# Physics Delhi Set 3 2016-2017 CBSE (Science) Class 12 Question Paper Solution

Physics [Delhi Set 3]
Date & Time: 14th March 2017, 12:30 pm
Duration: 3h

[1] 1

A long straight current carrying wire passes normally through the centre of circular loop. If the current through the wire increases, will there be an induced emf in the loop? Justify.

Concept: Cells, Emf, Internal Resistance
Chapter: [0.03] Current Electricity
[1] 2

How is the speed of em-waves in vacuum determined by the electric and magnetic field?

Concept: Electromagnetic Waves
Chapter: [0.08] Electromagnetic Waves
[1] 3

Name the junction diode whose I-V characteristics are drawn below:

Concept: V-I Characteristics (Linear and Non-linear)
Chapter: [0.03] Current Electricity
[1] 4

At a place, the horizontal component of earth's magnetic field is B and angle of dip is 60°. What is the value of horizontal component of the earth's magnetic field at equator?

Concept: Magnetic Field on the Axis of a Circular Current Loop
Chapter: [0.05] Moving Charges and Magnetism
[1] 5

Does the charge given to a metallic sphere depend on whether it is hollow or solid? Give reason for your answer.

Concept: Basic Properties of Electric Charge
Chapter: [0.02] Electric Charges and Fields
[2] 6 | Attempt any one
[2] 6.1

Define the distance of closest approach. An α-particle of kinetic energy 'K' is bombarded on a thin gold foil. The distance of the closest approach is 'r'. What will be the distance of closest approach for an α-particle of double the kinetic energy?

Concept: Alpha-particle Scattering and Rutherford’S Nuclear Model of Atom
Chapter: [0.13] Atoms
[2] 6.2

Write two important limitations of Rutherford nuclear model of the atom.

Concept: Alpha-particle Scattering and Rutherford’S Nuclear Model of Atom
Chapter: [0.13] Atoms
[2] 7

When are two objects just resolved? Explain.

Concept: Optical Instruments - The Microscope
Chapter: [0.09] Ray Optics and Optical Instruments

How can the resolving power of a compound microscope be increased? Use relevant formula to support your answer.

Concept: Optical Instruments - The Microscope
Chapter: [0.09] Ray Optics and Optical Instruments
[2] 8

What is the line of sight communication?

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

Why is it not possible to use sky waves for transmission of TV signals?

Concept: Satellite Communication
Chapter: [0.15] Communication Systems

Upto what distance can a signal be transmitted using an antenna of height 'h'?

Concept: Satellite Communication
Chapter: [0.15] Communication Systems
[2] 9

An α-particle and a proton are accelerated through the same potential difference. Find the ratio of their de Broglie wavelength.

Concept: de-Broglie Relation
Chapter: [0.11] Dual Nature of Radiation and Matter
[2] 10

How does Ampere-Maxwell law explain the flow of current through a capacitor when it is being charged by a battery?

Concept: Electric Charges
Chapter: [0.02] Electric Charges and Fields

Write the expression for the displacement current in terms of the rate of change of electric flux.

Concept: Electric Charges
Chapter: [0.02] Electric Charges and Fields
[3] 11 | Attempt any one
[3] 11.1

Write the underlying principle of a moving coil galvanometer.

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

Why is it necessary to introduce a radial magnetic field inside the coil of a galvanometer?

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

Why is it necessary to introduce a cylindrical soft iron core inside the coil of a galvanometer?

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

Obtain the expression for current sensitivity of moving coil galvanometer.

Concept: Moving Coil Galvanometer
Chapter: [0.05] Moving Charges and Magnetism
[3] 11.2

Define self-inductance of a coil.

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

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

Concept: Solenoid and the Toroid - the Solenoid
Chapter: [0.05] Moving Charges and Magnetism
[3] 12

A radioactive nucleus 'A' undergoes a series of decays as given below:

The mass number and atomic number of A2 are 176 and 71 respectively. Determine the mass and atomic numbers of A4 and A.

Chapter: [0.12] Nuclei

Write the basic nuclear process underlying β+ and β decays.

Chapter: [0.12] Nuclei
[3] 13

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] 14

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.

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

Radiation of frequency 1015 Hz is incident on two photosensitive surface P and Q. There is no photoemission from surface P. Photoemission occurs from surface Q but photoelectrons have zero kinetic energy. Explain these observations and find the value of work function for surface Q.

Concept: Einstein’s Equation - Particle Nature of Light
Chapter: [0.11] Dual Nature of Radiation and Matter
[3] 15

(i) Find the value of the phase difference between the current and the voltage in the series LCR circuit shown below. Which one leads in phase : current or voltage ?

(ii) Without making any other change, find the value of the additional capacitor C1, to be connected in parallel with the capacitor C, in order to make the power factor of the circuit unity.

Concept: Ac Voltage Applied to a Series Lcr Circuit
Chapter: [0.06] Alternating Current
[3] 16

(i)Obtain the expression for the torque vecτ experienced by an electric dipole of dipole moment vecP in a uniform electric field, vecE .

(ii) What will happen if the field were not uniform?

Concept: Electric Dipole
Chapter: [0.02] Electric Charges and Fields
[3] 17

Explain briefly with the help of necessary diagrams, the forward biasing of a p-n junction diode. Also draw characteristic curves.

