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# Physics Patna Set 3 2014-2015 CBSE (Science) Class 12 Question Paper Solution

Physics [Patna Set 3]
Marks: 100Date: 2014-2015 March

[1]1

The line AB in the ray diagram represents a lens State whether the lens is convex or concave.

Concept: Lenses
Chapter: [6.01] Ray Optics and Optical Instruments
[1]2

Distinguish between emf and terminal voltage of a cell.

Concept: Cells, Emf, Internal Resistance
Chapter: [2.01] Current Electricity
[1]3

Draw a graph to show variation of capacitive-reactance with frequency in an a.c. circuit.

Concept: Reactance and Impedance
Chapter: [4.01] Alternating Current
[1]4

What is the function of a 'Repeater' used in communication system?

Concept: Basic Terminology Used in Electronic Communication Systems
Chapter: [10.01] Communication Systems
[1]5 | Explain the following, giving reasons:

When monochromatic light is incident on a surface separating two media, the reflected and refracted light both have the same frequency as the incident frequency.

Concept: Refraction of Monochromatic Light
Chapter: [6.02] Wave Optics

When light travels from a rarer to a denser medium, the speed decreases. Does this decrease in speed imply a reduction in the energy carried by the wave?

Concept: Refraction of Monochromatic Light
Chapter: [6.02] Wave Optics

In the wave picture of light, the intensity of light is determined by the square of the amplitude of the wave. What determines the intensity in the photon picture of light?

Concept: Refraction of Monochromatic Light
Chapter: [6.02] Wave Optics
[2]6

The equivalent wavelength of a moving electron has the same value as that of a photon of energy 6 × 10–17 J. Calculate the momentum of the electron.

Concept: Heisenberg and De Broglie Hypothesis
Chapter: [8.02] Atoms
[2]7

What is ground wave communication?

Concept: Propagation of Electromagnetic Waves
Chapter: [10.01] Communication Systems

Explain why ground wave communication mode cannot be used for long-distance communication using high frequencies

Concept: Propagation of Electromagnetic Waves
Chapter: [10.01] Communication Systems
[2]8 | Attempt any one
[2]8.1

A ray of light passes through an equilateral glass prism such that the angle of incidence is equal to the angle of emergence and each of these angles is equal to 3/4 of angle of prism. Find the angle of deviation.

Concept: Dispersion by a Prism
Chapter: [6.01] Ray Optics and Optical Instruments
[2]8.2

Calculate the speed of light in a medium whose critical angle is 45°. Does critical angle for a given pair of media depend on the wavelength of incident light ? Give reason.

Concept: Snell’s Law
Chapter: [6.01] Ray Optics and Optical Instruments
[2]9

How does one explain, using de Broglie hypothesis, Bohr's second postulate of quantization of orbital angular momentum?

Concept: De Broglie’S Explanation of Bohr’S Second Postulate of Quantisation
Chapter: [8.02] Atoms
[2]10

In a meter bridge shown in the figure, the balance point is found to be 40 cm from end A. If a resistance of 10 Ω is connected in series with R, balance point is obtained 60 cm from A. Calculate the values of R and S.

Concept: Metre Bridge
Chapter: [2.01] Current Electricity
[3]11 | Attempt any one
[3]11.1

State Lenz's law. Illustrate, by giving an example, how this law helps in predicting the direction of the current in a loop in the presence of a changing magnetic flux.

Concept: Lenz’S Law and Conservation of Energy
Chapter: [4.02] Electromagnetic Induction

In a given coil of self-inductance of 5 mH, current changes from 4 A to 1 A in 30 ms. Calculate the emf induced in the coil.

Concept: Inductance - Self-Inductance
Chapter: [4.02] Electromagnetic Induction
[3]11.2

In what way is Gauss's law in magnetism different from that used in electrostatics ? Explain briefly.

Concept: Magnetism and Gauss’s Law
Chapter: [3.01] Magnetism and Matter

The Earth's magnetic field at the Equator is approximately 0.4 G. Estimate the Earth's magnetic dipole moment. Given : Radius of the Earth = 6400 km.

