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# Physics All India Set 3 2013-2014 CBSE (Arts) Class 12 Question Paper Solution

Physics [All India Set 3]
Marks: 70Academic Year: 2013-2014
Date: March 2014

[1]1

A conducting loop is held above a current carrying wire PQ as shown in the figure. Depict the direction of the current induced in the loop when the current in the wire PQ is constantly increasing.

Concept: Magnetic Force
Chapter: [3.02] Moving Charges and Magnetism
[1]2

The graph shows the variation of stopping potential with frequency of incident radiation for two photosensitive metals A and B. Which one of the two has higher value of work-function? Justify your answer.

Concept: Photoelectric Effect - Hertz’S Observations
Chapter: [7.01] Dual Nature of Radiation and Matter
[1]3

Using the concept of force between two infinitely long parallel current carrying conductors, define one ampere of current.

Concept: Force Between Two Parallel Currents, the Ampere
Chapter: [3.02] Moving Charges and Magnetism
[1]4

A biconvex lens made of a transparent material of refractive index 1.25 is immersed in water of refractive index 1.33. Will the lens behave as a converging or a diverging lens? Give reason.

Concept: Lenses
Chapter: [6.01] Ray Optics and Optical Instruments
[1]5

Why do the electric field lines never cross each other?

Concept: Electric Field Lines
Chapter: [1.02] Electric Charges and Fields
[1]6

To which part of the electromagnetic spectrum does a wave of frequency 5 × 1011 Hz belong?

Concept: Electromagnetic Spectrum
Chapter: [5.01] Electromagnetic Waves
[1]7

Why is it found experimentally difficult to detect neutrinos in nuclear β-decay?

Concept: Radioactivity - Law of Radioactive Decay
Chapter: [8.01] Nuclei
[1]8

Why is the use of A.C. voltage preferred over D.C. voltage ? Give two reasons.

Concept: Alternating Currents and Direct Currents
Chapter: [4.01] Alternating Current
[2]9

Considering the case of a parallel plate capacitor being charged, show how one is required to generalize Ampere's circuital law to include the term due to displacement current.

Concept: The Parallel Plate Capacitor
Chapter: [1.01] Electrostatic Potential and Capacitance
[2]10

Estimate the average drift speed of conduction electrons in a copper wire of cross-sectional area 2·5 × 10−7 m2 carrying a current of 2·7 A. Assume the density of conduction electrons to be 9 × 1028 m−3

Concept: Drift of Electrons and the Origin of Resistivity
Chapter: [2.01] Current Electricity
[2]11

A cell of emf 'E' and internal resistance 'r' is connected across a variable resistor 'R'. Plot a graph showing variation of terminal voltage 'V' of the cell versus the current 'I'. Using the plot, show how the emf of the cell and its internal resistance can be determined.

Concept: Cells, Emf, Internal Resistance
Chapter: [2.01] Current Electricity
[2]12

A parallel plate capacitor of capacitance C is charged to a potential V. It is then connected to another uncharged capacitor having the same capacitance. Find out the ratio of the energy stored in the combined system to that stored initially in the single capacitor.

Concept: Capacitors and Capacitance
Chapter: [1.01] Electrostatic Potential and Capacitance
[2]13 | Attempt Any One
[2]13.1

Using Rutherford's model of the atom, derive the expression for the total energy of the electron in hydrogen atom. What is the significance of total negative energy possessed by the electron?

Concept: Alpha-particle Scattering and Rutherford’S Nuclear Model of Atom
Chapter: [8.02] Atoms
[2]13.2

Using Bohr's postulates of the atomic model, derive the expression for radius of nth electron orbit. Hence obtain the expression for Bohr's radius.

Concept: Bohr'S Model for Hydrogen Atom
Chapter: [8.02] Atoms
[2]14

Show diagrammatically the behaviour of magnetic field lines in the presence of (i) paramagnetic and (ii) diamagnetic substances. How does one explain this distinguishing feature?

Concept: Magnetic Properties of Materials
Chapter: [3.01] Magnetism and Matter
[2]15

Draw the circuit diagram of a half wave rectifier and explain its working.

