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Physics (Theory) 2016-2017 ISC (Science) Class 12 Question Paper Solution

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Physics (Theory)
Marks: 70Academic Year: 2016-2017
Date & Time: 1st March 2017, 2:00 pm
Duration: 3h

Attempt all questions are compulsory. 


Part I
[20]1
[5]1.A | Choose the correct alternative (a),(b),(c) or (d) for each of the questions given below:
[1]1.A.i

The electrostatic potential energy of two-point charges, 1 µC each, placed 1 meter apart in the air is: 

9 ×103

9 ×109

9 ×10-3 J  

9 ×10-3 eV

Concept: Electrical Potential Energy of a System of Two Point Charges and of Electric Dipole in an Electrostatic Field
Chapter: [1.02] Electrostatic Potential, Potential Energy and Capacitance
[1]1.A.ii

A wire of resistance 'R' is cut into 'n' equal parts. These parts are then connected in parallel with each other. The equivalent resistance of the combination is : 

nR

R/n

n/R2

R/n2

Concept: Combination of Capacitors
Chapter: [1.02] Electrostatic Potential, Potential Energy and Capacitance
[1]1.A.iii

The magnetic susceptibility of platinum is 0.0001. It's relative permeability is: 

1.0000

0.9999

1.0001

0

Concept: Magnetic Properties of Materials
Chapter: [3.02] Magnetism and Matter
[1]1.A.iv

When a light wave travels from air to glass

its wavelength decreases.

its wavelength increases.

there is no change in wavelength.

its frequency decreases.

Concept: Wave Nature of Matter
Chapter: [7] Dual Nature of Radiation and Matter
[1]1.A.v

A radioactive substance decays to 1/16th of its initial mass in 40 days. The half-life of the substance, in days, is: 

20

10

5

2.5

Concept: Radioactivity - Introduction of Radioactivity
Chapter: [8.02] Nuclei
[15]1.B | Answer all questions given below briefly and to the point:
[1]1.B.i

Maximum torque acting on an electric dipole of moment  3×10-29  Cm in a uniform electric field E is 6 × 10-25 Nm. Find E.

Concept: Dipole in a Uniform External Field
Chapter: [1.01] Electric Charges and Fields
[1]1.B.ii

What is meant by the drift speed of free electrons?

Concept: Ohm’s Law
Chapter: [2] Current Electricity
[1]1.B.iii

On which conservation principle is Kirchoff's Second Law of electrical networks based?

Concept: Kirchhoff’s Rules
Chapter: [2] Current Electricity
[1]1.B.iv

Calculate magnetic flux density of the magnetic field at the centre of a circular coil of 50 turns, having a radius of 0.5m and carrying a current of 5 A.

Concept: Magnetic Flux
Chapter: [4.01] Electromagnetic Induction
[1]1.B.v

An a.c generator generates an emf 'ε' where ε = 314 Sin (50πt) volt. Calculate the frequency of emf ε.

Concept: Transformers
Chapter: [4.02] Alternating Current
[1]1.B.vi

With what type of source of light are cylindrical wave fronts associated?

Concept: Huygens Principle
Chapter: [6.02] Wave Optics
[1]1.B.vii

How is the fringe width of an interference pattern in Young's double-slit experiment affected if the two slits are brought closer to each other?

Concept: Interference of Light Waves and Young’S Experiment
Chapter: [6.02] Wave Optics
[1]1.B.viii

In a regular prism, what is the relation between angle of incidence and angle of emergence when it is in the minimum deviation position?

Concept: Dispersion by a Prism
Chapter: [6.01] Ray Optics and Optical Instruments
[1]1.B.ix

A converging lens of focal length 40 cm is kept in contact with a diverging lens of focal length 30 cm. Find the focal length of the combination .

Concept: Reflection of Light by Spherical Mirrors
Chapter: [6.01] Ray Optics and Optical Instruments
[1]1.B.x

How can the spherical aberration produced by a lens be minimized? 

