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# Physics Delhi (Set 2) 2019-2020 CBSE (Science) Class 12 Question Paper Solution

Physics [Delhi (Set 2)]
Marks: 70Academic Year: 2019-2020
Date: February 2020
Duration: 3h
• This question paper comprises four sections – A, B, C, and D.
• There are 37 questions in the question paper. All questions are compulsory.
• Section A:  Q. no. 1 to 20 are very short-answer type questions carrying 1 mark each.
• Section B: Q. no. 21 to 27 are short-answer type questions carrying 2 marks each.
• Section C: Q. no. 28 to 34 are long-answer type questions carrying 3 marks each.
• Section D: Q. no. 35 to 37 are also long answer type questions carrying 5 marks each.
• 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.

SECTION - A
[1]1

Choose the correct answer from given options
The wavelength and intensity of light emitted by a LED depend upon

forward bias and energy gap of the semiconductor

energy gap of the semiconductor and reverse bias

energy gap only

forward bias only

Concept: Special Purpose P-n Junction Diodes
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
[1]2

Choose the correct answer from given options
The graph showing the correct variation of linear momentum (p) of a charge particle with its de-Broglie wavelength (λ) is _____________.

Concept: Energy and Momentum
Chapter: [11.01] The Special Theory of Relativity
[1]3

Choose the correct answer from given options
The selectivity of a series LCR a.c. circuit is large, when

L is large and R is large

L is small and R is small

L is large and R is small

L = R

Concept: Ac Voltage Applied to a Series Lcr Circuit
Chapter: [4.01] Alternating Current
[1]4

Choose the correct answer from given options
Photo diodes are used to detect

gamma rays

IR rays

optical signals

Concept: Light Process and Photometry
Chapter: [6.01] Ray Optics and Optical Instruments
[1]5

Choose the correct answer from given options
The relationship between Brewster angle θ and speed of light v in the denser medium is

c cosθ = v

v tanθ = c

c = v tanθ

v cosθ = c

Concept: Brewster's Law
Chapter: [6.02] Wave Optics
[1]6

Choose the correct answer from given options
A biconvex lens of focal length f is cut into two identical plano-convex lenses. The focal length of each part will be

f

f/2

2f

4f

Concept: Lenses
Chapter: [6.01] Ray Optics and Optical Instruments
[1]7

Choose the correct answer from given options
The phase difference between the current and the voltage in series LCR circuit at resonance is

π

π/2

π/3

zero

Concept: Ac Voltage Applied to a Series Lcr Circuit
Chapter: [4.01] Alternating Current
[1]8

Choose the correct answer from given options
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

2 : 3

4 : 3

3: 4

3: 2

Concept: Einstein’S Photoelectric Equation: Energy Quantum of Radiation
Chapter: [7.01] Dual Nature of Radiation and Matter
[1]9

Choose the correct answer from given options
The electric flux through a closed Gaussian surface depends upon

Net charge enclosed and permittivity of the medium

Net, charge enclosed, permittivity of the medium and the size of the Gaussian surface

Net charge enclosed only

Permittivity of the medium only

Concept: Electric Flux
Chapter: [1.02] Electric Charges and Fields
[1]10

Choose the correct answer from given options
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

2r

sqrt(2r)

4r

r/sqrt(2)

Concept: The Earth’S Magnetism
Chapter: [3.01] Magnetism and Matter
[1]11

Fill in the blank.
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_________.

Concept: Electrostatics of Conductors
Chapter: [1.01] Electrostatic Potential and Capacitance
[1]12

Fill in the blank.
The ___________, a property of materials C, Si, and Ge depends upon the energy gap between their conduction and valence bands.

Concept: Electrical Resistivity and Conductivity
Chapter: [2.01] Current Electricity
[1]13

Fill in the blank.
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.

Concept: Application of Junction Diode as a Rectifier
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
[1]14

Fill in the blank.
The physical quantity having SI unit NC–1m is __________.

Concept: Electric Potential
Chapter: [1.01] Electrostatic Potential and Capacitance
[1]15

Fill in the blank.
A copper wire of the non-uniform area of cross-section is connected to a d.c. battery. The physical quantity, which remains constant along the wire is __________.

