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

Physics [Panchkula Set 3]
Date: March 2015

[1]1

Define mobility of a charge carrier

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

Write its (‘mobility’ of charge carriers) S.I. unit

Concept: Drift of Electrons and the Origin of Resistivity
Chapter: [2.01] Current Electricity
[1]2

In a series LCR circuit, VL = VC ≠ VR. What is the value of power factor?

Concept: Ac Voltage Applied to a Series Lcr Circuit
Chapter: [4.01] Alternating Current
[1]3

The focal length of an equiconvex lens is equal to the radius of curvature of either face. What is the refractive index of the material of the lens?

Concept: Lenses
Chapter: [6.01] Ray Optics and Optical Instruments
[1]4

Write a relation for polarisation vecPof a dielectric material in the presence of an external electric field vecE .

Concept: Dielectrics and Polarisation
Chapter: [1.01] Electrostatic Potential and Capacitance
[1]5

What happens when a forward bias is applied to a p-n junction?

Concept: Semiconductor Diode
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
[2]6
[1]6.1

Distinguish between ‘Analog’ and ‘Digital’ forms of communication.

Concept: Elements of a Communication System
Chapter: [10.01] Communication Systems
[1]6.2

Explain briefly two commonly used applications of the ‘Internet’.

Concept: Elements of a Communication System
Chapter: [10.01] Communication Systems
[2]7

Given the ground state energy E0 = - 13.6 eV and Bohr radius a0 = 0.53 Å. Find out how the de Broglie wavelength associated with the electron orbiting in the ground state would change when it jumps into the first excited state.

Concept: Energy Levels
Chapter: [8.02] Atoms
[2]8

State Bohr’s postulate of hydrogen atom which successfully explains the emission lines in the spectrum of hydrogen atom. Use Rydberg formula to determine the wavelength of Hα line. [Given: Rydberg constant R = 1.03 × 107 m−1]

Concept: Bohr'S Model for Hydrogen Atom
Chapter: [8.02] Atoms
[2]9

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]10 | Attempt Any One
[2]10.1

Write the important characteristic features by which the interference can be distinguished from the observed diffraction pattern.

Concept: Interference
Chapter: [6.02] Wave Optics
[2]10.2

Explain the basic differences between the construction and working of a telescope and a microscope

Concept: Optical Instruments - The Microscope
Chapter: [6.01] Ray Optics and Optical Instruments
[3]11

Light of intensity ‘I’ and frequency ‘v’ is incident on a photosensitive surface and causes photoelectric emission. What will be the effect on anode current when (i) the intensity of light is gradually increased. In each case, all other factors remain the same. Explain, giving justification in each case.

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

Light of intensity ‘I’ and frequency ‘v’ is incident on a photosensitive surface and causes photoelectric emission. What will be the effect on anode current when (ii) the frequency of incident radiation is increased. In each case, all other factors remain the same. Explain, giving justification in each case.

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

Light of intensity ‘I’ and frequency ‘v’ is incident on a photosensitive surface and causes photoelectric emission. What will be the effect on anode current when the anode potential is increased? In each case, all other factors remain the same. Explain, giving justification in each case.

Concept: Photoelectric Effect - Hertz’S Observations
Chapter: [7.01] Dual Nature of Radiation and Matter
[3]12

When is a transistor said to be in active state?

Concept: Junction Transistor - Transistor: Structure and Action
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits

Draw a circuit diagram of a p-n-p transistor and explain how it works as a transistor amplifier

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

Write clearly, why in the case of a transistor (i) the base is thin and lightly doped

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

Write clearly, why in the case of a transistor (ii) the emitter is heavily doped.

Concept: Transistor and Characteristics of a Transistor
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
[3]13

State three important factors showing the need for translating a low frequency signal into a high frequency wave before transmission

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

Draw a sketch of a sinusoidal carrier wave along with a modulating signal and show how these are superimposed to obtain the resultant amplitude modulated wave

Concept: Modulation and Its Necessity
Chapter: [10.01] Communication Systems
[3]14

You are given three circuit elements X, Y and Z. When the element X is connected across an a.c. source of a given voltage, the current and the voltage are in the same phase. When the element Y is connected in series with X across the source, voltage is ahead of the current in phase by π/4 . But the current is ahead of the voltage in phase by π/4when Z is connected in series with X across the source. Identify the circuit elements X, Y and Z. When all the three elements are connected in series across the same source, determine the impedance of the circuit.Draw a plot of the current versus the frequency of applied source and mention the significance of this plot.

