# Physics (Theory) Set 1 2015-2016 ISC (Science) Class 12 Question Paper Solution

Physics (Theory) [Set 1]
Date & Time: 4th March 2016, 2:00 pm
Duration: 3h

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

In Figure 1 below, a charge Q is fixed. Another charge q is moved along a circular arc MN of radius r around it, from the point M to the point N such that the length of the arc MN = l. The work done in this process is:

figure 1

zero

1/(4piε_0).(Qq)/r^2l

(Qq)/(2ε_0r^2)l

(Qq)/(2piε_0r^2)

Concept: Basic Properties of Electric Charge
Chapter: [1.01] Electric Charges and Fields
[1]1.A.ii

A carbon resistor has coloured bands as shown in Figure 2 below. The resistance of the resistor is:

figure 2

260 ± 10%

260 ± 5%

2600 ± 5%

2600 ± 10%

Concept: Temperature Dependence of Resistance
Chapter: [2] Current Electricity
[1]1.A.iii

A solenoid L and a resistor R are connected in series to a battery, through a switch. When the switch is put on, current I flowing through it varies with time t as shown in which of the graphs given below:

Concept: Combination of Resistors – Series and Parallel
Chapter: [2] Current Electricity
[1]1.A.iv

Two thin lenses having optical powers of -10D and+ 6D are placed in contact with each other. The focal length of the combination is:

+ 0·25 cm

- 0·25 cm

+ 0·25 m

- 0·25 m

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

The total energy of an electron in the ground state of the hydrogen atom is -13·6 eV. Its total energy, when a hydrogen atom is in the first excited state, is:

+ 13·6 eV

+ 3·4 eV

- 3·4 eV

- 54·4 eV

Concept: Alpha-particle Scattering and Rutherford’S Nuclear Model of Atom
Chapter: [8.01] Atoms
[15]1.B | Answer all questions given below briefly and to the point:
[1]1.B.i

A charged oil drop weighing 1.6 x 10-15 N is found to remain suspended in a uniform electric field of intensity 2 x 103 Nc-1. Find the charge on the drop.

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

For a metallic conductor, what is the relation between current density (J), conductivity (a) and electric field intensity E?

Concept: Electrical Resistivity and Conductivity
Chapter: [2] Current Electricity
[1]1.B.iii

In Figure 3 given below, find the value of resistance x for which points A and B are at the same potential:

figure 3

Concept: Electrical Resistivity and Conductivity
Chapter: [2] Current Electricity
[1]1.B.iv

Write the expression for the Lorentz force F in vector form.

Concept: Magnetic Force
Chapter: [3.01] Moving Charges and Magnetism [3.02] Magnetism and Matter
[1]1.B.v

A coil has a self-inductance of 0·05 Henry. Find the magnitude of the emf induced in it when the current flowing through it is changing at the rate of 100 As-1.

Concept: Electromagnetic Induction
Chapter: [4.01] Electromagnetic Induction
[1]1.B.vi

To which regions of the electromagnetic spectrum do the following wavelengths belong:
(a) 250 nm
(b) 1500 nm

Concept: Electromagnetic Spectrum
Chapter: [5] Electromagnetic Waves
[1]1.B.vii

What is the difference between polarised light and unpolarised light?

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

Name the principle on the basis of which optical fibres work.

Concept: Total Internal Reflection
Chapter: [6.01] Ray Optics and Optical Instruments
[1]1.B.ix

Calculate dispersive power of a transparent material given : nv = 1.56, nr = 1.54, ny = 1.55.

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

What is meant by short-sightedness ?

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

Two metals A and B have work functions 4 eV and 6 eV respectively. Which metal has a lower threshold wavelength for photoelectric effect?

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

Calculate angular momentum of an electron in the third Bohr orbit of a hydrogen atom.

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

In a nuclear reactor, what is the function of a moderator?

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

In our Nature, where is the nuclear fusion reaction taking place continuously?

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

What is the use of Zener diode?

