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# Board Paper of Class 12-Science 2020 Physics Delhi(Set 2) - Solutions

General Instructions:

(i) This question paper comprises four sections – A, B, C and D.
(ii) There are 37 questions in the question paper. All questions are compulsory.
(iii) Section A :  Q. no. 1 to 20 are very short-answer type questions carrying 1 mark each.
(iv) Section B : Q. no. 21 to 27 are short-answer type questions carrying 2 marks each.
(v) Section C : Q. no. 28 to 34 are long-answer type questions carrying 3 marks each.
(vi) Section D : Q. no. 35 to 37 are also long answer type questions carrying 5 marks each.
(vii) There is no overall choice in the question paper. However, an internal choice has been provided in two questions of one mark, two questions of two marks, one question of three marks and all the three questions of five marks. You have to attempt only one of the choices in such questions.
(viii) However, separate instructions are given with each section and question, wherever necessary.
(ix) Use of calculators and log tables is not permitted.
(x) You may use the following values of physical constants wherever necessary.

c = 3 × 108 m/s

h = 6.63 × 10–34 Js

e = 1.6 × 10–19 C

μo = 4π × 10–7 T m A–1

ε0 = 8.854 × 10–12 C2 N–1 m–2 = 9 × 109 N m2 C–2

Mass of electron (me) = 9.1 × 10–31 kg

Mass of neutron = 1.675 × 10–27 kg

Mass of proton = 1.673 × 10–27 kg

Avogadro’s number = 6.023 × 1023 per gram mole

Boltzmann constant = 1.38 × 10–23 JK–1

• Question 1
The wavelength and intensity of light emitted by a LED depend upon
(a) forward bias and energy gap of the semiconductor
(b) energy gap of the semiconductor and reverse bias
(c) energy gap only
(d) forward bias only VIEW SOLUTION

• Question 2
The graph showing the correct variation of linear momentum (p) of a charge particle with its de-Broglie wavelength (λ) is – VIEW SOLUTION

• Question 3
The selectivity of a series LCR a.c. circuit is large, when
(a) L is large and R is large
(b) L is small and R is small
(c) L is large and R is small
(d) L = R VIEW SOLUTION

• Question 4
Photo diodes are used to detect
(b) gamma rays
(c) IR rays
(d) optical signals VIEW SOLUTION

• Question 5
The relationship between Brewster angle θ and speed of light v in the denser medium is
(a) cosθ = v
(b) v tanθ = c
(c) c = v tanθ
(d) v cosθ = c VIEW SOLUTION

• Question 6
A biconvex lens of focal length f is cut into two identical plano-convex lenses. The focal length of each part will be
(a) f
(b) f/2
(c) 2f
(d) 4f VIEW SOLUTION

• Question 7
The phase difference between the current and the voltage in series LCR circuit at resonance is
(a) π
(b) π/2
(c) π/3
(d) zero VIEW SOLUTION

• Question 8
Photons of frequency v are incident on the surface of two metals A and B of threshold frequency 3/4 v and 2/3 v, respectively. The ratio of maximum kinetic energy of electrons emitted from A to that from B is
(a) 2 : 3
(b) 4 : 3
(c) 3 : 4
(d) 3 : 2 VIEW SOLUTION

• Question 9
The electric flux through a closed Gaussian surface depends upon
(a) Net charge enclosed and permittivity of the medium
(b) Net, charge enclosed, permittivity of the medium and the size of the Gaussian surface
(c) Net charge enclosed only
(d) Permittivity of the medium only VIEW SOLUTION

• Question 10
A charge particle after being accelerated through a potential difference 'V' enters in a uniform magnetic field and moves in a circle of radius r. If V is doubled, the radius of the circle will become
(a) 2r
(b) $\sqrt{2}r$
(c) 4r
(d) $\frac{r}{\sqrt{2}}$ VIEW SOLUTION

• Question 11
A point charge is placed at the centre of a hollow conducting sphere of internal radius 'r' and outer radius '2r'. The ratio of the surface charge density of the inner surface to that of the outer surface will be_________. VIEW SOLUTION

