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# Question Paper Solutions - Physics 2014 - 2015-CBSE 12th-Class 12 CBSE (Central Board of Secondary Education)

SubjectPhysics
Year2014 - 2015 (March)

#### Alternate Sets

Marks: 70
[1]1

Define capacitor reactance. Write its S.I units.

Chapter: [1.02] Electrostatic Potential, Potential Energy and Capacitance
Concept: Capacitors and Capacitance
[1]2

What is the electric flux through a cube of side 1 cm which encloses an electric dipole?

Chapter: [1.01] Electric Charges and Fields
Concept: Electric Flux
[1]3

A concave lens of refractive index 1.5 is immersed in a medium of refractive index 1.65. What is the nature of the lens?

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

How are side bands produced?

Chapter: [10.01] Communication Systems
Concept: Modulation and Its Necessity
[1]5

Graph showing the variation of current versus voltage for a material Ga As is shown in the figure. Identify the region of
(i) negative resistance

(ii) where Ohm's law is obeyed.

Chapter: [11] Current Electricity
Concept: Ohm’s Law
[2]6

A proton and an α-particle have the same de-Broglie wavelength. Determine the ratio of their accelerating potentials

Chapter: [7] Dual Nature of Radiation and Matter
Concept: de-Broglie Relation

A proton and an α-particle have the same de-Broglie wavelength Determine the ratio of  their speeds.

Chapter: [7.01] Dual Nature of Radiation and Matter
Concept: Wave Nature of Matter
[2]7

Show that the radius of the orbit in hydrogen atom varies as n2, where n is the principal quantum number of the atom.

Chapter: [8.01] Atoms
Concept: De Broglie’S Explanation of Bohr’S Second Postulate of Quantisation
[2]8

Distinguish between 'intrinsic' and 'extrinsic' semiconductors

Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
Concept: Extrinsic Semiconductor
[2]9 | Attempt Any One
[2]9.1

Use the mirror equation to show that an object placed between f and 2f of a concave mirror produces a real image beyond 2f.

Chapter: [6.01] Ray Optics and Optical Instruments
Concept: Ray Optics - Mirror Formula
[2]9.2

Find an expression for intensity of transmitted light when a polaroid sheet is rotated between two crossed polaroids. In which position of the polaroid sheet will the transmitted intensity be maximum?

Chapter: [6.02] Wave Optics
Concept: Polarisation
[2]10

Use Kirchhoff's rules to obtain conditions for the balance condition in a Wheatstone bridge.

Chapter: [2] Current Electricity
Concept: Kirchhoff’s Rules
[3]11

Identify the part of the electromagnetic spectrum which is suitable for radar system used in aircraft navigation.

Chapter: [14] Electromagnetic Waves
Concept: Electromagnetic Spectrum

Name the parts of the electromagnetic spectrum which is

used to treat muscular strain.

Write in brief, how these waves can be produced.

Chapter: [14] Electromagnetic Waves
Concept: Electromagnetic Spectrum

Name the parts of the electromagnetic spectrum which is

used as a diagnostic tool in medicine.

Write in brief, how these waves can be produced.

Chapter: [14] Electromagnetic Waves
Concept: Electromagnetic Spectrum
[3]12

A giant refracting telescope has an objective lens of focal length 15 m. If an eye piece of focal length 1.0 cm is used, what is the angular magnification of the telescope ?

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

If this telescope is used to view the moon, what is the diameter of the image of the moon formed by the objective lens ? the diameter of the moon is 3.48 × 106 m and the radius of lunar orbit is 3.8 × 108m.

Chapter: [6.01] Ray Optics and Optical Instruments
Concept: Optical Instruments - The Microscope
[3]13

Write Einstein's photoelectric equation and mention which important features in photoelectric effect can be explained with the help of this equation.

The maximum kinetic energy of the photoelectrons gets doubled when the wavelength of light incident on the surface changes from λ1 to λ2. Derive the expressions for the threshold wavelength λ0 and work function for the metal surface.

Chapter: [17] Electrons and Photons
Concept: Einstein’s Equation - Particle Nature of Light
[3]14 | Attempt Any One
[3]14.1

In the study of Geiger-Marsdon experiment on scattering of α particles by a thin foil of gold, draw the trajectory of α-particles in the coulomb field of target nucleus. Explain briefly how one gets the information on the size of the nucleus from this study.

