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# Physics Delhi Set 3 2013-2014 CBSE (Science) Class 12 Question Paper Solution

Physics [Delhi Set 3]
Date: March 2014

[1]1

Define the term 'drift velocity' of charge carriers in a conductor and write its relationship with the current flowing through it.

Concept: Electric Current
Chapter: [2.01] Current Electricity
[1]2

The carrier wave of a signal is given by C(t) = 3 sin (8πt) volt. The modulating signal is a square wave as shown. Find its modulation index.

Concept: Modulation and Its Necessity
Chapter: [10.01] Communication Systems
[1]3

Write the expression, in a vector form, for the Lorentz magnetic force $\vec{F}$ due to a charge moving with velocity $\vec{V}$ in a magnetic field $\vec{B}$. What is the direction of the magnetic force?

Concept: Force on a Moving Charge in Uniform Magnetic and Electric Fields
Chapter: [3.02] Moving Charges and Magnetism
[1]4

Plot a graph showing variation of current versus voltage for the material GaAs ?

Concept: Semiconductor Diode
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
[1]5

A convex lens is placed in contact with a plane mirror. A point object at a distance of 20 cm on the axis of this combination has its image coinciding with itself. What is the focal length of the lens?

Concept: Refraction at Spherical Surfaces and by Lenses - Combination of Thin Lenses in Contact
Chapter: [6.01] Ray Optics and Optical Instruments
[1]6

The figure given below shows the block diagram of a generalized communication system. Identify the element labelled 'X' and write its function.

Concept: Elements of a Communication System
Chapter: [10.01] Communication Systems
[1]7

Two spherical bobs, one metallic and the other of glass, of the same size are allowed to fall freely from the same height above the ground. Which of the two would reach earlier and why?

Concept: Eddy Currents
Chapter: [4.02] Electromagnetic Induction
[1]8

"For any charge configuration, equipotential surface through a point is normal to the electric field." Justify.

Concept: Applications of Gauss’s Law
Chapter: [1.02] Electric Charges and Fields
[2]9

An electric dipole of length 1 cm, which placed with its axis making an angle of 60° with uniform electric field, experience a torque of $6\sqrt{3} Nm$ . Calculate the potential energy of the dipole if it has charge ±2 nC.

Concept: Electric Dipole
Chapter: [1.02] Electric Charges and Fields
[2]10

State the underlying principle of a cyclotron. Write briefly how this machine is used to accelerate charged particles to high energies

Concept: Motion in Combined Electric and Magnetic Fields - Cyclotron
Chapter: [3.02] Moving Charges and Magnetism
[2]11

A capacitor 'C', a variable resistor 'R' and a bulb 'B' are connected in series to the ac mains in circuit as shown. The bulb glows with some brightness. How will the glow of the bulb change if (i) a dielectric slab is introduced between the plates of the capacitor, keeping resistance R to be the same; (ii) the resistance R is increased keeping the same capacitance?

Concept: Combination of Capacitors
Chapter: [1.01] Electrostatic Potential and Capacitance
[2]12

A proton and an alpha particle are accelerated through the same potential. Which one of the two has (i) greater value of de−Broglie wavelength associated with it and (ii) less kinetic energy. Give reasons to justify your answer.

Concept: de-Broglie Relation
Chapter: [7.01] Dual Nature of Radiation and Matter
[2]13

Out of the two magnetic materials, 'A' has relative permeability slightly greater than unity while 'B' has less than unity. Identify the nature of the materials 'A' and 'B'. Will their susceptibilities be positive or negative?

Concept: Magnetic Properties of Materials
Chapter: [3.01] Magnetism and Matter
[2]14

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

Concept: Kirchhoff’s Rules
Chapter: [2.01] Current Electricity
[2]15

Given a uniform electric filed $\vec{E} = 4 \times {10}^3 \ \hat{i} N/C$. Find the flux of this field through a square of 5 cm on a side whose plane is parallel to the Y-Z plane. What would be the flux through the same square if the plane makes a 30° angle with the x-axis?

Concept: Electric Flux
Chapter: [1.02] Electric Charges and Fields
[2]16
[1]16.1

Monochromatic light of frequency 5.0 × 1014 Hz is produced by a laser. The power emitted is 3.0 × 10–3 W. Estimate the number of photons emitted per second on an average by the source ?

Concept: Refraction of Monochromatic Light
Chapter: [6.02] Wave Optics
[1]16.2

Draw a plot showing the variation of photoelectric current versus the intensity of incident radiation on a given photosensitive surface.

Concept: Photoelectric Effect - Hertz’S Observations
Chapter: [7.01] Dual Nature of Radiation and Matter
[2]17 | Attempt any one of the following.
[2]17.1

Write the truth table for the combination of the gates shown. Name the gates used.

Concept: Digital Electronics and Logic Gates
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
[2]17.2

Identify the logic gates marked 'P' and 'Q' in the given circuit. Write the truth table for the combination.