Concept: Application of Junction Diode as a Rectifier
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
[2]16

Two monochromatic rays of light are incident normally on the face AB of an isosceles right-angled prism ABC. The refractive indices of the glass prism for the two rays '1' and '2' are respectively 1.38 and 1.52. Trace the path of these rays after entering through the prism.

Concept: Dispersion by a Prism
Chapter: [6.01] Ray Optics and Optical Instruments
[2]17

Draw a circuit diagram of a transistor amplifier in CE configuration.

Concept: Junction Transistor - Transistor as an Amplifier (Ce-configuration)
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits

Under what condition does the transistor act as an amplifier?

Concept: Junction Transistor - Transistor as an Amplifier (Ce-configuration)
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
[2]18

Write the functions of the following in communication systems:

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

Write the functions of the following in communication systems:

Demodulator

Concept: Basic Terminology Used in Electronic Communication Systems
Chapter: [10.01] Communication Systems
[3]19

A convex lens of focal length 20 cm is placed coaxially with a convex mirror of radius of curvature 20 cm. The two are kept 15 cm apart. A point object is placed 40 cm in front of the convex lens. Find the position of the image formed by this combination. Draw the ray diagram showing the image formation.

Concept: Concave Mirror
Chapter: [6.01] Ray Optics and Optical Instruments
[3]20

An electron microscope uses electrons accelerated by a voltage of 50 kV. Determine the de-Broglie wavelength associated with the electrons. Taking other factors, such as numerical aperture etc. to be same, how does the resolving power of an electron microscope compare with that of an optical microscope which used yellow light?

Concept: Davisson-Germer Experiment
Chapter: [7.01] Dual Nature of Radiation and Matter
[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
[3]22

Write two basic modes of communication

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

Explain the process of amplitude modulation.

Concept: Amplitude Modulation
Chapter: [10.01] Communication Systems

Draw a schematic sketch showing how amplitude modulated signal is obtained by superposing a modulating signal over a sinusoidal carrier wave.

Concept: Amplitude Modulation
Chapter: [10.01] Communication Systems
[3]23 | Attempt Any One
[3]23.1
[1]23.1.1

Why are the connections between the resistors in a meter bridge made of thick copper strips?

Concept: Metre Bridge
Chapter: [2.01] Current Electricity
[1]23.1.2

Why is it generally preferred to obtain the balance point in the middle of the meter bridge wire?

Concept: Metre Bridge
Chapter: [2.01] Current Electricity
[1]23.1.3

Which material is used for the meter bridge wire and why?

Concept: Metre Bridge
Chapter: [2.01] Current Electricity
[3]23.2

A resistance of R Ω draws current from a potentiometer as shown in the figure. The potentiometer has a total resistance Ro Ω. A voltage V is supplied to the potentiometer. Derive an expression for the voltage across R when the sliding contact is in the middle of the potentiometer.

Concept: Metre Bridge
Chapter: [2.01] Current Electricity
[3]24

For the past some time, Aarti had been observing some erratic body movement, unsteadiness and lack of coordination in the activities of her sister Radha, who also used to complain of severe headache occasionally. Aarti suggested to her parents to get a medical check-up of Radha. The doctor thoroughly examined Radha and diagnosed that she has a brain tumour.

(a) What, according to you, are the values displayed by Aarti?

(b) How can radioisotopes help a doctor to diagnose brain tumour?

Chapter: [8.01] Nuclei
[3]25

A rod of length l is moved horizontally with a uniform velocity 'v' in a direction perpendicular to its length through a region in which a uniform magnetic field is acting vertically downward. Derive the expression for the emf induced across the ends of the rod.

Concept: Magnetic Force
Chapter: [3.02] Moving Charges and Magnetism

How does one understand this motional emf by invoking the Lorentz force acting on the free charge carriers of the conductor? Explain.

Concept: Force Between Two Parallel Currents, the Ampere
Chapter: [3.02] Moving Charges and Magnetism
[3]26

Show, via a suitable diagram, how unpolarised light can be polarised by reflection.