From the relation R = R0 A1/3, where R0 is constant and A is the mass number of the nucleus, show that nuclear matter density is independent of A

Chapter: [8.02] Nuclei
Concept: Atomic Masses and Composition of Nucleus
[3]14.2

Distinguish between nuclear fission and fusion. Show how in both these processes energy is released. Calculate the energy release in MeV in the deuterium-tritium fusion reaction :

""_1^2H+_1^3H->_2^4He+n

Using the data :

m(""_1^2H) = 2.014102 u

m(""_1^3H) = 3.016049 u

m(""_2^4He) = 4.002603 u

mn = 1.008665 u

1u = 931.5 MeV/c2

Chapter: [8.02] Nuclei
Concept: Nuclear Energy - Nuclear Fusion – Energy Generation in Stars
[3]15

Draw a block diagram of a detector for AM signal and show, using necessary processes and the waveforms, how the original message signal is detected from the input AM wave.

Chapter: [10.01] Communication Systems
Concept: Amplitude Modulation
[3]16

A cell of emf 'E' and internal resistance 'r' is connected across a variable load resistor R. Draw the plots of the terminal voltage V versus (i) R and (ii) the current I.

It is found that when R = 4 Ω, the current is 1 A and when R is increased to 9 Ω, the current reduces to 0.5 A. Find the values of the emf E and internal resistance r.

Chapter: [11] Current Electricity
Concept: Cells, Emf, Internal Resistance
[3]17

Two capacitors of unknown capacitances C1 and C2 are connected first in series and then in parallel across a battery of 100 V. If the energy stored in the two combinations is 0.045 J and 0.25 J respectively, determine the value of C1 and C2. Also calculate the charge on each capacitor in parallel combination.

Chapter: [1.02] Electrostatic Potential and Capacitance
Concept: Combination of Capacitors
[3]18

Write the underlying principle of a moving coil galvanometer.

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

A galvanometer of resistance G is converted into a voltmeter to measure upto V volts by connecting a resistance R1 in series with the coil. If a resistance R2 is connected in series with it, then it can measures upto V/2 volts. Find the resistance, in terms of R1 and R2, required to be connected to convert it into a voltmeter that can read upto 2 V. Also find the resistance G of the galvanometer in terms of R1 and R2

Chapter: [3.01] Moving Charges and Magnetism
Concept: Moving Coil Galvanometer
[3]19

With what considerations in view, a photodiode is fabricated? State its working with the help of a suitable diagram.

Even though the current in the forward bias is known to be more than in the reverse bias, yet the photodiode works in reverse bias. What is the reason?

Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
Concept: Special Purpose P-n Junction Diodes
[3]20

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)

Define the terms : Input resistance How are these determined using typical input and output characteristics?

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

Define the terms :Current amplification factor. How are these determined using typical input and output characteristics?

Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
Concept: Junction Transistor - Transistor: Structure and Action
[3]21 | Answer the following questions :

In a double slit experiment using the light of wavelength 600 nm, the angular width of the fringe formed on a distant screen is 0.1°. Find the spacing between the two slits.?

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

Light of wavelength 5000 Å propagating in air gets partly reflected from the surface of water. How will the wavelengths and frequencies of the reflected and refracted light be affected?

Chapter: [9] Ray Optics
Concept: Refraction
[3]22

An inductor L of inductance XL is connected in series with a bulb B and an ac source. How would brightness of the bulb change when (i) number of turn in the inductor is reduced, (ii) an iron rod is inserted in the inductor and (iii) a capacitor of reactance XC = XL is inserted in series in the circuit? Justify your answer in each case.

Chapter: [4.02] Alternating Current
Concept: Reactance and Impedance
[4]23

A group of students while coming from the school noticed a box marked "Danger H.T. 2200 V" at a substation in the main street. They did not understand the utility of a such a high voltage, while they argued, the supply was only 220 V. They asked their teacher this question the next day. The teacher thought it to be an important question and therefore explained to the whole class.

(i) What device is used to bring the high voltage down to low voltage of a.c. current and what is the principle of its working ?