Concept: Semiconductor Diode
Chapter: [9] Electronic Devices
Section A
[5]2 | Attempt any FOUR questions of the following:
[3]2.A

Two-point charges Q1 = 400 μC and Q2 = 100 μC are kept fixed, 60 cm apart in a vacuum. Find the intensity of the electric field at the midpoint of the line joining Q1 and Q2.

Concept: Electric Dipole
Chapter: [1.01] Electric Charges and Fields
[2]2.B
[1]2.B.i

State Gauss’Law.

Concept: Applications of Gauss’s Law
Chapter: [1.01] Electric Charges and Fields
[1]2.B.ii

In an electric dipole, at which point is the electric potential zero ?

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

Obtain an expression for equivalent capacitance when three capacitors C1, C2 and C3 are connected in series.

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

A metallic wire has a resistance of 3.0 Ω at 0°C and 4.8 Ω at 150°C. Find the temperature coefficient of resistance of its material.

Concept: Temperature Dependence of Resistance
Chapter: [2] Current Electricity
[5]4
[3]4.A

In the circuit shown in the figure below, E1 and E2 are two cells having emfs 2 V and 3 V respectively, and negligible internal resistance. Applying Kirchhoff’s laws of electrical networks, find the values of currents l1 and I2.

Concept: Kirchhoff’s Rules
Chapter: [2] Current Electricity
[2]4.B

State how a moving coil galvanometer can be converted into an ammeter.

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

Draw a labelled circuit diagram of a potentiometer to measure the internal resistance ‘r’ of a cell. Write the working formula (derivation is not required).

Concept: Potentiometer
Chapter: [2] Current Electricity
[2]5.B
[1]5.B.i

Define Curie temperature.

Concept: Current Loop as a Magnetic Dipole and Its Magnetic Dipole Moment
Chapter: [3.02] Magnetism and Matter
[1]5.B.ii

If magnetic susceptibility of a certain magnetic material is 0 0001, find its relative permeability.

Concept: Current Loop as a Magnetic Dipole and Its Magnetic Dipole Moment
Chapter: [3.02] Magnetism and Matter
[5]6
[3]6.A
[2]6.A.i

Two infinitely long current carrying conductors X and Y are kept parallel to each other, 24 cm apart in a vacuum. They carry currents of 5A and 7A respectively, in the same direction, as shown in the figure below. Find the position of a neutral point, i.e., a point where resultant magnetic flux density is zero. (Ignore earth’s magnetic field).

Concept: Force on a Current - Carrying Conductor in a Uniform Magnetic Field
Chapter: [3.01] Moving Charges and Magnetism
[1]6.A.ii

If current through the conductor Y is reversed in direction, will neutral point lie between X and Y, to the left of X or to the right of Y ?

Concept: Junction Transistor - Transistor: Structure and Action
Chapter: [9] Electronic Devices
[2]6.B
[1]6.B.i

Define Ampere in terms of force between two current carrying conductors.

Concept: Force Between Two Parallel Currents, the Ampere
Chapter: [3.01] Moving Charges and Magnetism
[1]6.B.ii

What is the ideal transformer?

Concept: Transformers
Chapter: [4.02] Alternating Current
[5]7
[3]7.A

A coil having self-inductance of 0.7 H and resistance of 165 Ω is connected to an a.c. source of

275V,50Hz. If π = 22/7

Calculate:
(i) Reactance of the coil
(ii) Impedance of the coil ‘
(iii) Current flowing through the coil

Concept: Reactance and Impedance
Chapter: [4.02] Alternating Current
[2]7.B

Draw a labelled graph showing a variation of impedance of a series LCR circuit with frequency of the a.c. supply.

Concept: Ac Voltage Applied to a Series Lcr Circuit
Chapter: [4.02] Alternating Current
Section B
[5]8 | Attepmt any three questions:
[3]8.A

Derive Snell’s law of refraction using Huygens’s wave theory.

Concept: Snell’s Law
Chapter: [6.01] Ray Optics and Optical Instruments
[2]8.B

Monochromatic light of wavelength 650 nm falls normally on a slit of width 1.3 x 10-4 cm and the resulting Fraunhofer diffraction is obtained on a screen. Find the angular width of the . central maxima.