Concept: Current Loop as a Magnetic Dipole and Its Magnetic Dipole Moment
Chapter: [3.01] Magnetism and Matter
[3]12

How are electromagnetic waves produced? What is the source of energy of these waves?

Concept: Electromagnetic Waves
Chapter: [5.01] Electromagnetic Waves

Draw a schematic sketch of the electromagnetic waves propagating along the + x-axis. Indicate the directions of the electric and magnetic fields

Concept: Electromagnetic Waves
Chapter: [5.01] Electromagnetic Waves
[3]13

Obtain the relation between the decay constant and half life of a radioactive sample.

Chapter: [8.01] Nuclei

The half life of a certain radioactive material against \u0003α-decay is 100 days. After how much time, will the undecayed fraction of the material be 6.25%?

Chapter: [8.01] Nuclei
[3]14

How is a Zener diode fabricated?

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

Describe, with the help of a circuit diagram, the working of Zener diode as a voltage regulator.

Concept: Zener Diode as a Voltage Regulator
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
[3]15

Find the equivalent capacitance of the network shown in the figure, when each capacitor is of 1 μF. When the ends X and Y are connected to a 6 V battery, find out (i) the charge and (ii) the energy stored in the network.

Concept: Capacitors and Capacitance
Chapter: [1.01] Electrostatic Potential and Capacitance
[3]16

State the principle of working of a potentiometer.

Concept: Potentiometer
Chapter: [2.01] Current Electricity

Why is a potentiometer preferred over a voltmeter for measuring the emf of a cell?

Concept: Potentiometer
Chapter: [2.01] Current Electricity
[3]17

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

Two monochromatic beams, one red and the other blue, have the same intensity. In which case (i) the number of photons per unit area per second is larger, (ii) the maximum kinetic energy of the photoelectrons is more? Justify your answer.

Concept: Photoelectric Effect - Hertz’S Observations
Chapter: [7.01] Dual Nature of Radiation and Matter
[3]18

a) Give two reasons to explain why reflecting telescopes are preferred over refracting type.

Concept: Ray Optics - Mirror Formula
Chapter: [6.01] Ray Optics and Optical Instruments

Use mirror equation to show that convex mirror always produces a virtual image independent of the location of the object.

Concept: Ray Optics - Mirror Formula
Chapter: [6.01] Ray Optics and Optical Instruments
[3]19

Write the necessary conditions to obtain sustained interference fringes.

Concept: Interference
Chapter: [6.02] Wave Optics

In Young's double slit experiment, plot a graph showing the variation of fringe width versus the distance of the screen from the plane of the slits keeping other parameters same. What information can one obtain from the slope of the curve?

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

What is the effect on the fringe width if the distance between the slits is reduced keeping other parameters same?

Concept: Interference of Light Waves and Young’S Experiment
Chapter: [6.02] Wave Optics
[3]20

A series LCR circuit is connected across an a.c. source of variable angular frequency 'ω'. Plot a graph showing variation of current 'i' as a function of 'ω' for two resistances R1 and R2 (R1 > R2).

Answer the following questions using this graph :

(a) In which case is the resonance sharper and why?

(b) In which case in the power dissipation more and why?

Concept: Ac Voltage Applied to a Series Lcr Circuit
Chapter: [4.01] Alternating Current
[3]21

Draw the necessary energy band diagrams to distinguish between conductors, semiconductors and insulators.
How does the change in temperature affect the behaviour of these materials ? Explain briefly.

Concept: Energy Bands in Conductors, Semiconductors and Insulators
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits

A series LCR circuit is connected across an a.c. source of variable angular frequency 'ω'. Plot a graph showing variation of current 'i' as a function of 'ω' for two resistances R1 and R2 (R1 > R2).

Answer the following questions using this graph :

(a) In which case is the resonance sharper and why?

(b) In which case in the power dissipation more and why?

Concept: Ac Voltage Applied to a Series Lcr Circuit
Chapter: [4.01] Alternating Current
[3]22

What are the three basic units in communication systems ? Write briefly the function of each of these.

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

Write any three applications of the Internet used in communication systems.