Concept: Reflection of Light by Spherical Mirrors
Chapter: [6.01] Ray Optics and Optical Instruments
[1]1.B.xi

Calculate the momentum of a photon of energy 6 x I 0-19 J. 

Concept: Photoelectric Effect - Hertz’S Observations
Chapter: [7] Dual Nature of Radiation and Matter
[1]1.B.xii

According to Bohr, 'Angular momentum of an orbiting electron is quantized'. What is meant by this statement?

Concept: Bohr'S Model for Hydrogen Atom
Chapter: [8.01] Atoms
[1]1.B.xiii

Why nuclear fusion reaction is also called thermo-nuclear reaction?

Concept: Nuclear Energy - Nuclear Fusion – Energy Generation in Stars
Chapter: [8.02] Nuclei
[1]1.B.xiv

What is the minimum energy which a gamma-ray photon must possess in order to produce electron-positron pair?

Concept: Mass-energy and Nuclear Binding Energy - Nuclear Binding Energy
Chapter: [8.02] Nuclei
[1]1.B.xv

Show the variation of voltage with time, for a digital signal.

Concept: Digital Electronics and Logic Gates
Chapter: [9] Electronic Devices
PART II (50 marks)
[5]2
SECTION - A (20 marks) Answer any three questions.
[4]2.A

Show that electric potential at a point P, at a distance 'r' from a fixed point charge Q, is given by:
    `v=(1/(4pi∈_0))Q/r`.

Concept: Potential Due to a Point Charge
Chapter: [1.02] Electrostatic Potential, Potential Energy and Capacitance
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[1]2.B

The intensity of the electric field at a perpendicular distance of 0·5 m from an infinitely long line charge having linear charge density (λ) is 3-6 × 103 Vm-1. Find the value of λ.

Concept: Electric Field Lines
Chapter: [1.01] Electric Charges and Fields
[5]3
[3]3.A

Three capacitors C1 = 3μF, C2 = 6μF, and C3 = 10μF are connected to a 50 V battery as  shown in Figure  below:

Calculate:
(i) The equivalent capacitance of the circuit between points A and B.
(ii) The charge on C1.

Concept: Capacitors and Capacitance
Chapter: [1.02] Electrostatic Potential, Potential Energy and Capacitance
[2]3.B

Two resistors R1= 60 Ω and R2 = 90Ω are connected in parallel. If electric power consumed by the resistor R1 is15 W, calculate the power consumed by the resistor R2.

Concept: Combination of Resistors – Series and Parallel
Chapter: [2] Current Electricity
[5]4
[3]4.A

Figure below shows two resistors R1 and R2 connected to a battery having an emf of 40V and negligible internal resistance. A voltmeter having a resistance of. 300 Ω is used to measure the potential difference across R1 Find the reading of the voltmeter.

Concept: Potentiometer
Chapter: [2] Current Electricity
[2]4.B

A moving coil galvanometer has a coil of resistance 59 Ω. It shows a full-scale deflection for a current of 50 mA. How will you convert it to an ammeter having a range of 0 to 3A?

Concept: Moving Coil Galvanometer
Chapter: [3.01] Moving Charges and Magnetism
[5]5
[3]5.A

In a meter bridge circuit, resistance in the left-hand gap is 2 Ω and an unknown resistance  X is in the right-hand gap as shown in the figure below. The null point is found to be 40 cm from the left end of the Wire. What resistance should be connected to X so that the new null point is 50 cm from the left end of the wire?

Concept: Combination of Resistors – Series and Parallel
Chapter: [2] Current Electricity
[2]5.B

The horizontal component of the earth's magnetic field at a place is `1/sqrt(3)` time the vertical component. Determine the angle of dip at that place.

Concept: The Earth’s Magnetism
Chapter: [3.02] Magnetism and Matter
[5]6
[3]6.A

Using Ampere's circuital law, obtain an expression for the magnetic flux density 'B' at a point 'X' at a perpendicular distance 'r' from a long current-carrying conductor.
(Statement of the law is not required).