Concept: Electric Current
Chapter: [2.01] Current Electricity
[1]16

Answer the following question.
Write the conditions on path difference under which (i) constructive (ii) destructive interference occur in Young's double-slit experiment.

Concept: Interference of Light Waves and Young’S Experiment
Chapter: [6.02] Wave Optics
[1]17
[1]17.1

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

Concept: Induced Emf and Current
Chapter: [4.02] Electromagnetic Induction
OR
[1]17.2

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).

Concept: Reactance and Impedance
Chapter: [4.01] Alternating Current
[1]18

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

Concept: Propagation of Electromagnetic Waves
Chapter: [10.01] Communication Systems
[1]19
[1]19.1

An electron moves along +x direction. It enters into a region of uniform magnetic field. vecB directed along –z direction as shown in fig. Draw the shape of the trajectory followed by the electron after entering the field.

Concept: The Bar Magnet
Chapter: [3.01] Magnetism and Matter
OR
[1]19.2

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.

Concept: Magnetic Field Due to a Current Element, Biot-Savart Law
Chapter: [3.02] Moving Charges and Magnetism
[1]20

Define the term 'current sensitivity' of a moving coil galvanometer.

Concept: Moving Coil Galvanometer
Chapter: [3.02] Moving Charges and Magnetism
SECTION - B
[2]21
[1]21.1

Define one Bacquerel.

Concept: Radioactivity - Law of Radioactive Decay
Chapter: [8.01] Nuclei
[1]21.2

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.

Concept: Radioactivity - Law of Radioactive Decay
Chapter: [8.01] Nuclei
[2]22

Answer the following question.
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.

Concept: Diffraction - The Single Slit
Chapter: [6.02] Wave Optics
[2]23
[1]23.1

In the case of a photo electric effect experiment, explain the following facts, giving reasons.
The wave theory of light could not explain the existence of the threshold frequency.

Concept: Photoelectric Effect and Wave Theory of Light
Chapter: [7.01] Dual Nature of Radiation and Matter
[1]23.2

In the case of photoelectric effect experiment, explain the following facts, giving reasons.
The photoelectric current increases with increase of intensity of incident light.

Concept: Experimental Study of Photoelectric Effect
Chapter: [7.01] Dual Nature of Radiation and Matter
[2]24

Answer the following question.
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.

Concept: Electromagnetic Spectrum
Chapter: [5.01] Electromagnetic Waves
[2]25

Answer the following question.
Use Bohr's model of hydrogen atom to obtain the relationship between the angular momentum and the magnetic moment of the revolving electron.

Concept: Bohr'S Model for Hydrogen Atom
Chapter: [8.02] Atoms
[2]26
[2]26.1

Define the term 'wavefront of light'. A plane wavefront AB propagating from a 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 wavefront in the diagram.

Concept: Huygens Principle
Chapter: [6.02] Wave Optics
OR
[2]26.2

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.

Concept: Propagation of Electromagnetic Waves
Chapter: [10.01] Communication Systems
[2]27
[2]27.1

Answer the following question.
Obtain the expression for the energy stored in a capacitor connected across a dc battery. Hence define energy density of the capacitor

Concept: Energy Stored in a Capacitor
Chapter: [1.01] Electrostatic Potential and Capacitance
OR
[2]27.2

Answer the following question.
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.

Concept: Dipole in a Uniform External Field
Chapter: [1.02] Electric Charges and Fields [3.01] Magnetism and Matter
SECTION - C
[3]28
[2]28.1

Answer the following question.
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.

Concept: Relation Between Electric Field and Electrostatic Potential
Chapter: [1.01] Electrostatic Potential and Capacitance
[1]28.2

Answer the following question.
If an external electric field (E) is applied on the system, write the expression for the total energy of this system.

Concept: Relation Between Electric Field and Electrostatic Potential
Chapter: [1.01] Electrostatic Potential and Capacitance
[3]29
[3]29.1

What is a solar cell?

Concept: Special Purpose P-n Junction Diodes
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits

Answer the following question.
Draw solar cell V-I characteristics.

