# Physics All India Set 2 C 2015-2016 Science (English Medium) Class 12 Question Paper Solution

Physics [All India Set 2 C]
Marks: 70 Academic Year: 2015-2016
Date & Time: 5th March 2016, 10:30 am
Duration: 3h

[1] 1

In what way is the behaviour of a diamagnetic material different from that of a paramagnetic, when kept in an external magnetic field?

Concept: Magnetic Properties of Materials
Chapter: [0.04] Magnetism and Matter
[1] 2

The plot of the variation of potential difference across a combination of three identical cells in series, versus current is shown below. What is the emf and internal resistance of each cell ?

Concept: Cells, Emf, Internal Resistance
Chapter: [0.03] Current Electricity
[1] 3

Why does sun appear red at sunrise and sunset?

Concept: Applications of Scattering of Light
Chapter:
[1] 4

A charge 'q' is moved from a point A above a dipole of dipole moment 'p' to a point B below the dipole in equatorial plane without acceleration. Find the work done in the process.

Concept: Magnetic Dipole Moment of a Revolving Electron
Chapter: [0.04] Magnetism and Matter
[1] 5

Name the essential components of a communication system.

Concept: Elements of a Communication System
Chapter: [0.15] Communication Systems
[2] 6

Calculate the de-Broglie wavelength of the electron orbitting in the n = 2 state of hydrogen atom.

Concept: Heisenberg and De Broglie Hypothesis
Chapter: [0.13] Atoms
[2] 7

A battery of emf 12 V and internal resistance 2 Ω is connected to a 4 Ω resistor as shown in the figure.

(a) Show that a voltmeter when placed across the cell and across the resistor, in turn, gives the same reading.

(b) To record the voltage and the current in the circuit, why is voltmeter placed in parallel and ammeter in series in the circuit?

Concept: Cells, Emf, Internal Resistance
Chapter: [0.03] Current Electricity
[2] 8 | Attempt Any One
[2] 8.1

Define ionization energy.

Concept: De Broglie’s Explanation of Bohr’s Second Postulate of Quantisation
Chapter: [0.13] Atoms

How would the ionization energy change when the electron in the hydrogen atom is replaced by a particle of mass 200 times that of the electron but having the same charge?

Concept: De Broglie’s Explanation of Bohr’s Second Postulate of Quantisation
Chapter: [0.13] Atoms
[2] 8.2

Calculate the shortest wavelength of the spectral lines emitted in Balmer series.

[Given Rydberg constant, R = 107 m–1]

Concept: De Broglie’s Explanation of Bohr’s Second Postulate of Quantisation
Chapter: [0.13] Atoms
[2] 9

Define modulation index.

Concept: Modulation and Its Necessity
Chapter: [0.15] Communication Systems

Why is Modulation index kept low?

Concept: Modulation and Its Necessity
Chapter: [0.15] Communication Systems

What is the role of a bandpass filter?

Concept: Modulation and Its Necessity
Chapter: [0.15] Communication Systems
[2] 10

A ray PQ incident normally on the refracting face BA is refracted in the prism BAC made of material of refractive index 1.5. Complete the path of ray through the prism. From which face will the ray emerge? Justify your answer.

Concept: Dispersion by a Prism
Chapter: [0.09] Ray Optics and Optical Instruments
[3] 11

Derive an expression for  drift velocity of free electrons.

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

How does drift velocity of electrons in a metallic conductor vary with increase in temperature? Explain.

Concept: Drift of Electrons and the Origin of Resistivity
Chapter: [0.03] Current Electricity
[3] 12

(a) Write the basic nuclear process involved in the emission of β+ in a symbolic form, by a radioactive nucleus.

(b) In the reactions given below:

(i)""_16^11C->_y^zB+x+v

(ii)""_6^12C+_6^12C->_a^20 Ne + _b^c He

Find the values of x, y, and z and a, b and c.

Concept: Law of Radioactive Decay
Chapter: [0.12] Nuclei
[3] 13

Sketch the graphs showing variation of stopping potential with frequency of incident radiations for two photosensitive materials A and B having threshold frequencies vA > vB.

(i) In which case is the stopping potential more and why?

(ii) Does the slope of the graph depend on the nature of the material used? Explain.

Concept: Photoelectric Effect - Hertz’s Observations
Chapter: [0.11] Dual Nature of Radiation and Matter
[3] 14

State law of Malus.

