# Physics All India Set 3 N 2015-2016 CBSE (Commerce) Class 12 Question Paper Solution

Physics [All India Set 3 N]
Date & Time: 5th March 2016, 10:30 am
Duration: 3h

[1] 1

A signal of 5 kHz frequency is amplitude modulated on a carrier wave of frequency 2 MHz. What are the frequencies of the side bands produced?

Concept: Amplitude Modulation
Chapter: [0.15] Communication Systems
[1] 2

What can be the causes of helical motion of a charged particle?

Concept: Solenoid and the Toroid - the Toroid
Chapter: [0.05] Moving Charges and Magnetism
[1] 3

Define mobility of a charge carrier

Concept: Conductors and Insulators Related to Electric Field
Chapter: [0.01] Electrostatic Potential and Capacitance

What is its relation with relaxation time?

Concept: Drift of Electrons and the Origin of Resistivity
Chapter: [0.03] Current Electricity
[1] 4

What is the amount of work done in moving a point charge Q around a circular arc of radius ‘r’ at the centre of which another point charge ‘q’ is located?

Concept: Electric Field Due to a Point Charge
Chapter: [0.02] Electric Charges and Fields
[1] 5

Why can’t we see clearly through fog?

Concept: Some Natural Phenomena Due to Sunlight
Chapter: [0.09] Ray Optics and Optical Instruments

Name the phenomenon responsible for it.

Concept: Some Natural Phenomena Due to Sunlight
Chapter: [0.09] Ray Optics and Optical Instruments
[2] 6 | Attempt Any One
[2] 6.1

When is Hα line in the emission spectrum of hydrogen atom obtained? Calculate the frequency of the photon emitted during this transition.

Concept: Hydrogen Spectrum
Chapter: [0.13] Atoms
[2] 6.2

Calculate the wavelength of radiation emitted when electron in a hydrogen atom jumps from n = oo to n = 1.

Chapter: [0.11] Dual Nature of Radiation and Matter
[2] 7

When 5 V potential difference is applied across a wire of length 0.1 m, the drift speed of electrons is 2.5 x 10-4 m/s. If the electron density in the wire is 8 x 1028 m-3, calculate the resistivity of the material of the wire.

Concept: Potential Difference and Emf of a Cell
Chapter: [0.03] Current Electricity
[2] 8

A proton and an α -particle are accelerated through the same potential difference. Which one of the two has greater de-Broglie wavelength Justify your answer.

Concept: de-Broglie Relation
Chapter: [0.11] Dual Nature of Radiation and Matter

A proton and an α-particle are accelerated through the same potential difference. Which one of the two has less kinetic energy? Justify your answer.

Concept: de-Broglie Relation
Chapter: [0.11] Dual Nature of Radiation and Matter
[2] 9

A ray PQ incident on the refracting face BA is refracted in the prism BAC as shown in the figure and emerges from the other refracting face AC as RS such that AQ = AR. If the angle of prism A = 60° and refractive index of material of prism is sqrt3 . Calculate angle θ.

Concept: Dispersion by a Prism
Chapter: [0.09] Ray Optics and Optical Instruments
[2] 10

Why is base band signal not transmitted directly? Give any two reasons

Concept: Need for Modulation and Demodulation
Chapter: [0.15] Communication Systems
[3] 11
[2] 11.1

A screen is placed at a distance of 100 cm from an object. The image of the object is formed on the screen by a convex lens for two different locations of the lens separated by 20 cm. Calculate the focal length of the lens used.

Concept: Lenses
Chapter: [0.09] Ray Optics and Optical Instruments
[1] 11.2

A converging lens is kept coaxially in contact with a diverging lens – both the lenses being of equal focal length. What is the focal length of the combination?

Concept: Refraction at Spherical Surfaces and Lenses - Combination of Thin Lenses in Contact
Chapter: [0.09] Ray Optics and Optical Instruments
[3] 12
[2] 12.1

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

Chapter: [0.12] Nuclei
[1] 12.2

How is the mean life of a given radioactive nucleus related to the decay constant?

Chapter: [0.12] Nuclei
[3] 13

What is space wave propagation?

Concept: Propagation of Electromagnetic Waves
Chapter: [0.15] Communication Systems

State the factors which limit its range of propagation

Concept: Propagation of Electromagnetic Waves
Chapter: [0.15] Communication Systems

Derive an expression for the maximum line of sight distance between two antennas for space wave propagation.

Concept: Propagation of Electromagnetic Waves
Chapter: [0.15] Communication Systems
[3] 14
[2] 14.1

Distinguish between a conductor and a semi conductor on the basis of energy band diagram

Concept: Energy Bands in Conductors, Semiconductors and Insulators
Chapter: [0.14] Semiconductor Electronics - Materials, Devices and Simple Circuits
[1] 14.2

The following figure shows the input waveforms (A, B) and the output waveform (Y) of a gate. Identify the gate, write its truth table and draw its logic symbol.

