μ F, C₂ = C₃ = C₄ = 4μ F. (b) maximum charge supplied by the battery, and (c) total energy stored in the network.

The figure shows the graphical variation of the reactance of a capacitor with frequency of ac source. (a) Find the capacitance of the capacitor. (b) An ideal inductor has the same reactance at 100 Hz frequency as the capacitor has at the same frequency. Find the value of inductance of the inductor. (c) Draw the graph showing the variation of the reactance of this inductor with frequency.

What is the difference in the construction of an astronomical telescope and a compound microscope ? The focal lengths of the objective and eyepiece of a compound microscope are 1·25 cm and 5·0 cm, respectively. Find the position of the object relative to the objective in order to obtain an angular magnification of 30 when the final image is formed at the near point.

The maximum kinetic energy of the photoelectrons emitted is doubled when the wavelength of light incident on the photosensitive surface changes from λ₁ to λ₂. Deduce expressions for the threshold wavelength and work function for the metal surface in terms of λ₁ and λ₂.

(a) Use Gauss's law to show that due to a uniformly charged spherical shell of radius R, the electric field at any point situated outside the shell at a distance r from its centre is equal to the electric field at the same point, when the entire charge on the shell were concentrated at its centre. Also plot the graph showing the variation of electric field with r, for r ≤ R and r ≥ R. (b) Two point charges of +1μ C and +4μ C are kept 30 cm apart. How far from the +1μ C charge on the line joining the two charges, will the net electric field be zero ? OR (a) Two point charges q₁ and q₂ are kept r distance apart in a uniform external electric field vecE. Find the amount of work done in assembling this system of charges. (b) A cube of side 20 cm is kept in a region as shown in the figure. An electric field vecE exists in the region such that the potential at a point is given by V = 10x + 5, where V is in volt and x is in m. Find the (i) electric field vecE, and (ii) total electric flux through the cube.

(a) A circular loop of radius R carries a current I. Obtain an expression for the magnetic field at a point on its axis at a distance x from its centre. (b) A conducting rod of length 2 m is placed on a horizontal table in north-south direction. It carries a current of 5 A from south to north. Find the direction and magnitude of the magnetic force acting on the rod. Given that the Earth's magnetic field at the place is 0·6 × 10⁻⁴ T and angle of dip is (π)/(6). OR (a) Obtain the expression for the deflecting torque acting on the current carrying rectangular coil of a galvanometer in a uniform magnetic field. Why is a radial magnetic field employed in the moving coil galvanometer ? (b) Particles of mass 1·6 × 10⁻²⁷ kg and charge 1·6 × 10⁻¹⁹ C are accelerated in a cyclotron of dee radius 40 cm. It employs a magnetic field 0·4 T. Find the kinetic energy (in MeV) of the particle beam imparted by the accelerator.

(a) Derive lens maker's formula for a biconvex lens. (b) A point object is placed at a distance of 12 cm on the principal axis of a convex lens of focal length 10 cm. A convex mirror is placed coaxially on the other side of the lens at a distance of 10 cm. If the final image coincides with the object, sketch the ray diagram and find the focal length of the convex mirror. OR (a) What is a wavefront ? How does it propagate ? Using Huygens' principle, explain reflection of a plane wavefront from a surface and verify the laws of reflection. (b) A parallel beam of light of wavelength 500 nm falls on a narrow slit and the resulting diffraction pattern is obtained on a screen 1 m away. If the first minimum is formed at a distance of 2·5 mm from the centre of the screen, find the (i) width of the slit, and (ii) distance of first secondary maximum from the centre of the screen.