Concept: Law of Malus
Chapter: [0.1] Wave Optics

Draw a graph showing the variation of intensity (I) of polarised light transmitted by an analyzer with the angle (θ) between polariser and analyzer.

Concept: Plane Polarised Light
Chapter: [0.1] Wave Optics

What is the value of refractive index of a medium of polarising angle 60°?

Concept: Polarisation
Chapter: [0.1] Wave Optics
[3] 15

Define an equipotential surface.

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

Draw equipotential surfaces:

(1) in the case of a single point charge and

(2) in a constant electric field in Z-direction. Why are the equipotential surfaces about a single charge not equidistant?

(3) Can electric field exist tangential to an equipotential surface? Give reason

Concept: Equipotential Surfaces
Chapter: [0.01] Electrostatic Potential and Capacitance
[3] 16

Use Biot-Savart law to derive the expression for the magnetic field on the axis of a current carrying circular loop of radius R.

Draw the magnetic field lines due to a circular wire carrying current I.

Concept: Magnetic Field on the Axis of a Circular Current Loop
Chapter: [0.05] Moving Charges and Magnetism
[3] 17 | Attempt Any One
[3] 17.1

Define a wavefront.

Concept: Huygens' Principle
Chapter: [0.1] Wave Optics

State Huygen's principle.

Concept: Huygens' Principle
Chapter: [0.1] Wave Optics

Consider a plane wave front incident on a thin convex lens. Draw a proper diagram to show how the incident wave front traverses through the lens and after refraction focusses on the focal point of the lens, giving the shape of the emergent wave front.

Concept: Huygens' Principle
Chapter: [0.1] Wave Optics
[3] 17.2 | Explain the following, giving reasons:

When monochromatic light is incident on a surface separating two media, the reflected and refracted light both have the same frequency as the incident frequency.

Concept: Refraction of Monochromatic Light
Chapter: [0.1] Wave Optics

When light travels from a rarer to a denser medium, the speed decreases. Does this decrease in speed imply a reduction in the energy carried by the wave?

Concept: Refraction of Monochromatic Light
Chapter: [0.1] Wave Optics

In the wave picture of light, the intensity of light is determined by the square of the amplitude of the wave. What determines the intensity in the photon picture of light?

Concept: Refraction of Monochromatic Light
Chapter: [0.1] Wave Optics
[3] 18

For a CE-transistor amplifier, the audio signal voltage across the collector resistance of 2 kΩ is 2 V. Suppose the current amplification factor of the transistor is 100, find the input signal voltage and base current if the base resistance is 1 kΩ.

Concept: Junction Transistor - Transistor as an Amplifier (Ce-configuration)
Chapter: [0.14] Semiconductor Electronics - Materials, Devices and Simple Circuits
[3] 19

Why does a galvanometer show a momentary deflection at the time of charging or discharging a capacitor? Write the necessary expression to explain this observation.

Concept: Moving Coil Galvanometer
Chapter: [0.05] Moving Charges and Magnetism

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

Concept: Electromagnetic Spectrum
Chapter: [0.08] Electromagnetic Waves

Identify the part of the electromagnetic spectrum which is:

produced by bombarding a metal target by high speed electrons.

Concept: Electromagnetic Spectrum
Chapter: [0.08] Electromagnetic Waves
[3] 20

(i) Which mode of propagation is used by shortwave broadcast services having frequency range from a few MHz upto 30 MHz? Explain diagrammatically how long distance communication can be achieved by this mode.

(ii) Why is there an upper limit to frequency of waves used in this mode?

Concept: Propagation of Electromagnetic Waves
Chapter: [0.15] Communication Systems
[3] 21

(i) Explain with the help of a diagram the formation of depletion region and barrier potential in a pn junction.

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

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

Concept: Application of Junction Diode as a Rectifier
Chapter: [0.14] Semiconductor Electronics - Materials, Devices and Simple Circuits
[3] 22

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

Concept: Power in AC Circuit: the Power Factor
Chapter: [0.06] Alternating Current

A lamp is connected in series with an inductor and an AC source. What happens to the brightness of the lamp when the key is plugged in and an iron rod is inserted inside the inductor? Explain.