• Question 12
The ___________, a property of materials C, Si and Ge depends upon the energy gap between their conduction and valence bands. VIEW SOLUTION

• Question 13
The ability of a junction diode to __________ an alternating voltage, is based on the fact that it allows current to pass only when it is forward biased. VIEW SOLUTION

• Question 14
The physical quantity having SI unit NC–1m is __________. VIEW SOLUTION

• Question 15
A copper wire of non-uniform area of cross-section is connected to a d.c. battery. The physical quantity, which remains constant along the wire is __________. VIEW SOLUTION

• Question 16
Write the conditions on path difference under which (i) constructive (ii) destructive interference occur in Young's double slit experiment. VIEW SOLUTION

• Question 17
Plot a graph showing variation of induced e.m.f. with the rate of change of current flowing through a coil.

OR

A series combination of an inductor (L), capacitor (C) and a resistor (R) is connected across an ac source of emf of peak value E0, and angular frequency (ω). Plot a graph to show variation of impedance of the circuit with angular frequency (ω). VIEW SOLUTION

• Question 18
Depict the fields diagram of an electromagnetic wave propagating along positive X-axis with its electric field along Y-axis. VIEW SOLUTION

• Question 19
An electron moves along +x direction. It enters into a region of uniform magnetic field. $\stackrel{\to }{\mathrm{B}}$ directed along –z direction as show in fig. Draw the shape of trajectory followed by the electron after entering the field. OR

A square shaped current carrying loop MNOP is placed near a straight long current carrying wire AB as shown in the fig. The wire and the loop lie in the same plane. If the loop experiences a net force F towards the wire, find the magnitude of the force on the side 'NO' of the loop. VIEW SOLUTION

• Question 20
Define the term 'current sensitivity' of a moving coil galvanometer. VIEW SOLUTION

• Question 21
(a) Define one Bacquerel.
(b) A radioactive substance disintegrates into two types of daughter nuclei, one type with disintegration constant λ1 and the other type with disintegration constant λ2 . Determine the half-life of the radioactive substance. VIEW SOLUTION

• Question 22
In a single slit diffraction experiment, the width of the slit is increased. How will the (i) size and (ii) intensity of central bright band be affected? Justify your answer. VIEW SOLUTION

• Question 23
In case of photo electric effect experiment, explain the following facts, giving reasons.
(a) The wave theory of light could not explain the existence of the threshold frequency.
(b) The photo electric current increases with increase of intensity of incident light. VIEW SOLUTION

• Question 24
Gamma rays and radio waves travel with the same velocity in free space. Distinguish between them in terms of their origin and the main application. VIEW SOLUTION

• Question 25
Use Bohr's model of hydrogen atom to obtain the relationship between the angular momentum and the magnetic moment of the revolving electron. VIEW SOLUTION

• Question 26
Define the term 'wave front of light'. A plane wave front AB propagating from denser medium (1) into a rarer medium (2) is incident of the surface P1P2 separating the two media as shown in fig.
Using Huygen's principle, draw the secondary wavelets and obtain the refracted wave front in the diagram. OR

Light from a sodium lamp (S) passes through two polaroid sheets P1 and P2 as shown in fig. What will be the effect on the intensity of the light transmitted (i) by P1 and (ii) by P2 on the rotating polaroid P1 about the direction of propagation of light? Justify your answer in both cases. VIEW SOLUTION

• Question 27
Obtain the expression for the energy stored in a capacitor connected across a dc battery. Hence define energy density of the capacitor.

OR

Derive the expression for the torque acting on an electric dipole, when it is held in a uniform electric field. identify the orientation of the dipole in the electric field, in which it attains a stable equilibrium. VIEW SOLUTION

• Question 28
(a) Two point charges q1 and q2 are kept at a distance of r12 in air. Deduce the expression for the electrostatic potential energy of this system.
(b) If an external electric field (E) is applied on the system, write the expression for the total energy of this system. VIEW SOLUTION

• Question 29
What is a solar cell? Draw its V-I characteristics. Explain the three processes involved in its working.

