# Physics 55/3/3 2018-2019 CBSE (Science) Class 12 Question Paper Solution

Physics [55/3/3]
Date & Time: 5th March 2019, 2:30 pm
Duration: 3h

SECTION - A ( 5 marks )
[1]1

Define the term 'intensity of radiation' in terms of photon picture of light.

Concept: Experimental Study of Photoelectric Effect
Chapter: [7.01] Dual Nature of Radiation and Matter
[1]2
[1]2.a

State the condition under which a large magnification can be achieved in an astronomical telescope.

Concept: Optical Instruments - Telescope
Chapter: [6.01] Ray Optics and Optical Instruments
OR
[1]2.b

How does the angle of minimum deviation of a glass prism vary, if the incident violet light is replaced by red light? Give reason.

Concept: Refraction Through a Prism
Chapter: [6.01] Ray Optics and Optical Instruments
[1]3

Show variation of resistivity of copper as a function of temperature in a graph.

Concept: Electrical Resistivity and Conductivity
Chapter: [2.01] Current Electricity
[1]4
[1]4.a

Two metallic spheres A and B kept on insulating stands are in contact with each other. A positively charged  rod P is brought near the sphere A as shown in the figure. The two spheres are separated from each other,  and the rod P is removed. What will be the nature of charges on spheres A and B?

Concept: Ohm’s Law
Chapter: [2.01] Current Electricity
OR
[1]4.b

A metal sphere is kept on an insulting stands. A negatively charged rod is brought near it, then the sphere is  earthed as shown. On removing the earthing, and taking the negatively charged rod away, what will be the  nature of charge on the sphere? Give reason for your answer.

Concept: Ohm’s Law
Chapter: [2.01] Current Electricity
[1]5

Write the function of receiver in communication system.

Concept: Basic Terminology Used in Electronic Communication Systems
Chapter: [10.01] Communication Systems
SECTION - B (14 marks )
[2]6

Plot a graph showing the variation of undecayed nuclei N versus time t. From the graph, find out how one  can determine the half-life and average life of the radioactive nuclei.

Chapter: [8.01] Nuclei
[2]7
[1]7.a

Which property of nuclear force explains the constancy of binding energy per nucleon ((BE)/A) for nuclei in the range 20< A < 170 ?

Concept: Mass-energy and Nuclear Binding Energy - Nuclear Binding Energy
Chapter: [8.01] Nuclei
[1]7.b

Complete the following nuclear reactions :

(i) "_15^32P ->  "_z^AX + bar(e) + bar(v)

(ii) "_6^12 C +"_6^12C ->  "_2^A Y +  "_4^2 He

Chapter: [8.01] Nuclei
[2]8

Obtain Bohr’s quantisation condition for angular momentum of electron orbiting in nth orbit in hydrogen  atom on the basis of the wave picture of an electron using de Broglie hypothesis.

Concept: Bohr'S Model for Hydrogen Atom
Chapter: [8.02] Atoms
[2]9

The figure show a network of five capacitors connected to a 10V battery. Calculate the charge acquired by the 5μF capacitor.

Concept: Capacitors and Capacitance
Chapter: [1.01] Electrostatic Potential and Capacitance
[2]10
[2]10.a

A .0.5m long solenoid of 10 turns/cm has area of cross-section 1cm2 . Calculate the voltage induced across  its ends if the current in the solenoid is changed from 1A to 2A in 0.1s.

Chapter: [4.02] Electromagnetic Induction
OR
[2]10.b

A small flat search coil of area 5cm2 with 140 closely wound turns is placed between the poles of a  powerful magnet producing magnetic field 0.09T and then quickly removed out of the field region. Calculate:

(a) Change of magnetic flux through the coil, and

(b) emf induced in the coil.

Concept: Induced Emf and Current
Chapter: [4.02] Electromagnetic Induction
[2]11

In the case of a concave mirror of focal length f , when an object is kept between f and 2 f , show that its image is formed beyond 2 f .

Concept: Concave Mirror
Chapter: [6.01] Ray Optics and Optical Instruments
[2]12
[1]12.a

Five point charges, each of charge +q are placed on five vertices of a regular hexagon of side 'l '. Find the magnitude of the resultant force on a charge -q placed at the centre of the hexagon.

