# Physics Foreign Set 2 2015-2016 CBSE (Commerce) Class 12 Question Paper Solution

Physics [Foreign Set 2]
Date & Time: 5th March 2016, 10:30 am
Duration: 3h

[1] 1

The charging current for a capacitor is 0.25 A.  What is the displacement current across its plates?

Concept: Displacement Current
Chapter: [0.08] Electromagnetic Waves
[1] 2

Give one example of use of eddy currents.

Concept: Eddy Currents
Chapter: [0.07] Electromagnetic Induction
[1] 3

Define Electric Flux. Write its SI unit.

Concept: Electric Flux
Chapter: [0.02] Electric Charges and Fields
[1] 4

A point charge Q is placed at point O as shown in the figure. The potential difference VA – VB positive. Is the charge Q negative or positive?

Concept: Potential Energy of a System of Charges
Chapter: [0.01] Electrostatic Potential and Capacitance
[1] 5

Depict the behaviour of magnetic field lines in the presence of a diamagnetic material?

Concept: Motion in a Magnetic Field
Chapter: [0.05] Moving Charges and Magnetism
[2] 6

The wavelength λ of a photon and the de-Broglie wavelength of an electron have the same value. Show that energy of a photon in (2λmc/h) times the kinetic energy of electron; where m, c and h have their usual meaning.

Concept: de-Broglie Relation
Chapter: [0.11] Dual Nature of Radiation and Matter
[2] 7

Distinguish between polarized and unpolarized light. Does the intensity of polarized light emitted by a polaroid depend on its orientation? Explain briefly.
The vibration in a beam of polarized light make an angle of 60° with the axis of the polaroid sheet. What percentage of light is transmitted through the sheet?

Concept: Plane Polarised Light
Chapter: [0.1] Wave Optics
[2] 8

A metal rod of square cross-sectional area A having length l has current I flowing through it when a potential difference of V volt is applied across its ends (figure I). Now the rod is cut parallel to its length into two identical pieces and joined as shown in figure II. What potential difference must be maintained across the length of 2l. so that the current in the rod is still I?

Concept: Electric Potential Difference
Chapter: [0.01] Electrostatic Potential and Capacitance
[2] 9 | Attempt any one of the following.
[2] 9.1
[1] 9.1.1

State Bohr postulate of hydrogen atom that gives the relationship for the frequency of emitted photon in a transition.

Concept: Bohr’s Model for Hydrogen Atom
Chapter: [0.13] Atoms
[1] 9.1.2

An electron jumps from fourth to first orbit in an atom. How many maximum number of spectral lines can be emitted by the atom? To which series these lines correspond?

Concept: Atomic Spectra
Chapter: [0.13] Atoms
[2] 9.2

Use de-Broglie's hypothesis to write the relation for the nth radius of Bohr orbit in terms of Bohr's quantization condition of orbital angular momentum ?

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

A device X used in communication system can convert one form of energy into another. Name the device X. Explain the function of a repeater in a communication system.

Concept: Basic Terminology Used in Electronic Communication Systems
Chapter: [0.15] Communication Systems
[3] 11 | Attempt any one of the following.
[3] 11.1

Three rays (1, 2, 3) of different colours fall normally on one of the sides of an isosceles right angled prism as shown. The refractive index of prism for these rays is 1.39, 1.47 and 1.52 respectively. Find which of these rays get internally reflected and which get only refracted from AC. Trace the paths of rays. Justify your answer with the help of necessary calculations.

Concept: Refraction Through a Prism
Chapter: [0.09] Ray Optics and Optical Instruments
[3] 11.2

Describe an activity to show that the colours of white light splitted by a glass prism can be recombined to get white light by another identical glass prism. Also, draw a ray diagram to show the recombination of the spectrum of white light.

Concept: Dispersion by a Prism
Chapter: [0.09] Ray Optics and Optical Instruments
[3] 12 | Attempt any one of the following.
[2] 12.1

Describe the working principle of a solar cell. Mention three basic processes involved in the generation of emf.

Concept: Potential Difference and Emf of a Cell
Chapter: [0.03] Current Electricity
[1] 12.2

Why are Si and GaAs preferred materials for solar cells?

Concept: Potential Difference and Emf of a Cell
Chapter: [0.03] Current Electricity
[3] 13

In the following arrangement of capacitors, the energy stored in the 6 µF capacitor is E. Find the value of the following :
(i) Energy stored in 12 µF capacitor.
(ii) Energy stored in 3 µF capacitor.
(iii) Total energy drawn from the battery.

Concept: Energy Stored in a Capacitor
Chapter: [0.01] Electrostatic Potential and Capacitance
[3] 14 | Attempt any one of the following.
[1] 14.1

Define 'activity' of a radioactive substance ?

Chapter: [0.12] Nuclei
[2] 14.2

Two different radioactive elements with half lives T1 and T2 have N1 and N2 undecayed atoms respectively present at a given instant. Derive an expression for the ratio of their activities at this instant in terms of N1 and N2 ?

Chapter: [0.12] Nuclei
[3] 15 | Attempt any one of the following.
[2] 15.1

Plot a graph showing the variation of photoelectric current with intensity of light. The work function for the following metals is given:
Na: 2.75 eV and Mo : 4.175 eV.
Which of these will not give photoelectron emission from a radiation of wavelength 3300

$A^\circ$ from a laser beam? What happens if the source of laser beam is brought closer?