Concept: Digital Electronics and Logic Gates
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
[2]18

For a single slit of width "a", the first minimum of the interference pattern of a monochromatic light of wavelength λ occurs at an angle of λa. At the same angle of λa, we get a maximum for two narrow slits separated by a distance "a". Explain.

Concept: Fraunhofer Diffraction Due to a Single Slit
Chapter: [6.02] Wave Optics
[3]19
[1]19.1

State Ampere’s circuital law

Concept: Ampere’s Circuital Law
Chapter: [3.02] Moving Charges and Magnetism
[2]19.2

Two long coaxial insulated solenoids, S1 and S2 of equal lengths are wound one over the other as shown in the figure. A steady current "I" flow thought the inner solenoid S1 to the other end B, which is connected to the outer solenoid S2 through which the same current "I" flows in the opposite direction so as to come out at end A. If n1 and n2 are the number of turns per unit length, find the magnitude and direction of the net magnetic field at a point (i) inside on the axis and (ii) outside the combined system

Concept: Solenoid and the Toroid - the Solenoid
Chapter: [3.02] Moving Charges and Magnetism
[3]20

A 12.3 eV electron beam is used to bombard gaseous hydrogen at room temperature. Upto which energy level the hydrogen atoms would be excited?
Calculate the wavelengths of the second member of Lyman series and second member of Balmer series.

Concept: Energy Levels
Chapter: [8.02] Atoms
[3]21 | Attempt any one of the following.
[3]21.1
[2]21.1.1

Obtain the expression for the energy stored per unit volume in a charged parallel plate capacitor.

Concept: Energy Stored in a Capacitor
Chapter: [1.01] Electrostatic Potential and Capacitance
[1]21.1.2

The electric field inside a parallel plate capacitor is E. Find the amount of work done in moving a charge q over a closed loop a b c d a.

Concept: Combination of Capacitors
Chapter: [1.01] Electrostatic Potential and Capacitance
[3]21.2
[1]21.2.1

Derive the expression for the capacitance of a parallel plate capacitor having plate area A and plate separation d.

Concept: Capacitance of a Parallel Plate Capacitor with and Without Dielectric Medium Between the Plates
Chapter: [1.01] Electrostatic Potential and Capacitance
[2]21.2.2

Two charged spherical conductors of radii R1 and R2 when connected by a conducting wire acquire charges q1 and q2 respectively. Find the ratio of their surface charge densities in terms of their radii.

Concept: Conductors and Insulators Related to Electric Field
Chapter: [1.01] Electrostatic Potential and Capacitance
[5]22
[2]22.1

A mobile phone lies along the principal axis of a concave mirror. Show, with the help of a suitable diagram, the formation of its image. Explain why magnification is not uniform.

Concept: Concave Mirror
Chapter: [6.01] Ray Optics and Optical Instruments
[1]22.2

Suppose the lower half of the concave mirror's reflecting surface is covered with an opaque material. What effect this will have on the image of the object? Explain

Concept: Concave Mirror
Chapter: [6.01] Ray Optics and Optical Instruments
[3]23
[1]23.1

Draw a labelled ray diagram showing the formation of a final image by a compound microscope at least distance of distinct vision

Concept: Optical Instruments - The Microscope
Chapter: [6.01] Ray Optics and Optical Instruments
[2]23.2

The total magnification produced by a compound microscope is 20. The magnification produced by the eye piece is 5. The microscope is focussed on a certain object. The distance between the objective and eyepiece is observed to be 14 cm. If least distance of distinct vision is 20 cm, calculate the focal length of the objective and the eye piece.

Concept: Optical Instruments - The Microscope
Chapter: [6.01] Ray Optics and Optical Instruments
[3]24
[1]24.1

Name the em waves which are produced during radioactive decay of a nucleus. Write their frequency range ?

Concept: Elementary Facts About Electromagnetic Wave Uses
Chapter: [5.01] Electromagnetic Waves
[1]24.2

Welders wear special glass goggles while working. Why? Explain.

Concept: Electromagnetic Induction
Chapter: [4.02] Electromagnetic Induction
[1]24.3

Why are infrared waves often called as heat waves? Given their one application.

Concept: Electromagnetic Waves
Chapter: [5.01] Electromagnetic Waves
[3]25

A potentiometer wire of length 1 m has a resistance of 5 Ω. It is connected to a 8 V battery in series with a resistance of 15 Ω. Determine the emf of the primary cell which gives a balance point at 60 cm.

Concept: Potentiometer
Chapter: [2.01] Current Electricity
[3]26
[1]26.1

Derive the mathematical expression for law of radioactive decay for a sample of a radioactive nucleus

Chapter: [8.01] Nuclei
[2]26.2

Write symbolically the process expressing the β+ decay of ""_11^22Na. Also write the basic nuclear process underlying this decay.