Concept: Polarisation
Chapter: [6.02] Wave Optics

Two polaroids P1 and P2 are placed with their pass axes perpendicular to each other. Unpolarised light of intensity I0 is incident on P1. A third polaroid P3 is kept in between P1 and P2 such that its pass axis makes an angle of 60° with that of P1. Determine the intensity of light transmitted through P1, P2 and P3.

Concept: Polarisation
Chapter: [6.02] Wave Optics
[3]27

A voltage V = V0 sin ωt is applied to a series LCR circuit. Derive the expression for the average power dissipated over a cycle. Under what condition (i) no power is dissipated even though the current flows through the circuit, (ii) maximum power is dissipated in the circuit?

Concept: Ac Voltage Applied to a Series Lcr Circuit
Chapter: [4.01] Alternating Current
[5]28 | Attempt Any One
[5]28.1

In Young's double slit experiment, describe briefly how bright and dark fringes are obtained on the screen kept in front of a double slit. Hence obtain the expression for the fringe width.

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

The ratio of the intensities at minima to the maxima in the Young's double slit experiment is 9 : 25. Find the ratio of the widths of the two slits.

Concept: Interference of Light Waves and Young’S Experiment
Chapter: [6.02] Wave Optics
[5]28.2

Describe briefly how a diffraction pattern is obtained on a screen due to a single narrow slit illuminated by a monochromatic source of light. Hence obtain the conditions for the angular width of secondary maxima and secondary minima.

Concept: Fraunhofer Diffraction Due to a Single Slit
Chapter: [6.02] Wave Optics

Two wavelengths of sodium light of 590 nm and 596 nm are used in turn to study the diffraction taking place at a single slit of aperture 2 × 10−6 m. The distance between the slit and the screen is 1·5 m. Calculate the separation between the positions of first maxima of the diffraction pattern obtained in the two cases.

Concept: Fraunhofer Diffraction Due to a Single Slit
Chapter: [6.02] Wave Optics
[5]29 | Attempt Any One
[5]29.1
[3]29.1.1

Deduce an expression for the frequency of revolution of a charged particle in a magnetic field and show that it is independent of velocity or energy of the particle.

Concept: Motion in Combined Electric and Magnetic Fields - Cyclotron
Chapter: [3.02] Moving Charges and Magnetism
[2]29.1.2

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
[5]29.2
[3]29.2.1

Draw a labelled diagram of a moving coil galvanometer. Describe briefly its principle and working.

Concept: Moving Coil Galvanometer
Chapter: [3.02] Moving Charges and Magnetism
[2]29.2.2 | Answer the following:
[1]29.2.2.1

Why is it necessary to introduce a cylindrical soft iron core inside the coil of a galvanometer?

Concept: Moving Coil Galvanometer
Chapter: [3.02] Moving Charges and Magnetism
[1]29.2.2.2

Increasing the current sensitivity of a galvanometer may not necessarily increase its voltage sensitivity. Explain, giving reason.

Concept: Moving Coil Galvanometer
Chapter: [3.02] Moving Charges and Magnetism
[5]30 | Attempt Any One
[5]30.1

Draw a labelled diagram of Van de Graaff generator. State its working principle to show how by introducing a small charged sphere into a larger sphere, a large amount of charge can be transferred to the outer sphere. State the use of this machine and also point out its limitations.

Concept: Electric Potential Difference
Chapter: [1.01] Electrostatic Potential and Capacitance
[5]30.2

Deduce the expression for the torque acting on a dipole of dipole moment vecp in the presence of a uniform electric field vecE

Concept: Dipole in a Uniform External Field
Chapter: [1.02] Electric Charges and Fields [3.01] Magnetism and Matter

Consider two hollow concentric spheres, S1 and S2, enclosing charges 2Q and 4Q respectively as shown in the figure. (i) Find out the ratio of the electric flux through them. (ii) How will the electric flux through the sphere S1 change if a medium of dielectric constant 'εr' is introduced in the space inside S1 in place of air ? Deduce the necessary expression

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

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