Concept: Superposition Principle of Forces
Chapter: [1.02] Electric Charges and Fields
[1]12.b

A smple pendulum consists of a small sphere of mass m suspended by a thread of length l. The sphere  carries a positive charge q. The pendulum is placed in a uniform electric field of strength E directed vertically  downwards. Find the period of oscillation of the pendulum due to the electrostatic force acting on the  sphere, neglecting the effect of the gravitational force.

Concept: Electric Charges
Chapter: [1.02] Electric Charges and Fields
SECTION - C
[3]13
[3]13.a
[1]13.a.a

An ac circuit as shown in the figure has an inductor of inductance L and a resistor or resistance R  connected in series. Using the phasor diagram, explain why the voltage in the circuit will lead the  current in phase.

Concept: Ac Voltage Applied to a Series Lcr Circuit
Chapter: [4.01] Alternating Current
[1]13.a.b

The potential difference across the resistor is 160V and that across the inductor is 120V. Find the  effective value of the applied voltage. If the effective current in the circuit be 1.0 A, calculate the total impedance of the circuit.

Concept: Ac Voltage Applied to a Series Lcr Circuit
Chapter: [4.01] Alternating Current
[1]13.a.c

What will be the potential difference in the circuit when direct current is passed through the circuit?

Concept: Ac Voltage Applied to a Series Lcr Circuit
Chapter: [4.01] Alternating Current
OR
[3]13.b
[3]13.b.a

An ac circuit consists of a series combination of circuit elements X and Y. The current is ahead of the voltage  in phase by pi /4 . If element X is a pure resistor of 100Ω ,

(a)  name the circuit element Y.
(b)  calculate the rms value of current, if rms value of voltage is  141V.
(c)  what will happen if the ac source is replaced by a dc source ?

Concept: Combination of Capacitors
Chapter: [1.01] Electrostatic Potential and Capacitance
[3]14

Write the range of the wavelength of the following electromagnetic radiations:
(a) Infrared rays
(b) Ultraviolet rays
(c) γ -rays

Write one use of each of the above.

Concept: Electromagnetic Spectrum
Chapter: [5.01] Electromagnetic Waves
[3]15
[3]15.a
[1.5]15.a.a

Plot a graph to show the variation of stopping potential with frequency of incident radiation in relation to photoelectric effect.

Concept: Photoelectric Effect and Wave Theory of Light
Chapter: [7.01] Dual Nature of Radiation and Matter
[1.5]15.a.b

Use Einstein’s photoelectric equation to show how from this graph,
(i) Threshold frequency, and (ii) Planck’s constant can be determined.

Concept: Einstein’S Photoelectric Equation: Energy Quantum of Radiation
Chapter: [7.01] Dual Nature of Radiation and Matter
OR
[3]15.b
[1]15.b.a

How does one explain the emission of electrons from a photosensitive surface with the help of Einstein’s photoelectric equation?

Concept: Einstein’S Photoelectric Equation: Energy Quantum of Radiation
Chapter: [7.01] Dual Nature of Radiation and Matter
[1]15.b.b

Work function of aluminium is 4.2 eV. If two photons each of energy 2.5 eV are incident on its surface, will  the emission of electrons take place? Justify your answer.

Concept: Photoelectric Effect and Wave Theory of Light
Chapter: [7.01] Dual Nature of Radiation and Matter
[1]15.b.c

The stopping potential in an experiment on photoelectric effect is 1.5V. What is the maximum kinetic energy of the photoelectrons emitted? Calculate in Joules.

Concept: Photoelectric Effect and Wave Theory of Light
Chapter: [7.01] Dual Nature of Radiation and Matter
[3]16
[1.5]16.a

Explain how an unpolarised light gets polarised when incident on the interface separating the two  transparent media.

Concept: Plane Polarised Light
Chapter: [6.02] Wave Optics
[1.5]16.b

Green light is incident at the polarising angle on a certain transparent medium. The angle of refraction is 30° . Find
(i) polarising angle, and
(ii) refractive index of the medium.