OR

Draw the circuit diagram of a full wave rectifier. Explain its working showing its input and output waveforms. VIEW SOLUTION

• Question 30
(a) Define internal resistance of a cell.
(b) A cell of emf E and internal resistance r is connected across a variable resistor R. Plot the shape of graphs showing variation of terminal voltage V with (i) R and (ii) circuit current I. VIEW SOLUTION

• Question 31
Calculate the de-Broglie wavelength associated with the electron revolving in the first excited state of hydrogen atom. The ground state energy of the hydrogen atom is –13.6 eV. VIEW SOLUTION

• Question 32
When a conducting loop of resistance 10 Ω and area 10 cm2 is removed from an external magnetic field acting normally, the variation of induced current-I in the loop with time t is as shown in the figure.

Find the
(a) total charge passed through the loop.
(b) change in magnetic flux through the loop
(c) magnitude of the field applied VIEW SOLUTION

• Question 33
Draw the curve showing the variation of binding energy per nucleon with the mass number of nuclei. Using it explain the fusion of nuclei lying on ascending part and fission of nuclei lying on descending part of this curve. VIEW SOLUTION

• Question 34
An optical instrument uses a lens of 100 D for the objective lens and 50 D for its eye piece. When the tube length is kept at 20 cm, the final image is formed at infinity.
(a) Identify the optical instrument.
(b) Calculate the magnification produced by the instrument. VIEW SOLUTION

• Question 35
(a) Write two important characteristics of equipotential surfaces.
(b) A thin circular ring of radius r is charged uniformly so that its linear charge density becomes λ. Derive an expression for the electric field at a point P at a distance x from it along the axis of the ring. Hence, prove that at large distances (x >> r), the ring behaves as a point charge.

OR

(a) State Gauss's law on electrostatics and drive an expression for the electric field due to a long straight thin uniformly charged wire (linear charge density λ) at a point lying at a distance r from the wire.
(b) The magnitude of electric field (in NC–1) in a region varies with the distance r(in m) as
E = 10r + 5
By how much does the electric potential increase in moving from point at r = 1 1 m to a point at r = 10 m. VIEW SOLUTION

• Question 36
(a) Define the term 'focal length of a mirror'. With the help of a ray diagram, obtain the relation between its focal length and radius of curvature.
(b) Calculate the angle of emergence (e) of the ray of light incident normally on the face AC of a glass prism ABC of refractive index $\sqrt{3}$. How will the angle of emergence change qualitatively, if the ray of light emerges from the prism into a liquid of refractive index 1.3 instead of air? OR

(a) Define the term 'resolving power of a telescope'. How will the resolving power be effected with the increase in
(i) Wavelength of light used.
(ii) Diameter of the objective lens.

(b) A screen is placed 80 cm from an object. The image of the object on the screen is formed by a convex lens placed between them at two different locations separated by a distance 20 cm. determine the focal length of the lens. VIEW SOLUTION

• Question 37
(a) Show that an ideal inductor does not dissipate power in an ac circuit.
(b) The variation of inductive reactance (XL) of an inductor with the frequency (f) of the ac source of 100 V and variable frequency is shown in the fig. (i) Calculate the self-inductance of the inductor.
(ii) When this inductor is used in series with a capacitor of unknown value and resistor of 10 Ω at 300 s–1, maximum power dissipation occurs in the circuit. Calculate the capacitance of the capacitor.

OR

(a) A conductor of length 'l' is rotated about one of its ends at a constant angular speed 'ω' in a plane perpendicular to a uniform magnetic field B. Plot graphs to show variations of the emf induced across the ends of the conductor with (i) angular speed ω and (ii) length of the conductor l.

(b) Two concentric circular loops of radius 1 cm and 20 cm are placed coaxially.
(i) Find mutual inductance of the arrangement.
(ii) If the current passed through the outer loop is changed at a rate of 5 A/ms, find the emf induced in the inner loop. Assume the magnetic field on the inner loop to be uniform.
VIEW SOLUTION
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