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# Question Paper Solutions for Physics All India Set 2 2014-2015 CBSE (Science) Class 12

#### Alternate Sets

Marks: 70
[1]1

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

Chapter: [4.02] Alternating Current
Concept: Reactance and Impedance
[1]2

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

Chapter: [10.01] Communication Systems
Concept: Basic Terminology Used in Electronic Communication Systems
[1]3

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

Chapter: [6.02] Ray Optics and Optical Instruments
Concept: Lenses
[1]4

The field lines of a negative point charge are as shown in the figure. Does the kinetic energy of a small negative charge increase or decrease in going from B to A?

Chapter: [1.01] Electric Charges and Fields
Concept: Electric Field Due to a Point Charge
[1]5

Distinguish between emf and terminal voltage of a cell.

Chapter: [2.01] Current Electricity
Concept: Cells, Emf, Internal Resistance
[2]6

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

Chapter: [8.01] Atoms
Concept: De Broglie’S Explanation of Bohr’S Second Postulate of Quantisation
[2]7

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.

Chapter: [2.01] Current Electricity
Concept: Metre Bridge
[2]8

What is ground wave communication?

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

Explain why this mode cannot be used for long distance communication using high frequencies.

Chapter: [10.01] Communication Systems
Concept: Propagation of Electromagnetic Waves
[2]9

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.

Chapter: [8.01] Atoms
Concept: Heisenberg and De Broglie Hypothesis
[2]10 | Attempt any one of the following.
[2]10.1

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.

Chapter: [6.02] Ray Optics and Optical Instruments
Concept: Dispersion by a Prism
[2]10.2

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.

Chapter: [6.02] Ray Optics and Optical Instruments
Concept: Snell’s Law
[3]11

How is a Zener diode fabricated?

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

Explain, with the help of a circuit diagram, the principle and working of a Zener diode as voltage regular.

Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
Concept: Zener Diode as a Voltage Regulator
[3]12

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.

Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
Concept: Energy Bands in Conductors, Semiconductors and Insulators
[3]13 | Attempt any one of the following.
[2]13.1

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

Chapter: [10.01] Communication Systems
Concept: Elements of a Communication System
[1]13.2

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

Chapter: [10.01] Communication Systems
Concept: Elements of a Communication System
[3]14 | Attempt any one of the following.
[1]14.1

Write the necessary conditions to obtain sustained interference fringes.

Chapter: [6.01] Wave Optics
Concept: Interference
[1]14.2

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?

Chapter: [6.01] Wave Optics
Concept: Interference of Light Waves and Young’S Experiment
[1]14.3

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

Chapter: [6.01] Wave Optics
Concept: Interference of Light Waves and Young’S Experiment
[3]15

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?

Chapter: [4.02] Alternating Current
Concept: Ac Voltage Applied to a Series Lcr Circuit
[3]16 | Attempt any one of the following.
[1]16.1

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

Chapter: [6.02] Ray Optics and Optical Instruments
Concept: Ray Optics - Mirror Formula
[2]16.2

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

Chapter: [6.02] Ray Optics and Optical Instruments
Concept: Ray Optics - Mirror Formula
[3]17 | Attempt any one of the following.
[3]17.1

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.

Chapter: [4.01] Electromagnetic Induction
Concept: Lenz’S Law and Conservation of Energy

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.

Chapter: [4.01] Electromagnetic Induction
Concept: Inductance - Self-Inductance
[3]17.2

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

Chapter: [3.02] Magnetism and Matter
Concept: Magnetism and Gauss’s Law

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.

Chapter: [3.02] Magnetism and Matter
Concept: Current Loop as a Magnetic Dipole and Its Magnetic Dipole Moment
[3]18

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

Chapter: [5.01] Electromagnetic Waves
Concept: Electromagnetic Waves

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

Chapter: [5.01] Electromagnetic Waves
Concept: Electromagnetic Waves

Write the relation between the velocity of propagation and the magnitudes of electric and magnetic fields.

Chapter: [3.01] Moving Charges and Magnetism
Concept: Motion in Combined Electric and Magnetic Fields - Velocity Selector
[3]19

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

Chapter: [8.02] Nuclei
Concept: Radioactivity - Law of Radioactive Decay

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: [8.02] Nuclei
[3]20

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.

Chapter: [1.02] Electrostatic Potential and Capacitance
Concept: Capacitors and Capacitance
[3]21

State the underlying principle of a potentiometer ?

Chapter: [2.01] Current Electricity
Concept: Potentiometer

Write two factors by which current sensitivity of a potentiometer can be increased.

Chapter: [2.01] Current Electricity
Concept: Potentiometer

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

Chapter: [2.01] Current Electricity
Concept: Potentiometer
[3]22 | Attempt any one of the following.
[1]22.1

Define the term 'intensity of radiation' in terms of photon picture of light.

Chapter: [7.01] Dual Nature of Radiation and Matter
Concept: Experimental Study of Photoelectric Effect
[2]22.2

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.

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

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

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

Chapter: [2.01] Current Electricity
Concept: Kirchhoff’s Rules
[3]24.1.2

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.

Chapter: [4.01] Electromagnetic Induction
Concept: Induced Emf and Current
[5]24.2
[2]24.2.1

"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.

Chapter: [1.01] Electric Charges and Fields
Concept: Electric Flux
[3]24.2.2

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'.

Chapter: [1.01] Electric Charges and Fields
Concept: Electric Field - Introduction of Electric Field
[5]25 | Attempt any one of the following.
[3]25.1
[3]25.1.1

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.

Chapter: [6.02] Ray Optics and Optical Instruments
Concept: Rarer and Denser Medium
[2]25.1.2

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

Chapter: [6.01] Wave Optics
Concept: Polarisation

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.

Chapter: [6.01] Wave Optics
Concept: Brewster's Law
[5]25.2

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.

Chapter: [6.02] Ray Optics and Optical Instruments
Concept: Magnification
[5]26 | Attempt any one of the following.
[5]26.1

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.

Chapter: [3.01] Moving Charges and Magnetism
Concept: Force Between Two Parallel Currents, the Ampere
[5]26.2

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.

Chapter: [4.01] Electromagnetic Induction
Concept: Inductance - Mutual Inductance

State any two causes of energy loss in actual transformers.

Chapter: [4.01] Electromagnetic Induction
Concept: Eddy Currents
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