Concept: Polarisation
Chapter: [6.02] Wave Optics
[3]17

In the circuit shown in the figure, find the value of the current shown in the ammeter A.

Concept: Force on a Current - Carrying Conductor in a Uniform Magnetic Field
Chapter: [3.02] Moving Charges and Magnetism
[3]18
[1.5]18.a

Explain briefly, using a proper diagram, the difference in behaviour of a conductor and a dielectric in  the presence of external electric field.

Concept: Dielectrics and Polarisation
Chapter: [1.01] Electrostatic Potential and Capacitance
[1.5]18.b

Define the term polarization of a dielectric and write the expression for a linear isotropic dielectric in  terms of electric field.

Concept: Dielectrics and Polarisation
Chapter: [1.01] Electrostatic Potential and Capacitance
[3]19
[1.5]19.a

Obtain the expression for the current flowing through a conductor having number density of the electron n, area of cross-section A in terms of the drift velocity vd

Concept: Drift of Electrons and the Origin of Resistivity
Chapter: [2.01] Current Electricity
[1.5]19.b

How does the resistivity of a semiconductor change with rise of temperature ? Explain.

Concept: Resistivity of Various Materials
Chapter: [2.01] Current Electricity
[3]20
[3]20.a
[1.5]20.a.a

(i) Write the truth tables of the logic gates marked P and Q in the given circuit.
(ii) Write the truth table for the circuit.

Concept: Digital Electronics and Logic Gates
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
[1.5]20.a.b

Why are NOR gates considered as universal gates?

Concept: Digital Electronics and Logic Gates
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
OR
[3]20.b
[1.5]20.b.a

Explain how a potential barrier is developed in a p-n junction diode.

Concept: Special Purpose P-n Junction Diodes
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
[1.5]20.b.b

Draw the circuit arrangement for studying the V-I characteristics of a p-n junction diode in reverse bias. Plot the V-I characteristics in this case.

Concept: Special Purpose P-n Junction Diodes
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
[3]21
[1]21.a

How are eddy currents generated in a conductor which is subjected to a magnetic field?

Concept: Eddy Currents
Chapter: [4.02] Electromagnetic Induction
[1]21.b

Write two examples of their useful applications .

Concept: Eddy Currents
Chapter: [4.02] Electromagnetic Induction
[1]21.c

How can the disadvantages of eddy currents be minimized?

Concept: Eddy Currents
Chapter: [4.02] Electromagnetic Induction
[3]22
[3]22.a

A converging beam of light travelling in air converges at a point P as shown in the figure. When a glass sphere  of refractive index 1 . 5 is introduced in between the path of the beam, calculate the new position of the image. Also draw the ray diagram for the image formed.

Concept: Refraction
Chapter: [6.01] Ray Optics and Optical Instruments
[3]22.b

A point ‘O’ marked on the surface of a glass sphere of diameter 20 cm is viewed through glass from the position directly opposite to the point O. If the refractive index of the glass is 1.5, find the position of the image formed. Also, draw the ray diagram for the formation of the image.

Concept: Refraction
Chapter: [6.01] Ray Optics and Optical Instruments
[3]23
[1]23.a

What do you mean by bandwidth of a signal? Give its importance.

Concept: Bandwidth of Signals
Chapter: [10.01] Communication Systems
[1]23.b

Differentiate between Analog and Digital communication.

Concept: Bandwidth of Signals
Chapter: [10.01] Communication Systems
[1]23.c

Write the functions of transducer and repeater ?

Concept: Elements of a Communication System
Chapter: [10.01] Communication Systems
[3]24

Define the dipole moment of a magnetic dipole. Write its S.I. unit.
Obtain the expression for the torque acting on a magnetic dipole placed in an external unifrom magnetic field.

Concept: Magnetic Field Intensity Due to a Magnetic Dipole (Bar Magnet) Along Its Axis
Chapter: [3.01] Magnetism and Matter
SECTION - D (15 Marks )
[5]25
[5]25.a
[2]25.a.a

Draw a circuit diagram of an n-p-n transistor with its emitter-base junction forward biased and basecollector  junction reverse biased. Briefly describe its working.

Explain how a transistor in its active state exhibits a low resistance at its emitter-base junction and high  resistance at its base-collector junction.