Concept: Ampere’s Circuital Law
Chapter: [3.01] Moving Charges and Magnetism
[2]6.B

PQ is a long straight conductor carrying a current of 3A as shown in Figure below. An electron moves with a velocity of 2 x 107 ms-1 parallel to it. Find the force acting on the electron.

Concept: Magnetic Force
Chapter: [3.01] Moving Charges and Magnetism [3.02] Magnetism and Matter
[5]7
[3]7.A
[1.5]7.A.i

AB and CD are two parallel conductors kept 1 m apart and connected by a resistance R of 6 Ω as shown in Figure below. They are placed in a magnetic field B = 3 × 10-2 T which is perpendicular to the plane of the conductors and directed into the paper. A Wire. MN is placed over AB and CD. and then made to slide with a velocity 2 ms-1 (Neglect the resistance of AB, CD, and MN.)

Calculate the induced c.urrent flowing through the resistor R.

Concept: Conductors and Insulators Related to Electric Field
Chapter: [1.02] Electrostatic Potential, Potential Energy and Capacitance
[1.5]7.A.ii

In an ideal transformer, an output of 66 kV is required when an input voltage of 220 V is available. If the primary has 300 turns, how many turns should the secondary have?

Concept: Transformers
Chapter: [4.02] Alternating Current
[2]7.B

In a series, LCR circuit, obtain an expression for the resonant frequency,

Concept: Ac Voltage Applied to a Series Lcr Circuit
Chapter: [4.02] Alternating Current
SECTION - B (15 marks) Answer any three questions.
[5]8
[3]8.A
[1.5]8.A.i

State any one property which is common to all electromagnetic waves.

Concept: Electromagnetic Waves
Chapter: [5] Electromagnetic Waves
[1.5]8.A.ii

Arrange the following electromagnetic waves in increasing order of their frequencies (i.e. begin with the lowest frequency):
Visible light, y rays, X rays, microwaves, radio waves, infrared radiations, and ultraviolet radiation.

Concept: Electromagnetic Spectrum
Chapter: [5] Electromagnetic Waves
[2]8.B
[1]8.B.i

What is meant by diffraction of light? 

Concept: de-Broglie Relation
Chapter: [7] Dual Nature of Radiation and Matter
[1]8.B.ii

In Fraunhofer diffraction, what kind of a source of light is used and where is it situated?

Concept: Fraunhofer Diffraction Due to a Single Slit
Chapter: [6.02] Wave Optics
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[5]9
[3]9.A

In Young's double slit experiment using monochromatic light of wavelength 600 nm, 5th bright fringe is at a distance of 0·48 mm from the centre of the pattern. If the screen is at a distance of 80 cm from the plane of the two slits, calculate:
(i) Distance between the two slits.
(ii) Fringe width, i.e. fringe separation.

Concept: Interference of Light Waves and Young’S Experiment
Chapter: [6.02] Wave Optics
[2]9.B

(i) State Brewster's law.
(ii) Find Brewster's angle for a transparent liquid having refractive index 1·5.

Concept: Brewster's Law
Chapter: [6.02] Wave Optics
[5]10
[2]10.A

Find a critical angle for glass and water pair, given the refractive index of glass, is 1 ·62 and that of water is 1 ·33.

Concept: Snell’s Law
Chapter: [6.01] Ray Optics and Optical Instruments
[3]10.B

Starting with an expression for refraction at a single spherical surface, obtain Lens Maker's Formula.

Concept: Lensmaker's Formula
Chapter: [6.01] Ray Optics and Optical Instruments
[5]11
[3]11.A

compound microscope consists of two convex lenses of focal length 2 cm and 5 cm. When an object is kept at a distance of 2.1 cm from the objective, a virtual and magnified image is fonned 25 cm from the eye piece.  Calculate the magnifying power of the microscope.