Concept: Reactance and Impedance
Chapter: [4.01] Alternating Current
[3]15

plot a graph showing the variation of current density (j) versus the electric field (E) for two conductors of different materials. What information from this plot regarding the properties of the conducting material, can be obtained which can be used to select suitable materials for use in making (i) standard resistance and (ii) connecting wires in electric circuits?

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

Electron drift speed is estimated to be of the order of mm s−1. Yet large current of the order of few amperes can be set up in the wire. Explain briefly.

Concept: Ampere’s Circuital Law
Chapter: [3.02] Moving Charges and Magnetism
[3]16

State Biot – Savart law.

Concept: Magnetic Field Due to a Current Element, Biot-Savart Law
Chapter: [3.02] Moving Charges and Magnetism

Deduce the expression for the magnetic field at a point on the axis of a current carrying circular loop of radius ‘R’ distant ‘x’ from the centre. Hence, write the magnetic field at the centre of a loop.

Concept: Magnetic Force
Chapter: [3.02] Moving Charges and Magnetism
[3]17

What dose a polaroid consist of?

Concept: Polarisation
Chapter: [6.02] Wave Optics

Show, using a simple polaroid, that light waves are transverse in nature. Intensity of light coming out of a polaroid does not change irrespective of the orientation of the pass axis of the polaroid. Explain why.

Concept: Polarisation
Chapter: [6.02] Wave Optics
[3]18 | Attempt Any One
[3]18.1

How is a Zener diode fabricated?

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

What causes the setting up of high electric field even for small reverse bias voltage across the diode?

Concept: Semiconductor Diode
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]18.2

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

If a small voltage is applied to a p-n junction diode, how will the barrier potential be affected when it is (i) forward biased

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

If a small voltage is applied to a p-n junction diode, how will the barrier potential be affected when it is(ii) reveres biased?

Concept: Special Purpose P-n Junction Diodes
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
[3]19

Arrange the following electromagnetic wave in the order of their increasing wavelength:

(a) γ- rays

(b) Microwaves

(c) X-rays

Concept: Electromagnetic Waves
Chapter: [5.01] Electromagnetic Waves

How are infra-red waves produced?

Concept: Electromagnetic Spectrum
Chapter: [5.01] Electromagnetic Waves

What role dose infra-red radiation play in maintain the Earth’s warmth

Concept: Electromagnetic Spectrum
Chapter: [5.01] Electromagnetic Waves

What role dose infra-red radiation play in physical therapy?

Concept: Electromagnetic Spectrum
Chapter: [5.01] Electromagnetic Waves
[3]20

Explain briefly the process of charging a parallel plate capacitor when it is connected across a d.c. battery

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

A capacitor of capacitance ‘C’ is charged to ‘V’ volts by a battery. After some time the battery is disconnected and the distance between the plates is doubled. Now a slab of dielectric constant, 1 < k < 2, is introduced to fill the space between the plates. How will the following be affected? (a) The electric field between the plates of the capacitor Justify your answer by writing the necessary expressions.

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

A capacitor of capacitance ‘C’ is charged to ‘V’ volts by a battery. After some time the battery is disconnected and the distance between the plates is doubled. Now a slab of dielectric constant, 1 < k < 2, is introduced to fill the space between the plates. How will the following be affected? (b) The energy stored in the capacitor Justify your answer by writing the necessary expressions

Concept: Capacitors and Capacitance
Chapter: [1.01] Electrostatic Potential and Capacitance
[3]21

Write symbolically the nuclear βdecay process of ""_6^11C Is the decayed product X an isotope or isobar of (""_6^11C)? Given the mass values m (""_6^11C) = 11.011434 u and m (X) = 11.009305 u. Estimate the Q-value in this process.

Concept: Mass-energy and Nuclear Binding Energy - Nuclear Binding Energy
Chapter: [8.01] Nuclei
[3]22

An object is placed 15 cm in front of a convex lens of focal length 10 cm. Find the nature and position of the image formed. Where should a concave mirror of radius of curvature 20 cm be placed so that the final image is formed at the position of the object itself?