(ii) Is it possible to use this device for bringing down the high dc voltage to the low voltage? Explain

(iii) Write the values displayed by the students and the teacher.

Chapter: [4.02] Alternating Current
Concept: Transformers
[5]24 | Attempt Any One
[5]24.1

State Ampere’s circuital law

Chapter: [14] Magnetic Effects of Electric Current
Concept: Ampere’s Circuital Law

Use this law to obtain the expression for the magnetic field inside an air cored toroid of average radius 'r', having 'n' turns per unit length and carrying a steady current I.

Chapter: [3.01] Moving Charges and Magnetism
Concept: Solenoid and the Toroid - the Solenoid

An observer to the left of a solenoid of N turns each of cross section area 'A' observes that a steady current I in it flows in the clockwise direction. Depict the magnetic field lines due to the solenoid specifying its polarity and show that it acts as a bar magnet of magnetic moment m = NIA.

Chapter: [3.01] Moving Charges and Magnetism
Concept: Solenoid and the Toroid - the Solenoid
[5]24.2

Define mutual inductance and write its S.I. units

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

Derive an expression for the mutual inductance of two long co-axial solenoids of same length wound one over the other,

Chapter: [3.01] Moving Charges and Magnetism
Concept: Solenoid and the Toroid - the Solenoid

In an experiment, two coils c1 and c2 are placed close to each other. Find out the expression for the emf induced in the coil c1 due to a change in the current through the coil c2.

Chapter: [4.01] Electromagnetic Induction
Concept: Inductance - Mutual Inductance
[5]25 | Attempt Any One
[5]25.1

Using Huygens's construction of secondary wavelets explain how a diffraction pattern is obtained on a screen due to a narrow slit on which a monochromatic beam of light is incident normally.

Chapter: [6.02] Wave Optics
Concept: Huygens Principle

Show that the angular width of the first diffraction fringe is half that of the central fringe.

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

Explain why the maxima at theta=(n+1/2)lambda/a become weaker and weaker with increasing n

Chapter: [11] Interference and Diffraction
Concept: Fraunhofer Diffraction Due to a Single Slit
[5]25.2

A point object 'O' is kept in a medium of refractive index n1 in front of a convex spherical surface of radius of curvature R which separates the second medium of refractive index n2 from the first one, as shown in the figure

Draw the ray diagram showing the image formation and deduce the relationship between the object distance and the image distance in terms of n1, n2 and R.

When the image formed above acts as a virtual object for a concave spherical surface separatig the medium n2 from n1 (n2 > n1), draw this ray diagram and write the similar (similar to (a)) relation. Hence obtain the expression for the lens maker's formula.

Chapter: [9] Ray Optics
Concept: Lensmaker's Formula
[5]26 | Attempt Any One
[5]26.1

An electric dipole of dipole momentvecp consists of point charges +q and −q separated by a distance 2a apart. Deduce the expression for the electric field vecE due to the dipole at a distance x from the centre of the dipole on its axial line in terms of the dipole moment vecp. Hence show that in the limit x>> a, vecE->2vecp"/"(4piepsilon_0x^3)

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

Given the electric field in the region vecE=2xhati, find the net electric flux through the cube and the charge enclosed by it.

Chapter: [1.01] Electric Charges and Fields
Concept: Electric Flux
[5]26.2

Distinguish with the help of a suitable diagram, the difference in the behaviour of a conductor and a dielectric placed in an external electric field.

Chapter: [12] Electrostatics
Concept: Dielectrics and Polarisation

Define the terms polarization of a dielectric and write its relation with susceptibility.

Chapter: [12] Electrostatics
Concept: Dielectrics and Polarisation

A thin metallic spherical shell of radius R carries a charge Q on its surface. A point chargeQ/2 is placed at its centre C and an other charge +2Q is placed outside the shell at a distance x from the centre as shown in the figure. Find (i) the force on the charge at the centre of shell and at the point A, (ii) the electric flux through the shell.

Chapter: [1.01] Electric Charges and Fields
Concept: Uniformly Charged Infinite Plane Sheet and Uniformly Charged Thin Spherical Shell (Field Inside and Outside)
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