Concept: Optical Instruments - The Microscope
Chapter: [6.01] Ray Optics and Optical Instruments
[2]11.B

(i) What is meant by resolving power of a telescope?
(ii) State any one method of increasing the resolving power of an astronomical telescope.

Concept: Optical Instruments - Telescope
Chapter: [6.01] Ray Optics and Optical Instruments
SECTION-C(15 marks) Answer any three questions.
[5]12
[3]12.A
[1.5]12.A.i

Plot a labelled graph of IVsl where Vs is stopping potential versus frequency f of the incident radiation. 

Concept: Photoelectric Effect - Hertz’S Observations
Chapter: [7] Dual Nature of Radiation and Matter
[1.5]12.A.ii

State how will you use this graph to detennine the value of Planck's constant.

Concept: Photoelectric Effect - Hertz’S Observations
Chapter: [7] Dual Nature of Radiation and Matter
[2]12.B
[1]12.B.i

Find the de Broglie wavelength of electrons moving with a speed of 7 x 106 ms -1 

Concept: de-Broglie Relation
Chapter: [7] Dual Nature of Radiation and Matter
[1]12.B.ii

Describe in brief what is observed when moving electrons are allowed to fall on a thin graphite film and the emergent beam falls on a fluorescent screen.

Concept: de-Broglie Relation
Chapter: [7] Dual Nature of Radiation and Matter
[5]13
[3]13.A

Draw energy level diagram for a hydrogen atom, showing the first four energy levels corresponding to n=1, 2, 3 and 4. Show transitions responsible for:
(i) Absorption spectrum of Lyman series.
(ii) The emission spectrum of the Balmer series.

Concept: Bohr'S Model for Hydrogen Atom
Chapter: [8.01] Atoms
[2]13.B

(i) Find the maximum frequency of X-rays produced by an X-ray tube operating at a tube potential of 66 kV.
(ii) State any one difference between characteristic X-rays and continuous X-rays.

Concept: Continuous and Characteristics X-rays
Chapter: [7] Dual Nature of Radiation and Matter
[5]14
[3]14.A

Obtain a relation between the half-life of a radioactive substance and decay constant (λ).

Concept: Radioactivity - Law of Radioactive Decay
Chapter: [8.02] Nuclei
[2]14.B

Calculate mass defect and binding energy per nucleon .of `"_10^20 Ne`, given 

Mass of `"_10^20 Ne= 19.992397` u

Mass of `"_1^1H = 1.007825`  u

Mass of `"_0^1n = 1.008665` u

Concept: Mass-energy and Nuclear Binding Energy - Nuclear Binding Energy
Chapter: [8.02] Nuclei
[5]15
[3]15.A

With reference to a semiconductor diode, what is meant by: 
(i) Forward bias
(ii) Reverse bias
(iii) Depletion region

Concept: Semiconductor Diode
Chapter: [9] Electronic Devices
[2]15.B

Draw a diagram to show how NAND gates can be combined to obtain an OR gate. (Truth table is not, required)

Useful Constants and Relations:

1. Charge of a proton (e) =1.6 × 10-19C
2. Planck's constant (h) = 6·6 × 10-34 Js
3. Mass of an electron (m) = 9·1× 10-31 kg
4. Permittivity of vacuum (∈0) =8 · 85 × 10-12 Fm-1 
5.   `(1/(4pi∈_0))` =9 ×109 mF-1
6. Permeability of vacuum 0) = 4π × 10-7 Hm-1
7.   `((mu_0)/(4pi))` =1 × 10-7 Hm-1
8. Speed of light in vacuum (c) = 3× 108 ms-1
9. Unified atomic mass unit (u)

= 931 MeV

10. Electron volt (leV) = 1.6 × 10-19 J
Concept: Digital Electronics and Logic Gates
Chapter: [9] Electronic Devices
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CISCE previous year question papers Class 12 Physics (Theory) with solutions 2016 - 2017

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