Concept: Concave Mirror
Chapter: [6.01] Ray Optics and Optical Instruments
[4]23

Ajit had a high tension tower erected on his farm land. He kept complaining to the authorities to remove it as it was occupying a large portion of his land. His uncle, who was a teacher, explained to him the need for erecting these towers for efficient transmission of power. As Ajit realised its significance, he stopped complaining.

(a) Why is it necessary to transport power at high voltage?

(b) A low power factor implies large power loss. Explain.

(c) Write two values each displayed by Ajit and his uncle.

Concept: Power in Ac Circuit: the Power Factor
Chapter: [4.01] Alternating Current
[5]24 | Attempt Any One
[5]24.1

Deduce the expression for the potential energy of a system of two charges q1 and q2 located vec(r_1) and vec(r_2), respectively, in an external electric field.

Concept: Potential Energy in an External Field - Potential Energy of a System of Two Charges in an External Field
Chapter: [1.01] Electrostatic Potential and Capacitance

Three point charges, + Q + 2Q and – 3Q are placed at the vertices of an equilateral triangle ABC of side l. If these charges are displaced to the mid-point A1, B1 and C1, respectively, find the amount of the work done in shifting the charges to the new locations.

Concept: Electrical Potential Energy of a System of Two Point Charges and of Electric Dipole in an Electrostatic Field
Chapter: [1.01] Electrostatic Potential and Capacitance
[5]24.2

Define electric flux.

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

Write its (electric flux.) S.I unit.

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

State and explain Gauss’s law.

Concept: Gauss’s Law
Chapter: [1.02] Electric Charges and Fields

Find out the outward flux to a point charge +q placed at the centre of a cube of side ‘a’. Why is it found to be independent of the size and shape of the surface enclosing it? Explain.

Concept: Electric Flux
Chapter: [1.02] Electric Charges and Fields
[5]25 | Attempt Any One
[5]25.1

Obtain an expression for the energy stored in a solenoid of self-inductance ‘L’ when the current through it grows from zero to ‘I’.

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

A square loop MNOP of side 20 cm is placed horizontally in a uniform magnetic field acting vertically downwards as shown in the figure. The loop is pulled with a constant velocity of 20 cm s−1 till it goes out of the field.

(i) Depict the direction of the induced current in the loop as it goes out of the field. For how long would the current in the loop persist?

(ii) Plot a graph showing the variation of magnetic flux and induced emf as a function of time.

Concept: Magnetic Flux
Chapter: [4.02] Electromagnetic Induction
[5]25.2

Draw the magnetic field lines due to a circular loop area vecA carrying current I. Show that it acts as a bar magnet of magnetic moment vecm = IvecA .

Concept: Magnetic Field Intensity Due to a Magnetic Dipole (Bar Magnet) Along Its Axis
Chapter: [3.01] Magnetism and Matter

Derive the expression for the magnetic field due to a solenoid of length ‘2l’, radius ‘a’ having ’n’ number of turns per unit length and carrying a steady current ‘I’ at a point
on the axial line, distance ‘r’ from the centre of the solenoid. How does this expression compare with the axial magnetic field due to a bar magnet of magnetic moment ‘m’?

Concept: Solenoid and the Toroid - the Solenoid
Chapter: [3.02] Moving Charges and Magnetism
[5]26 | Attempt Any One
[5]26.1

In young’s double slit experiment, deduce the conditions for obtaining constructive and destructive interference fringes. Hence, deduce the expression for the fringe width.

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

Show that the fringe pattern on the screen is actually a superposition of slit diffraction from each slit.

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

What should be the width of each slit to obtain 10 maxima of the double slit pattern within the central maximum of the single slit pattern, for green light of wavelength 500 nm, if the separation between two slits is 1 mm?

Concept: Width of Central Maximum
Chapter: [6.02] Wave Optics
[5]26.2

Two thin convex lenses L1 and L2 of focal lengths f1 and f2, respectively, are placed coaxially in contact. An object is placed at a point beyond the focus of lens L1. Draw a ray diagram to show the image formation by the combination and hence derive the expression for the focal length of the combined system.

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

A ray PQ incident on the face AB of a prism ABC, as shown in the figure, emerges from the face AC such that AQ = AR.

Draw the ray diagram showing the passage of the ray through the prism. If the angle of the prism is 60° and refractive index of the material of prism is sqrt3 , determine the values of angle of incidence and angle of deviation

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

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## CBSE previous year question papers Class 12 Physics with solutions 2014 - 2015

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