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

Physics (Theory)
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
[1]1.B.xii

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

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:

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
[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
[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 (μ0) = 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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