Concept: Power in AC Circuit: the Power Factor
Chapter: [0.06] Alternating Current
[4] 23

Ram is a student of class X in a village school. His uncle gifted him a bicycle with a dynamo fitted in it. He was very excited to get it. While cycling during night, he could light the bulb and see the objects on the road. He, however, did not know how this device works. he asked this question to his teacher. The teacher considered it an opportunity to explain the working to the whole class.

Answer the following questions:
(a) State the principle and working of a dynamo.

(b) Write two values each displayed by Ram and his school teacher.

Concept: Magnetic Flux
Chapter: [0.07] Electromagnetic Induction
[5] 24 | Attempt Any One
[5] 24.1

(i) Derive the mathematical relation between refractive indices n1 and n2 of two radii and radius of curvature R for refraction at a convex spherical surface. Consider the object to be a point since lying on the principle axis in rarer medium of refractive index n1 and a real image formed in the denser medium of refractive index n2. Hence, derive lens maker's formula.

(ii) Light from a point source in air falls on a convex spherical glass surface of refractive index 1.5 and radius of curvature 20 cm. The distance of light source from the glass surface is 100 cm. At what position is the image formed?

Concept: Lensmaker's Formula
Chapter: [0.09] Ray Optics and Optical Instruments
[5] 24.2

Draw a labeled ray diagram to obtain the real image formed by an astronomical telescope in normal adjustment position. Define its magnifying power.

Concept: Optical Instruments: Telescope
Chapter: [0.09] Ray Optics and Optical Instruments

You are given three lenses of power 0.5 D, 4 D, and 10 D to design a telescope.

1) Which lenses should be used as objective and eyepiece? Justify your answer.

2) Why is the aperture of the objective preferred to be large?

Concept: Optical Instruments: Telescope
Chapter: [0.09] Ray Optics and Optical Instruments
[5] 25 | Attempt Any One
[5] 25.1

Use Gauss's law to find the electric field due to a uniformly charged infinite plane sheet. What is the direction of field for positive and negative charge densities?

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 ratio of the potential differences that must be applied across the parallel and series combination of two capacitors C1 and C2 with their capacitances in the ratio 1 : 2 so that the energy stored in the two cases becomes the same.

Concept: Uniformly Charged Infinite Plane Sheet and Uniformly Charged Thin Spherical Shell (Field Inside and Outside)
Chapter: [0.02] Electric Charges and Fields
[5] 25.2

(i) If two similar large plates, each of area A having surface charge densities +σ and –σ are separated by a distance d in air, find the expressions for

(a) field at points between the two plates and on outer side of the plates. Specify the direction of the field in each case.

(b) the potential difference between the plates.

(c) the capacitance of the capacitor so formed.

(ii) Two metallic spheres of Radii R and 2R are charged so that both of these have same surface charge density σ. If they are connected to each other with a conducting wire, inn which direction will the charge flow and why?

Concept: Electric Field - Introduction of Electric Field
Chapter: [0.02] Electric Charges and Fields
[5] 26 | Attempt Any One
[5] 26.1

Draw a labeled diagram of a step-down transformer.

Concept: Transformers
Chapter: [0.06] Alternating Current

State the principle of the step-down transformer and its working.

Concept: Transformers
Chapter: [0.06] Alternating Current

Express the turn ratio in terms of voltages.

Concept: Transformers
Chapter: [0.06] Alternating Current

Find the ratio of primary and secondary currents in terms of turn ratio in an ideal transformer.

Concept: Transformers
Chapter: [0.06] Alternating Current

How much current is drawn by the primary of a transformer connected to 220 V supply when it delivers power to a 110 V − 550 W refrigerator?

Concept: Transformers
Chapter: [0.06] Alternating Current
[5] 26.2

Explain the meaning of the term mutual inductance.

Concept: Inductance - Mutual Inductance
Chapter: [0.07] Electromagnetic Induction

Consider two concentric circular coils, one of radius r1 and the other of radius r2 (r1 < r2) placed coaxially with centers coinciding with each other. Obtain the expression for the mutual inductance of the arrangement.

Concept: Inductance - Mutual Inductance
Chapter: [0.07] Electromagnetic Induction

A rectangular coil of area A, having the number of turns N is rotated at 'f' revolutions per second in a uniform magnetic field B, the field being perpendicular to the coil. Prove that the maximum emf induced in the coil is 2 πf NBA.

Concept: Induced e.m.f. and Induced Current
Chapter: [0.07] Electromagnetic Induction

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