Concept: Digital Electronics and Logic Gates
Chapter: [0.14] Semiconductor Electronics - Materials, Devices and Simple Circuits
[3] 15
[1] 15.1

Name two important processes that occur during the formation of a pn junction

Concept: Special Purpose P-n Junction Diodes
Chapter: [0.14] Semiconductor Electronics - Materials, Devices and Simple Circuits
[2] 15.2

Draw the circuit diagram of a full wave rectifier along with the input and output waveforms

Concept: Diode as a Rectifier
Chapter: [0.14] Semiconductor Electronics - Materials, Devices and Simple Circuits

Briefly explain how the output voltage/current is unidirectional.

Concept: Application of Junction Diode as a Rectifier
Chapter: [0.14] Semiconductor Electronics - Materials, Devices and Simple Circuits
[3] 16

State two important properties of photon which are used to write Einstein’s photoelectric equation.

Concept: Einstein’s Equation - Particle Nature of Light
Chapter: [0.11] Dual Nature of Radiation and Matter

Define (i) stopping potential and (ii) threshold frequency, using Einstein’s equation and drawing necessary plot between relevant quantities.

Concept: Einstein’s Equation - Particle Nature of Light
Chapter: [0.11] Dual Nature of Radiation and Matter
[3] 17
[2] 17.1

Derive Snell’s law on the basis of Huygen’s wave theory when the light is travelling from a denser to a rarer medium.

Concept: Snell’s Law
Chapter: [0.09] Ray Optics and Optical Instruments
[1] 17.2

Draw the sketches to differentiate between plane wavefront and spherical wavefront.

Concept: Introduction of Wave Optics
Chapter: [0.1] Wave Optics
[3] 18

How are electromagnetic waves produced? What is the source of energy of these waves?

Concept: Electromagnetic Waves
Chapter: [0.08] Electromagnetic Waves

Write mathematical expressions for electric and magnetic fields of an electromagnetic wave propagating along the z-axis.

Concept: Electromagnetic Waves
Chapter: [0.08] Electromagnetic Waves

Write any two important properties of electromagnetic waves.

Concept: Electromagnetic Waves
Chapter: [0.08] Electromagnetic Waves
[3] 19
[2] 19.1

When an AC source is connected to an ideal capacitor, show that the average power supplied by the source over a complete cycle is zero

Concept: Capacitors and Capacitance
Chapter: [0.01] Electrostatic Potential and Capacitance
[1] 19.2

A bulb is connected in series with a variable capacitor and an AC source as shown. What happens to the brightness of the bulb when the key is plugged in and capacitance of the capacitor is gradually reduced?

Concept: Capacitors and Capacitance
Chapter: [0.01] Electrostatic Potential and Capacitance
[3] 20 | Attempt Any One
[3] 20.1

State Ampere’s circuital law

Concept: Ampere’s Circuital Law
Chapter: [0.05] Moving Charges and Magnetism

Use this law to find magnetic field due to straight infinite current carrying wire.

Concept: Magnetic Force
Chapter: [0.05] Moving Charges and Magnetism

How are the magnetic field lines different from the electrostatic field lines?

Concept: The Bar Magnet
Chapter: [0.04] Magnetism and Matter
[3] 20.2

State the principle of a cyclotron.

Concept: Motion in Combined Electric and Magnetic Fields - Cyclotron
Chapter: [0.05] Moving Charges and Magnetism

Show that the time period of revolution of particles in a cyclotron is independent of their speeds. Why is this property necessary for the operation of a cyclotron?

Concept: Motion in Combined Electric and Magnetic Fields - Cyclotron
Chapter: [0.05] Moving Charges and Magnetism
[3] 21

Two identical cells of emf 1.5 V each joined in parallel, supply energy to an external circuit consisting of two resistances of 7 Ω each joined in parallel. A very high resistance voltmeter reads the terminal voltage of cells to be 1.4 V. Calculate the internal resistance of each cell.

Concept: Cells, Emf, Internal Resistance
Chapter: [0.03] Current Electricity
[3] 22

Find the electric field intensity due to a uniformly charged spherical shell at a point (i) outside the shell. Plot the graph of electric field with distance from the centre of the shell.

Concept: Uniformly Charged Infinite Plane Sheet and Uniformly Charged Thin Spherical Shell (Field Inside and Outside)
Chapter: [0.02] Electric Charges and Fields

Find the electric field intensity due to a uniformly charged spherical shell at a point (ii) inside the shell. Plot the graph of electric field with distance from the centre of the shell.