Concept: Refraction of Monochromatic Light
Chapter: [6.02] Wave Optics
[5]9
[4]9.A

In Young’s double-slit experiment, show that:

beta = (lambda "D")/"d" where the terms have their usual meaning.

Concept: Interference of Light Waves and Young’S Experiment
Chapter: [6.02] Wave Optics
[1]9.B

A ray of ordinary light is travelling in air. It is incident on air glass pair at a polarising angle of 56°. Find the angle of refraction in glass.

Concept: Polarisation
Chapter: [6.02] Wave Optics
[5]10
[3]10.A

Find the angle of incidence at which a ray of monochromatic light should be incident on the first surface AB of a regular glass prism ABC so that the emergent ray grazes the adjacent surface AC. (Refractive Index of glass = 1 .56)

Concept: Refraction of Monochromatic Light
Chapter: [6.02] Wave Optics
[2]10.B

State how the focal length of a glass lens (Refractive Index 1.5) changes when it is completely immersed in:

(i) Water (Refractive Index 1.33)
(ii) A liquid (Refractive Index 1.65)

Concept: Reflection of Light by Spherical Mirrors
Chapter: [6.01] Ray Optics and Optical Instruments
[5]11
[2]11.A

A convex lens of a focal length 5 cm is used as a simple microscope. Where should an object be placed so that the image formed by it lies at the least distance of distinct vision (D = 25 cm)?

Concept: Optical Instruments - The Microscope
Chapter: [6.01] Ray Optics and Optical Instruments
[3]11.B

Draw a labelled ray diagram showing the formation of an image by a refracting telescope when the final image lies at infinity.

Concept: Optical Instruments - Telescope
Chapter: [6.01] Ray Optics and Optical Instruments
Section C
[5]12 | Attempt any three questions.
[3]12.A

Monochromatic fight of wavelength 198 nm is incident on the surface of a metallic cathode whose work function is 2.5 eV How much potential difference must be applied between the cathode and the anode of a photocell to just stop the photocurrent from flowing?

Concept: Refraction of Monochromatic Light
Chapter: [6.02] Wave Optics
[2]12.B
[1]12.B.i

State de Broglie hypothesis

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

What conclusion can be drawn from Davisson and Germer’s experiment ?

Concept: Davisson-Germer Experiment
Chapter: [7] Dual Nature of Radiation and Matter
[5]13
[3]13.A
[2]13.A.i

How are various lines of Lyman series formed? Explain on the basis of Bohr’s theory.

Concept: Bohr'S Model for Hydrogen Atom
Chapter: [8.01] Atoms
[1]13.A.ii

Calculate the shortest wavelength of electromagnetic radiation present in Balmer series of hydrogen spectrum.

Concept: Electromagnetic Spectrum
Chapter: [5] Electromagnetic Waves
[2]13.B

State the effect of the following changes on the X-rays emitted by Coolidge X-ray tube :

(i) High voltage between cathode and anode is increased.
(ii) Filament temperature is increased.

Concept: Continuous and Characteristics X-rays
Chapter: [7] Dual Nature of Radiation and Matter
[5]14
[3]14.A
[2]14.A.i

Half-life of a certain radioactive material is 8 hours.

Find the disintegration constant of this material.

Chapter: [8.02] Nuclei
[1]14.A.ii

Half life of a certain radioactive material is 8 hours.
If one starts with 600 g of this substance, how much of it will disintegrate in one day?

Chapter: [8.02] Nuclei
[2]14.B

Sketch a graph showing the variation of binding energy per nucleon of a nucleus with its mass number.

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

Draw a circuit diagram for the common emitter transistor amplifier. What is meant by phase reversal?

Concept: Junction Transistor - Transistor as an Amplifier (Ce-configuration)
Chapter: [9] Electronic Devices
[2]15.B

Write the truth table of the following circuit. Name the gate represented by this circuit.

Concept: Junction Transistor - Transistor as an Amplifier (Ce-configuration)
Chapter: [9] Electronic Devices

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