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Physics (Theory) 2013-2014 ISC (Commerce) Class 12 Question Paper Solution

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Physics (Theory)
Marks: 70Academic Year: 2013-2014
Date: March 2014

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 intensity of the electric field at a point at a perpendicular distance ‘r’ from an infinite line charge, having linear charge density ‘λ’ is given by:

E = `(1/(4π∈_0))λ/r`

E = `(1/(4π∈_0)) (2λ)/r`

E = `(1/(4π∈_0)) λ/r^2`

E = `(1/(4π∈_0)) (2λ)/r^2`

Concept: Electric Field Due to a Point Charge
Chapter: [1.01] Electric Charges and Fields
[1]1.A.ii

If R1 and R2 are filament resistances of a 200 W and a 100 W bulb respectively, designed to operate on the same voltage, then:

R1 = R2

R2 = 2R1

R2 = 4R1

R1 = 4R2

Concept: Resistivity of Various Materials
Chapter: [2] Current Electricity
[1]1.A.iii

A metallic wire having a length of 2 m and weight of 4 x 10-3 N is found to remain at rest in a uniform and transverse magnetic field of 2 x 10-4 T. Current flowing through the wire is:

10 A

5 A

2 A

1 A

Concept: Electric Current
Chapter: [2] Current Electricity
[1]1.A.iv

When a beam of white light is passed through sodium vapors and then through a spectrometer, spectrum so obtained has two dark lines present in the yellow region. This spectrum is called:

band spectrum

continuous spectrum

absorption spectrum of sodium

emission spectrum of sodium

Concept: Hydrogen Spectrum
Chapter: [8.01] Atoms
[1]1.A.v

If l3 and l2 represent angular momenta of an orbiting electron in III and II Bohr orbits respectively, then l3: l2 is :

3: 2

9: 4

2: 3

4: 9

Concept: Bohr'S Model for Hydrogen Atom
Chapter: [8.01] Atoms
[15]1.B | Answer all questions given below briefly and to the point:
[1]1.B.i

A parallel plate air capacitor has a capacitance of  5 μF. It becomes 50 μF when a dielectric medium occupies the entire space between its two plates. What is the dielectric constant of the medium?

Concept: Capacitance of a Parallel Plate Capacitor with and Without Dielectric Medium Between the Plates
Chapter: [1.02] Electrostatic Potential, Potential Energy and Capacitance
[1]1.B.ii

Find the emf of the battery shown in the figure:

Concept: Cells, Emf, Internal Resistance
Chapter: [2] Current Electricity
[1]1.B.iii

Two substances A and B have their relative permeability slightly greater and slightly less than 1 respectively. What do you conclude about A and B as far as their magnetic materials are concerned?

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

When does a moving charged particle nor experience any force while moving through a uniform magnetic field?

Concept: Force on a Moving Charge in Uniform Magnetic and Electric Fields
Chapter: [3.01] Moving Charges and Magnetism
[1]1.B.v

What is the turns ratio i.e., transformer ratio, ns: np, in an ideal transformer which in-creases ac voltage from 220 V to 33000 V?

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

What is meant by coherent sources of light?

Concept: Coherent and Incoherent Sources and Sustained Interference of Light
Chapter: [6.02] Wave Optics
[1]1.B.vii

A ray of light is incident on a transparent medium at a polarizing angle. What is the angle between the reflected ray and the refracted ray?

Concept: Polarisation
Chapter: [6.02] Wave Optics
[1]1.B.viii

Name the physical principle on which the working of optical fibers is based.

Concept: Ray Optics - Mirror Formula
Chapter: [6.01] Ray Optics and Optical Instruments
[1]1.B.ix

What is meant by shortsightedness?

Concept: Lenses
Chapter: [6.01] Ray Optics and Optical Instruments
[1]1.B.x

How does focal length of a convex lens change with increase in wavelength of incident light?

Concept: Magnification
Chapter: [6.01] Ray Optics and Optical Instruments
[1]1.B.xi

With reference to the photoelectric effect, what is meant by threshold wavelength?

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

The half-life of a certain radioactive element is 3.465 days. Find its disintegration constant.

Concept: Radioactivity - Introduction of Radioactivity
Chapter: [8.02] Nuclei
[1]1.B.xiii

Binding energy per nucleon for helium nucleus (2 He) is 7.0 MeV Find value of mass defect for helium nucleus

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

Write one balanced reaction representing nuclear fusion.

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

Draw the truth table of a NOR gate.