Concept: Uniformly Charged Infinite Plane Sheet and Uniformly Charged Thin Spherical Shell (Field Inside and Outside)
Chapter: [0.02] Electric Charges and Fields
[4] 23

Seema’s uncle was advised by his doctor to have an MRI (Magnetic Resonance Imaging) scan of his brain. Her uncle felt it to be expensive and wanted to postpone it. When Seema learnt about this, she took the help of her family and also approached the doctor, who also offered a substantial discount. She then convinced her uncle to undergo the test to enable the doctor to know the condition of his brain. The information thus obtained greatly helped the doctor to treat him properly.

Based on the above paragraph, answer the following questions:

(a) What according to you are the values displayed by Seema, her family and the doctor?

(b) What could be the possible reason for MRI test to be so expensive?

(c) Assuming that MRI test was performed using a magnetic field of 0.1 T, find the minimum and maximum values of the force that the magnetic field could exert on a proton (charge = 1.6 x 10-19 C) moving with a speed of 104 m/s.

Concept: Magnetic Force
Chapter: [0.05] Moving Charges and Magnetism
[5] 24 | Attempt Any One
[5] 24.1

When a bar magnet is pushed towards (or away) from the coil connected to a galvanometer, the pointer in the galvanometer deflects. Identify the phenomenon causing this deflection and write the factors on which the amount and direction of the deflection depends. State the laws describing this phenomenon.

Concept: Electromagnetic Induction
Chapter: [0.07] Electromagnetic Induction [0.07] Electromagnetic Induction

Sketch the change in flux, emf and force when a conducting rod PQ of resistance R and length l moves freely to and fro between A and C with speed v on a rectangular conductor placed in uniform magnetic field as shown in the figure

Concept: Force on a Current - Carrying Conductor in a Uniform Magnetic Field
Chapter: [0.05] Moving Charges and Magnetism
[5] 24.2

In a series LCR circuit connected to an a.c. source of voltage v = vmsinωt, use phasor diagram to derive an expression for the current in the circuit. Hence, obtain the expression for the power dissipated in the circuit. Show that power dissipated at resonance is maximum

Concept: Ac Voltage Applied to a Series Lcr Circuit
Chapter: [0.06] Alternating Current
[5] 25 | Attempt Any One
[5] 25.1

Why does unpolarised light from a source show a variation in intensity when viewed through a polaroid which is rotated?

Concept: Some Natural Phenomena Due to Sunlight
Chapter: [0.09] Ray Optics and Optical Instruments

Show with the help of a diagram, how unpolarised light from Sun gets linearly polarised by scattering.

Concept: Some Natural Phenomena Due to Sunlight
Chapter: [0.09] Ray Optics and Optical Instruments

Three identical polaroid sheets P1, P2 and P3 are oriented so that the pass axis of P2 and P3 are inclined at angles of 60° and 90° respectively with the pass axis of P1. A monochromatic source S of unpolarised light of intensity I0 is kept in front of the polaroid sheet P1 as shown in the figure. Determine the intensities of light as observed by the observer at O, when polaroid P3 is rotated with respect to P2 at angles θ = 30° and 60°.

Concept: Polarisation
Chapter: [0.1] Wave Optics
[5] 25.2

Derive an expression for path difference in Young’s double slit experiment and obtain the conditions for constructive and destructive interference at a point on the screen.

Concept: Interference of Light Waves and Young’S Experiment
Chapter: [0.1] Wave Optics

The intensity at the central maxima in Young’s double slit experiment is I0. Find out the intensity at a point where the path difference is lambda/6,lambda/4 and lambda/3.

Concept: Interference of Light Waves and Young’S Experiment
Chapter: [0.1] Wave Optics
[5] 26 | Attempt Any One
[5] 26.1

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.

Concept: Dielectrics and Polarisation
Chapter: [0.01] Electrostatic Potential and Capacitance

A capacitor of capacitance C is charged fully by connecting it to a battery of emf E. It is then disconnected from the battery. If the separation between the plates of the capacitor is now doubled, how will the following change?

(i) charge stored by the capacitor.

(ii) Field strength between the plates.

(iii) Energy stored by the capacitor.

Concept: Capacitors and Capacitance
Chapter: [0.01] Electrostatic Potential and Capacitance

How does polarised dielectric modify the original external field?

Concept: Dielectrics and Polarisation
Chapter: [0.01] Electrostatic Potential and Capacitance
[5] 26.2

Explain why, for a charge configuration, the equipotential surface through a point is normal to the electric field at that point

Concept: Applications of Gauss’s Law
Chapter: [0.02] Electric Charges and Fields

Draw a sketch of equipotential surfaces due to a single charge (-q), depicting the electric field lines due to the charge

Concept: Equipotential Surfaces
Chapter: [0.01] Electrostatic Potential and Capacitance

Obtain an expression for the work done to dissociate the system of three charges placed at the vertices of an equilateral triangle of side ‘a’ as shown below.

Concept: Forces Between Multiple Charges
Chapter: [0.02] Electric Charges and Fields

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