Concept: Elements of a Communication System
Chapter: [10.01] Communication Systems
[4]23

During a thunderstorm the 'live' wire of the transmission line fell down on the ground from the poles in the street. A group of boys, who passed through, noticed it and some of them wanted to place the wire by the side. As they were approaching the wire and trying to lift the cable, Anuj noticed it and immediately pushed them away, thus preventing them from touching the live wire. During pushing some of them got hurt. Anuj took them to a doctor to get them medical aid.

Based on the above paragraph, answer the following questions :
(a) Write the two values which Anuj displayed during the incident.
(b) Why is it that a bird can sit on a suspended 'live' wire without any harm whereas touching it on the ground can give a fatal shock ?
(c) The electric power from a power plant is set up to a very high voltage before transmitting it to distant consumers. Explain, why.

Concept: Electric Current
Chapter: [2.01] Current Electricity
[5]24 | Attempt any one
[5]24.1
[2]24.1.1

Use Huygens' principle to show the propagation of a plane wavefront from a denser medium to a rarer medium. Hence find the ratio of the speeds of wavefronts in the two media.

Concept: Rarer and Denser Medium
Chapter: [6.01] Ray Optics and Optical Instruments
[3]24.1.2

Why does an unpolarised light incident on a polaroid get linearly polarised ?

Concept: Polarisation
Chapter: [6.02] Wave Optics

Derive the expression of Brewster's law when unpolarised light passing from a rarer to a denser medium gets polarised on reflection at the inteface.

Concept: Brewster's Law
Chapter: [6.02] Wave Optics
[5]24.2

A biconvex lens with its two faces of equal radius of curvature R is made of a transparent medium of refractive index μ1. It is kept in contact with a medium of refractive index μ2 as shown in the figure.

(a) Find the equivalent focal length of the combination.
(b) Obtain the condition when this combination acts as a diverging lens.
(c) Draw the ray diagram for the case μ1 > (μ2 + 1) / 2, when the object is kept far away from the lens. Point out the nature of the image formed by the system.

Concept: Magnification
Chapter: [6.01] Ray Optics and Optical Instruments
[5]25 | Attempt any one
[5]25.1

Two infinitely long straight parallel wires, '1' and '2', carrying steady currents I1 and I2 in the same direction are separated by a distance d. Obtain the expression for the magnetic field vecBdue to the wire '1' acting on wire '2'. Hence find out, with the help of a suitable diagram, the magnitude and direction of this force per unit length on wire '2' due to wire '1'. How does the nature of this force changes if the currents are in opposite direction? Use this expression to define the S.I. unit of current.

Concept: Force Between Two Parallel Currents, the Ampere
Chapter: [3.02] Moving Charges and Magnetism
[5]25.2

Draw a necessary arrangement for winding of primary and secondary coils in a step-up transformer. State its underlying principle and derive the relation between the primary and secondary voltages in terms of number of primary and secondary turns. Mention the two basic assumptions used in obtaining the above relation.

Concept: Inductance - Mutual Inductance
Chapter: [4.02] Electromagnetic Induction

State any two causes of energy loss in actual transformers.

Concept: Eddy Currents
Chapter: [4.02] Electromagnetic Induction
[5]26 | Attempt any one
[5]26.1

State the two Kirchhoff’s rules used in electric networks. How are there rules justified?

Concept: Kirchhoff’s Rules
Chapter: [2.01] Current Electricity

Two cells of emf E1 and E2 and internal resistances r1 and r2 are connected in parallel. Derive the expression for the (i) emf and (ii) internal resistance of a single equivalent cell which can replace this combination.

Concept: Induced Emf and Current
Chapter: [4.02] Electromagnetic Induction
[5]26.2

"The outward electric flux due to charge +Q is independent of the shape and size of the surface which encloses is." Give two reasons to justify this statement.

Concept: Electric Flux
Chapter: [1.02] Electric Charges and Fields

Two identical circular loops 1 and 2 of radius R each have linear charge densities −λ and +λ C/m respectively. The loops are placed coaxially with their centres Rsqrt3 distance apart. Find the magnitude and direction of the net electric field at the centre of loop 1.

Concept: Electric Field - Introduction of Electric Field
Chapter: [1.02] Electric Charges and Fields

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