Concept: Digital Electronics and Logic Gates
Chapter: [9] Electronic Devices
PART II SECTION A
[50]2 | Answer any two questions
[3]2.A

An electric dipole of dipole moment `vecP` is placed in a uniform electric field `vecE` with its axis inclined to the field. Write an expression for the torque `vecT` experienced by the dipole in vector form. Show diagrammatically how the dipole should be kept in the electric field so that the torque acting on it is:
(i) maximum
(ii) Zero

Concept: Dipole in a Uniform External Field
Chapter: [1.01] Electric Charges and Fields
[3]2.B

You are provided with 8 μF capacitors. Show with the help of a diagram how you will arrange minimum number of them to get a resultant capacitance of 20 μF.

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

Define temperature coefficient of resistance of the material of a conductor. 

Concept: Temperature Dependence of Resistance
Chapter: [2] Current Electricity
[6]3 | Answer any two questions
[3]3.A

Draw a labelled circuit diagram of a potentiometer to compare emfs of two cells. Write the working formula (Derivation not required).

Concept: Potentiometer
Chapter: [2] Current Electricity
[3]3.B

How much resistance should be connected to 15 Ω resistor shown in the circuit in figure below so that the points M and N are at the same potential:

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

With reference to free-electron theory of conductivity, explain the terms:
(a) Drift speed
(b) Relaxation time

Concept: Conductivity and Conductance;
Chapter: [2] Current Electricity

What is the colour code of a carbon resistor having a resistance of 470 Ω  and a tolerance of 5%?

Concept: Electrical Resistivity and Conductivity
Chapter: [2] Current Electricity
[9]4
[2]4.A

State Tangent Law in magnetism.

Concept: The Earth’s Magnetism
Chapter: [3.02] Magnetism and Matter

At a certain temperature, a ferromagnetic material becomes paramagnetic. What is this temperature called?

Concept: Magnetisation and Magnetic Intensity
Chapter: [3.02] Magnetism and Matter
[3]4.B

State Biot Savart law. 

Concept: Magnetic Field Due to a Current Element, Biot-Savart Law
Chapter: [3.01] Moving Charges and Magnetism

Find magnetic flux density at a point on the axis of a long solenoid having 5000 tums/m when it carrying a current of 2 A.

Concept: Magnetic Flux
Chapter: [4.01] Electromagnetic Induction
[4]4.C

An alternating emf of 110 V is applied to a circuit containing a resistance R of 80 Ω and an inductor L in series. The current is found to lag behind the supply voltage by an angle 8 = tan-1 (3/4). Find the :
(i) Inductive reactance
(ii) Impedance of the circuit
(iii) Current flowing in the circuit
(iv) If the inductor has a coefficient of self-inductance of 0.1 H, what is the frequency of the applied emf?

Concept: Induced Emf and Current
Chapter: [4.01] Electromagnetic Induction
SECTION B
[8]5 | Answer any two questions
[2]5.A

Name the part of the electromagnetic spectrum which is: 
Suitable for radar systems used in aircraft navigation.

Concept: Electromagnetic Spectrum
Chapter: [5] Electromagnetic Waves

Name the part of the electromagnetic spectrum which is: 
Produced by bombarding a metal target with high electrons.

Concept: Electromagnetic Spectrum
Chapter: [5] Electromagnetic Waves
[3]5.B

In Young’s double-slit experiment, using monochromatic light, fringes are obtained on a screen placed at some distance from the slits. If the screen is moved by 5 x 10-2 m towards the slits, the change in the fringe width is 3 x 10-5 m. If the distance between the two slits is 10-3 m, calculate the wavelength of the light used.

Concept: Interference of Light Waves and Young’S Experiment
Chapter: [6.02] Wave Optics
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[3]5.C

State Brewster’s law of polarization of light.

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

How will you identify with the help of an experiment whether a given beam of light is of polarized light or of unpolarized light?

Concept: Plane Polarised Light
Chapter: [6.02] Wave Optics
[8]6
[2]6.A

 A narrow beam of monochromatic light, PQ, is incident normally on one face of an equiangular glass prism of refractive index 1.45. When the prism is immersed in a certain liquid, the ray makes a grazing emergence along the other face (See figure). Find the refractive index of this liquid. 

Concept: Dispersion by a Prism
Chapter: [6.01] Ray Optics and Optical Instruments
[3]6.B

When two thin lenses of focal lengths f1 and f2 are kept coaxially and in contact, prove that their combined focal length ‘f’ is given by: `1/f = 1/f_1 + 1/f_2`

Concept: Lenses
Chapter: [6.01] Ray Optics and Optical Instruments
[3]6.C

The figure below shows the positions of a point object O, two lenses, a plane mirror and the final image I which coincides with the object. The focal length of the convex lens is 20 cm. Calculate the focal length of the concave lens.