Concept: Junction Transistor - Basic Transistor Circuit Configurations and Transistor Characteristics
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
[3]25.a.b

Derive the expression for the voltage gain of a transistor amplifier in CE configuration in terms of the  load resistance RL, current gain a βa and input resistance.
Explain why input and output voltages are in opposite phase.

Concept: Junction Transistor - Basic Transistor Circuit Configurations and Transistor Characteristics
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
(OR)
[5]25.b
[3]25.b.a

Write the important considerations which are to be taken into account while fabricating a p-n junction diode to be used as a Light Emitting Diode (LED). What should be the order of band gap of an LED,  if it is required to emit light in the visible range? Draw a circuit diagram and explain its action.

Concept: Special Purpose P-n Junction Diodes
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
[2]25.b.b

Draw the V-I characteristics of an LED. State two advantages of LED lamps over convertional  incandescent lamps.

Concept: Special Purpose P-n Junction Diodes
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
[5]26
[5]26.a
[3]26.a.a

State and explain the law used to determine magnetic field at a point due to a current element. Derive  the expression for the magnetic field due to a circular current carrying loop of radius r at its centre.

Concept: Magnetic Field Due to a Current Element, Biot-Savart Law
Chapter: [3.02] Moving Charges and Magnetism
[2]26.a.b

A long wire with a small current element of length 1 cm is placed at the origin and carries a current of 10 A along the X-axis. Find out the magnitude and direction of the magnetic field due to the element on  the Y-axis at a distance 0.5 m from it.

Concept: Magnetic Field Due to a Current Element, Biot-Savart Law
Chapter: [3.02] Moving Charges and Magnetism
(OR)
[5]26.b
[3]26.b.a

Derive the expression for the magnetic field due to a current carrying coil of radius r at a distance x from the centre along the X-axis.

Concept: Magnetic Field Due to a Current Element, Biot-Savart Law
Chapter: [3.02] Moving Charges and Magnetism
[2]26.b.b

A straight wire carrying a current of 5 A is bent into a semicircular arc of radius 2 cm as shown in the  figure. Find the magnitude and direction of the magnetic field at the centre of the arc.

Concept: Magnetic Field Due to a Current Element, Biot-Savart Law
Chapter: [3.02] Moving Charges and Magnetism
[5]27
[5]27.a
[1]27.a.a

Can the interference pattern be produced by two independent monochromatic sources of light? Explain.

Concept: Refraction of Monochromatic Light
Chapter: [6.02] Wave Optics
[2]27.a.b

The intensity at the central maximum (O) in a Young’s double slit experimental set-up shown in the figure is IO. If the distance OP equals one-third of the fringe width of the pattern, show that the intensity at point P, would equal (I_0)/4.

Concept: Interference
Chapter: [6.02] Wave Optics
[2]27.a.c

In Young’s double slit experiment, the slits are separated by 0.5 mm and screen is placed 1.0 m away  from the slit. It is found that the 5th bright fringe is at a distance of 4.13 mm from the 2nd dark fringe.  Find the wavelength of light used.

Concept: Interference
Chapter: [6.02] Wave Optics
OR
[5]27.b
[2]27.b.a

Derive the relation a sin θ = λ  for the first minimum of the diffraction pattern produced due to a single slit of width ‘a’ using light of wavelength λ.

Concept: Diffraction - Seeing the Single Slit Diffraction Pattern
Chapter: [6.02] Wave Optics
[2]27.b.b

State with reason, how the linear width of the central maximum will be affected if
(i) monochromatic yellow  light is replaced with red light, and
(ii) distance between the slit and the screen is increased.

Concept: Refraction of Monochromatic Light
Chapter: [6.02] Wave Optics
[1]27.b.c

Using the monochromatic light of same wavelength in the experimental set-up of the diffraction pattern  as well as in the interference pattern where the slit separation is 1 mm, 10 interference fringes are found  to be within the central maximum of the diffraction pattern. Determine the width of the single slit, if the  screen is kept at the same distance from the slit in the two cases.

Concept: Diffraction - Seeing the Single Slit Diffraction Pattern
Chapter: [6.02] Wave Optics

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