Chapter: [8.01] Nuclei

Is the nucleus formed in the decay of the nucleus ""_11^22Na, an isotope or isobar?

Concept: Mass-energy and Nuclear Binding Energy - Nuclear Binding Energy
Chapter: [8.01] Nuclei
[3]27

When Sunita, a class XII student, came to know that her parents are planning to rent out the top floor of their house to a mobile company she protested. She tried hard to convince her parents that this move would be a health hazard.
Ultimately her parents agreed :

(1) In what way can the setting up of transmission tower by a mobile company in a residential colony prove to be injurious to health?

(2) By objecting to this move of her parents, what value did Sunita display?

(3) Estimate the range of e.m. waves which can be transmitted by an antenna of height 20 m. (Given radius of the earth = 6400 km)

Concept: Electromagnetic Waves
Chapter: [5.01] Electromagnetic Waves
[5]28 | Attempt any one of the following.
[5]28.1
[2]28.1.1

Explain with the help of a diagram, how depletion region and potential barrier are formed in a junction diode.

Concept: Special Purpose P-n Junction Diodes
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
[3]28.1.2

Using the necessary circuit diagrams, show how the V-I characteristics of a p-n junction are obtained in

Forward biasing

How are these characteristics made use of in rectification?

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

Using the necessary circuit diagrams, show how the V-I characteristics of a p-n junction are obtained in

Reverse biasing

How are these characteristics made use of in rectification?

Concept: Special Purpose P-n Junction Diodes
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
[5]28.2
[2]28.2.1

Differentiate between three segments of a transistor on the basis of their size and level of doping.

Concept: Transistor and Characteristics of a Transistor
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
[1]28.2.2

How is a transistor biased to be in active state?

Concept: Transistor and Characteristics of a Transistor
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
[2]28.2.3

With the help of necessary circuit diagram, describe briefly how n-p-n transistor in CE configuration amplifies a small sinusoidal input voltage. Write the expression for the ac current gain

Concept: Junction Transistor - Transistor as an Amplifier (Ce-configuration)
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
[5]29 | Attempt any one of the following.
[5]29.1
[3]29.1.1

'Two independent monochromatic sources of light cannot produce a sustained interference pattern'. Give reason.

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

Light waves each of amplitude "a" and frequency "ω", emanating from two coherent light sources superpose at a point. If the displacements due to these waves are given by y1 = a cos ωt and y2 = a cos(ωt + ϕ) where ϕ is the phase difference between the two, obtain the expression for the resultant intensity at the point.

Concept: Principle of Superposition of Waves,
Chapter: [6.02] Wave Optics
[2]29.1.2

In Young's double slit experiment, using monochromatic light of wavelength λ, the intensity of light at a point on the screen where path difference is λ, is K units. Find out the intensity of light at a point where path difference is λ/3.

Concept: Interference of Light Waves and Young’S Experiment
Chapter: [6.02] Wave Optics
[5]29.2
[2]29.2.1

How does one demonstrate, using a suitable diagram, that unpolarised light when passed through a Polaroid gets polarised?

Concept: Polarisation
Chapter: [6.02] Wave Optics
[3]29.2.2

A beam of unpolarised light is incident on a glass-air interface. Show, using a suitable ray diagram, that light reflected from the interface is totally polarised, when μ = tan iB, where μ is the refractive index of glass with respect to air and iB is the Brewster's angle.

Concept: Polarisation
Chapter: [6.02] Wave Optics
[5]30 | Attempt any one of the following.
[5]30.1
[2]30.1.1

Describe a simple experiment (or activity) to show that the polarity of emf induced in a coil is always such that it tends to produce a current which opposes the change of magnetic flux that produces it.

Concept: Lenz’S Law and Conservation of Energy
Chapter: [4.02] Electromagnetic Induction
[3]30.1.2

The current flowing through an inductor of self inductance L is continuously increasing. Plot a graph showing the variation of

Magnetic flux versus the current

Concept: Magnetic Flux
Chapter: [4.02] Electromagnetic Induction

The current flowing through an inductor of self-inductance L is continuously increasing. Plot a graph showing the variation of

Induced emf versus dI/dt

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

The current flowing through an inductor of self inductance L is continuously increasing. Plot a graph showing the variation of Magnetic potential energy stored versus the current.

Concept: Inductance - Mutual Inductance
Chapter: [4.02] Electromagnetic Induction
[5]30.2
[4]30.2.1

Draw a schematic sketch of an ac generator describing its basic elements. State briefly its working principle. Show a plot of variation of

(i) Magnetic flux and

(ii) Alternating emf versus time generated by a loop of wire rotating in a magnetic field.

Concept: Magnetic Flux
Chapter: [4.02] Electromagnetic Induction
[1]30.2.2

Why is choke coil needed in the use of fluorescent tubes with ac mains?

Concept: Alternating Currents
Chapter: [4.01] Alternating Current

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