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# Question Paper Solutions for Physics All India Set 3 2013-2014 CBSE (Commerce) Class 12

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Physics
All India Set 3
2013-2014 March
Marks: 70

[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.

Chapter: [3.01] Moving Charges and Magnetism
Concept: Magnetic Force
[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.

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

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

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

Chapter: [6.02] Ray Optics and Optical Instruments
Concept: Lenses
[1]5

Why do the electric field lines never cross each other?

Chapter: [1.01] Electric Charges and Fields
Concept: Electric Field Lines
[1]6

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

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

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

Chapter: [8.02] Nuclei
Concept: Radioactivity - Law of Radioactive Decay
[1]8

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

Chapter: [4.02] Alternating Current
Concept: Alternating Currents and Direct Currents
[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.

Chapter: [1.02] Electrostatic Potential and Capacitance
Concept: The Parallel Plate Capacitor
[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

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

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

Chapter: [1.02] Electrostatic Potential and Capacitance
Concept: Capacitors 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?

Chapter: [8.01] Atoms
Concept: Alpha-particle Scattering and Rutherford’S Nuclear Model of Atom
[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.

Chapter: [8.01] Atoms
Concept: Bohr'S Model for Hydrogen Atom
[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?

Chapter: [3.02] Magnetism and Matter
Concept: Magnetic Properties of Materials
[2]15

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

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

Chapter: [6.02] Ray Optics and Optical Instruments
Concept: Dispersion by a Prism
[2]17

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

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

Under what condition does the transistor act as an amplifier?

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

Write the functions of the following in communication systems:

Receiver

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

Write the functions of the following in communication systems:

Demodulator

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

Chapter: [6.02] Ray Optics and Optical Instruments
Concept: Concave Mirror
[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?

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

Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
Concept: Energy Bands in Conductors, Semiconductors and Insulators
[3]22

Write two basic modes of communication

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

Explain the process of amplitude modulation.

Chapter: [10.01] Communication Systems
Concept: Amplitude Modulation

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

Chapter: [10.01] Communication Systems
Concept: Amplitude Modulation
[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?

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

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

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

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

Chapter: [2.01] Current Electricity
Concept: Metre Bridge
[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.

Chapter: [2.01] Current Electricity
Concept: Metre Bridge
[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: [8.02] Nuclei
Concept: Radioactivity - Introduction of Radioactivity
[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.

Chapter: [3.01] Moving Charges and Magnetism
Concept: Magnetic Force

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

Chapter: [3.01] Moving Charges and Magnetism
Concept: Force Between Two Parallel Currents, the Ampere
[3]26

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

Chapter: [6.01] Wave Optics
Concept: Polarisation

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.

Chapter: [6.01] Wave Optics
Concept: Polarisation
[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?

Chapter: [4.02] Alternating Current
Concept: Ac Voltage Applied to a Series Lcr Circuit
[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.

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

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.

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

Chapter: [6.01] Wave Optics
Concept: Fraunhofer Diffraction Due to a Single Slit

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.

Chapter: [6.01] Wave Optics
Concept: Fraunhofer Diffraction Due to a Single Slit
[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.

Chapter: [3.01] Moving Charges and Magnetism
Concept: Motion in Combined Electric and Magnetic Fields - Cyclotron
[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

Chapter: [3.01] Moving Charges and Magnetism
Concept: Motion in Combined Electric and Magnetic Fields - Cyclotron
[5]29.2
[3]29.2.1

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

Chapter: [3.01] Moving Charges and Magnetism
Concept: Moving Coil Galvanometer
[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?

Chapter: [3.01] Moving Charges and Magnetism
Concept: Moving Coil Galvanometer
[1]29.2.2.2

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

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

Chapter: [1.02] Electrostatic Potential and Capacitance
Concept: Electric Potential Difference
[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

Chapter: [3.02] Magnetism and Matter
Concept: Dipole in a Uniform External Field

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

Chapter: [1.01] Electric Charges and Fields
Concept: Dipole in a Uniform External Field

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

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

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