Concept: Ray Optics - Mirror Formula
Chapter: [6.01] Ray Optics and Optical Instruments
[8]7
[4]7.A

 What is meant by the dispersive power of transparent material?

Concept: Dispersion by a Prism
Chapter: [6.01] Ray Optics and Optical Instruments

 Show that, two thin lenses kept in contact, form an achromatic doublet if they satisfy the condition: `ω/f + (w')/(f') = 0`
where the terms have their usual meaning.

Concept: Lenses
Chapter: [6.01] Ray Optics and Optical Instruments
[2]7.B

Define the magnifying power of a microscope in terms of visual angle.

Concept: Optical Instruments - The Microscope
Chapter: [6.01] Ray Optics and Optical Instruments

What is the advantage of a compound microscope over a simple microscope?

Concept: Optical Instruments - The Microscope
Chapter: [6.01] Ray Optics and Optical Instruments
[2]7.C

An astronomical telescope uses two lenses of powers 10 dioptres and 1 dioptre. If the final image of a distant object is formed at infinity, calculate the length of the telescope

Concept: Optical Instruments - Telescope
Chapter: [6.01] Ray Optics and Optical Instruments
[8]8
[3]8.A

Answer the following questions with reference to Millikan’s oil drop experiment:
(i) What is an atomiser?
(ii)What is the use of an X-ray tube?
(iii) What is the unique property shown by the charge of an oil drop?

Concept: Materials, Devices and Simple Circuits
Chapter: [9] Electronic Devices
[3]8.B

Write Einstein’s photoelectric equation.

Concept: Einstein’s Equation - Particle Nature of Light
Chapter: [7] Dual Nature of Radiation and Matter

If the frequency of the incident radiation is increased from 4 × 1015 Hz to 8 × 1015 Hz, by how much will the stopping potential for a given photosensitive surface go up?

Concept: Photoelectric Effect - Hertz’S Observations
Chapter: [7] Dual Nature of Radiation and Matter
[2]8.C

What are matter waves?

Concept: Wave Nature of Matter
Chapter: [7] Dual Nature of Radiation and Matter

 Show with the help of a labelled graph how their wavelength (λ) varies with their linear momentum (p).

Concept: Wave Nature of Matter
Chapter: [7] Dual Nature of Radiation and Matter
[8]9
[3]9.A

The energy levels of an atom of a certain element are shown in the given figure. Which one of the transitions A, B, C, D or E will result in the emission of photons of electromagnetic radiation of wavelength 618.75 nm? Support your answer with mathematical calculations.

Concept: Electromagnetic Waves
Chapter: [5] Electromagnetic Waves
[2]9.B

Voltage applied between cathode and anode of an X-ray tube is 18 kV. Calculate the minimum wavelength of the X-rays produced.

Concept: Zener Diode as a Voltage Regulator
Chapter: [9] Electronic Devices
[3]9.C

In a nuclear reactor, what is the function of:
(i) The moderator
(ii) The control rods
(iii) The coolant

Concept: Mass-energy and Nuclear Binding Energy - Nuclear Binding Energy
Chapter: [8.02] Nuclei
[8]10
[3]10.A

The atomic mass of Uranium `""_92^238"U"` is 238.0508 u, while that of Thorium `""_90^234"Th"` is 234.0436u, and that of Helium `""_2^4"He"` "is 4.0026u. Alpha decay converts `""_92^238"U"` into `""_92^234"Th"` as, shown below:

`""_92^238"U" -> ( ""_90^234"Th" + ""_2^4"He" + "Energy" )`

Concept: Atomic Masses and Composition of Nucleus
Chapter: [8.02] Nuclei

What is a neutrino?

Concept: Atomic Masses and Composition of Nucleus
Chapter: [8.02] Nuclei
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[3]10.B

In semiconductor physics, what is meant by: 
(i) rectifier
(ii) an amplifier
(iii) an oscillator

Concept: Semiconductor Diode
Chapter: [9] Electronic Devices
[2]10.C

With the help of a diagram, show how you can use several NAND gates to obtain an OR gate.

Useful Constants and Relations :

1. Speed of Light in Vacuum      (c) = 3.00 x 108 m/s
2. Charge of a proton (e) = 1.60 x 10-19C
3. Planck's Constant (h) = 6.6 x 10-34 Js
4. Permeability of vacuum 0) = 4π x 10-7 Hm-1
5. Electron Volt (1eV ) = 1.6 x 10 
6. Unified Atomic Mass Unit (1u) = 931 MeV
  (π) = 3.14
  ( ln 2 ) = 0.693
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 2013 - 2014

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