Concept: p-n Junction
Chapter: [0.14] Semiconductor Electronics - Materials, Devices and Simple Circuits

Explain briefly with the help of necessary diagrams, the  reverse biasing of a p-n junction diode. Also draw characteristic curves.

Concept: p-n Junction
Chapter: [0.14] Semiconductor Electronics - Materials, Devices and Simple Circuits
[3] 18

A ray of light passing from air through an equilateral glass prism undergoes minimum deviation when the angle of incidence is 3/4 th of the angle of prism. Calculate the speed of light in the prism.

Concept: Refraction Through a Prism
Chapter: [0.09] Ray Optics and Optical Instruments
[3] 19

Define the term 'amplitude modulation'

Concept: Amplitude Modulation
Chapter: [0.15] Communication Systems

Explain any two factors which justify the need for modulating a low frequency base-band signal.

Concept: Amplitude Modulation
Chapter: [0.15] Communication Systems
[3] 20

Two identical capacitors of 12 pF each are connected in series across a battery of 50 V. How much electrostatic energy is stored in the combination? If these were connected in parallel across the same battery, how much energy will be stored in the combination now?

Also find the charge drawn from the battery in each case.

Concept: Capacitors and Capacitance
Chapter: [0.01] Electrostatic Potential and Capacitance
[3] 21

Write the expression for the force vecF acting on a particle of mass m and charge q moving with velocity vecV in a magnetic field vecB , Under what conditions will it move in (i) a circular path and (ii) a helical path?

Concept: Force on a Moving Charge in Uniform Magnetic and Electric Fields
Chapter: [0.05] Moving Charges and Magnetism

Show that the kinetic energy of the particle moving in a magnetic field remains constant.

Concept: Force on a Moving Charge in Uniform Magnetic and Electric Fields
Chapter: [0.05] Moving Charges and Magnetism
[3] 22

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.

Concept: Metre Bridge
Chapter: [0.03] Current Electricity
[4] 23

Mrs. Rashmi Singh broke her reading glasses. When she went to the shopkeeper to order new spects, he suggested that she should get spectacles with plastic lenses instead of glass lenses. On getting the new spectacles, she found that the new ones were thicker than the earlier ones. She asked this question to the shopkeeper but he could not offer satisfactory explanation for this. At home, Mrs. Singh raised the same question to her daughter Anuja who explained why plastic lenses were thicker.

(a) Write two qualities displayed each by Anuja and her mother.

(b) How do you explain this fact using lens maker's formula?

Concept: Lensmaker's Formula
Chapter: [0.09] Ray Optics and Optical Instruments
[5] 24 | Attempt any one
[5] 24.1

Derive an expression for  drift velocity of free electrons.

Concept: Drift of Electrons and the Origin of Resistivity
Chapter: [0.03] Current Electricity
A wire whose cross-sectional area is increasing linearly from its one end to the other, is connected across a battery of V volts.
Which of the following quantities remain constant in the wire?
(a) drift speed
(b) current density
(c) electric current
(d) electric field
Concept: Drift of Electrons and the Origin of Resistivity
Chapter: [0.03] Current Electricity
[5] 24.2

State the two Kirchhoff’s rules used in electric networks. How are there rules justified?

Concept: Kirchhoff’s Rules
Chapter: [0.03] Current Electricity

The current is drawn from a cell of emf E and internal resistance r connected to the network of resistors each of resistance r as shown in the figure. Obtain the expression for

1. the current draw from the cell and
2. the power consumed in the network.

Concept: Kirchhoff’s Rules
Chapter: [0.03] Current Electricity
[5] 25 | Attempt any one
[5] 25.1

Draw a labelled diagram of an ac generator.

Concept: Ac Generator
Chapter: [0.07] Electromagnetic Induction

Obtain the expression for the emf induced in the rotating coil of N turns each of cross-sectional area A, in the presence of a magnetic field vecB .

Concept: Ac Generator
Chapter: [0.07] Electromagnetic Induction
[5] 25.2

Draw a necessary arrangement for winding of primary and secondary coils in a step-up transformer. State its underlying principle and derive the relation between the primary and secondary voltages in terms of number of primary and secondary turns. Mention the two basic assumptions used in obtaining the above relation.

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

A power transmission line feeds input power at 2200 V to a step-down transformer with its primary windings having 300 turns. Find the number of turns in the secondary to get the power output at 220 V.

Concept: Transformers
Chapter: [0.06] Alternating Current
[5] 26 | Attempt any one
[5] 26.1

Distinguish between unpolarized light and linearly polarized light

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

Why does an unpolarised light incident on a polaroid get linearly polarised ?

Concept: Polarisation
Chapter: [0.1] Wave Optics

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.

Concept: Polarisation
Chapter: [0.1] Wave Optics
[5] 26.2

Explain two features to distinguish between the interference pattern in Young's double slit experiment with the diffraction pattern obtained due to a single slit.

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

A monochromatic light of wavelength 500 nm is incident normally on a single slit of width 0.2 mm to produce a diffraction pattern. Find the angular width of the central maximum obtained on the screen.

Estimate the number of fringes obtained in Young's double slit experiment with fringe width 0.5 mm, which can be accommodated within the region of total angular spread of the central maximum due to single slit.

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

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