Concept: Special Purpose P-n Junction Diodes
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits

Answer the following question.
Explain the three processes involved in solar cell working.

Concept: Special Purpose P-n Junction Diodes
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
OR
[3]29.2

Draw the circuit diagram of a full wave rectifier along with the input and output waveforms

Concept: Diode as a Rectifier
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
[3]30
[2]30.1

Define internal resistance of a cell.

Concept: Measurement of Internal Resistance of a Cell
Chapter: [2.01] Current Electricity
[1]30.2

Answer the following question.
A cell of emf E and internal resistance r is connected across a variable resistor R. Plot the shape of graphs showing a variation of terminal voltage V with (i) R and (ii) circuit current I.

Concept: Cells, Emf, Internal Resistance
Chapter: [2.01] Current Electricity
[3]31

Answer the following question.
Calculate the de-Broglie wavelength associated with the electron revolving in the first excited state of the hydrogen atom. The ground state energy of the hydrogen atom is – 13.6 eV.

Concept: Bohr'S Model for Hydrogen Atom
Chapter: [8.02] Atoms
[3]32

Answer the following question.
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

Concept: Magnetic Flux
Chapter: [4.02] Electromagnetic Induction
[3]33

Answer the following question.
Draw the curve showing the variation of binding energy per nucleon with the mass number of nuclei. Using it explains the fusion of nuclei lying on the ascending part and fission of nuclei lying on the descending part of this curve.

Concept: Mass-energy and Nuclear Binding Energy - Nuclear Binding Energy
Chapter: [8.01] Nuclei
[3]34

Answer the following question.
An optical instrument uses a lens of 100 D for the objective lens and 50 D for its eyepiece. 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.

Concept: Refraction at Spherical Surfaces and by Lenses - Refraction by a Lens
Chapter: [6.01] Ray Optics and Optical Instruments
SECTION - D
[5]35
[3]35.1

Answer the following question.
Write two important characteristics of equipotential surfaces.

Concept: Equipotential Surfaces
Chapter: [1.01] Electrostatic Potential and Capacitance
[2]35.2

Answer the following question.
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.

Concept: Current Density
Chapter: [2.01] Current Electricity
OR
[3]35.3

Answer the following question.
State Gauss's law on electrostatics and drive 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.

Concept: Gauss’s Law
Chapter: [1.02] Electric Charges and Fields
[2]35.4

Answer the following question.
The magnitude of the 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 = 11 m to a point at r = 10 m.

Concept: Potential Due to a Point Charge
Chapter: [1.01] Electrostatic Potential and Capacitance
[5]36
[3]36.1

Define the term 'focal length of a mirror'.

Concept: Refraction at Spherical Surfaces and by Lenses - Refraction at Spherical Surfaces
Chapter: [6.01] Ray Optics and Optical Instruments

Answer the following question.
With the help of a ray diagram, obtain the relation between its focal length and radius of curvature.

Concept: Reflection of Light by Spherical Mirrors
Chapter: [6.01] Ray Optics and Optical Instruments
[2]36.2

Answer the following question.
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?

Concept: Refraction Through a Prism
Chapter: [6.01] Ray Optics and Optical Instruments
OR
[3]36.3

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.

Concept: Optical Instruments - Telescope
Chapter: [6.01] Ray Optics and Optical Instruments
[2]36.4

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.

Concept: Thin Lens Formula
Chapter: [6.01] Ray Optics and Optical Instruments
[5]37
[3]37.1

Answer the following question.
Show that an ideal inductor does not dissipate power in an ac circuit.

Concept: Power in Ac Circuit: the Power Factor
Chapter: [4.01] Alternating Current
[2]37.2

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 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.

Concept: Capacitors and Capacitance
Chapter: [1.01] Electrostatic Potential and Capacitance
[3]37.3

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.

Concept: Cells, Emf, Internal Resistance
Chapter: [2.01] Current Electricity
[2]37.4

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.

Concept: Magnetic Field Due to a Current Element, Biot-Savart Law
Chapter: [3.02] Moving Charges and Magnetism

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