Concept: Photoelectric Effect - Hallwachs’ and Lenard’s Observations
Chapter: [0.11] Dual Nature of Radiation and Matter
[1] 15.2

Define the term "cut off frequency" in photoelectric emission. The threshod frequency of a metal is f. When the light of frequency 2f is incident on the metal plate, the maximum velocity of photo-electrons is v1. When the frequency of the incident radiation is increased to 5f, the maximum velocity of phto-electrons is v2. Find the ratio v1 : v2.

Concept: Photoelectric Effect and Wave Theory of Light
Chapter: [0.11] Dual Nature of Radiation and Matter
[3] 16 | Attempt any one of the following.
[1] 16.1

Distinguish between point to point and broadcast modes of communication. Give an example of each.

Concept: Basic Terminology Used in Electronic Communication Systems
Chapter: [0.15] Communication Systems
[2] 16.2

Explain the basic concept of mobile telephoning.

Concept: Elements of a Communication System
Chapter: [0.15] Communication Systems
[3] 17

Two identical coils P and Q each of radius R are lying in perpendicular planes such that they have a common centre. Find the magnitude and direction of the magnetic field at the common centre of the two coils, if they carry currents equal to I and $\sqrt{3}$ I respectively.

Concept: Motion in a Magnetic Field
Chapter: [0.05] Moving Charges and Magnetism
[3] 18 | Attempt any one of the following.
[1] 18.1

Define self-inductance. Write its SI units.

Concept: Inductance - Self Inductance
Chapter: [0.07] Electromagnetic Induction
[2] 18.2

A long solenoid with 15 turns per cm has a small loop of area 2.0 cmplaced inside the solenoid normal to its axis. If the current carried by the solenoid changes steadily from 2.0 A to 4.0 A in 0.1 s, what is the induced emf in the loop while the current is changing?

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

(a) Define torque acting on a dipole of dipole moment $\vec{p}$ placed in a uniform electric field $\vec{E}$ Express it in the vector from and point out the direction along which it acts.

Express it in the vector from and point out the direction along which it acts.
(b) What happens if the field is non-uniform?
(c) What would happen if the external field
$\vec{E}$  is increasing (i) parallel to $\vec{p}$ and (ii) anti-parallel to $\vec{p}$ ?
Concept: Electric Dipole
Chapter: [0.02] Electric Charges and Fields
[3] 19

(a) Define torque acting on a dipole of dipole moment $\vec{p}$ placed in a uniform electric field $\vec{E}$ Express it in the vector from and point out the direction along which it acts.

Express it in the vector from and point out the direction along which it acts.
(b) What happens if the field is non-uniform?
(c) What would happen if the external field
$\vec{E}$  is increasing (i) parallel to $\vec{p}$ and (ii) anti-parallel to $\vec{p}$ ?
Concept: Electric Dipole
Chapter: [0.02] Electric Charges and Fields
[3] 20 | Attempt any one of the following.
[1] 20.1

A point charge q moving with speed v enters a uniform magnetic field B that is acting into the plane of the paper as shown. What is the path followed by the charge q and in which plane does it move?

Concept: Motion in a Magnetic Field
Chapter: [0.05] Moving Charges and Magnetism
[1] 20.2

How does the path followed by the charge get affected if its velocity has a component parallel to $\vec{B}$ .

Concept: Motion in Combined Electric and Magnetic Fields - Velocity Selector
Chapter: [0.05] Moving Charges and Magnetism
[1] 20.3

If an electric field $\vec{E}$ is also applied such that the particle continues moving along the original straight line path, what should be the magnitude and direction of the electric field $\vec{E}$ ?

Concept: Motion in a Magnetic Field
Chapter: [0.05] Moving Charges and Magnetism
[3] 21 | Attempt any one of the following.
[1] 21.1

Which segment of electromagnetic waves has highest frequency? How are these waves produced? Give one use of these waves.

Concept: Electromagnetic Waves
Chapter: [0.08] Electromagnetic Waves
[2] 21.2

Which em waves lie near the high frequency end of visible part of em spectrum? Give its one use. In what way this component of light has harmful effects on humans?

Concept: Electromagnetic Waves
Chapter: [0.08] Electromagnetic Waves
[3] 22

In the following diagram, an object 'O' is placed 15 cm in front of a convex lens L1 of focal length 20 cm and the final image is formed at I at a distance of 80 cm from the second lens L2. Find the focal length of the lens L2.

Concept: Thin Lens Formula
Chapter: [0.09] Ray Optics and Optical Instruments
[4] 23

Gautam went for a vacation to the village where his grandmother lived. His grandmother took him to watch 'nautanki' one evening. They noticed a blackbox connected to the mike lying nearby. Gautam's grandmother did not know what that box was. When she asked this question to Gautam, he explained to her that it was an amplifier.
(i) Which values were displayed by the grandmother? How can inculcation of these values in students be promoted?
(ii) What is the function of an amplifier?
(iii) Which basic electronic device is used in the amplifier?

Concept: Junction Transistor - Transistor as an Amplifier (Ce-configuration)
Chapter: [0.14] Semiconductor Electronics - Materials, Devices and Simple Circuits
[5] 24 | Attempt any one of the following.
[3] 24.1

A 2 µF capacitor, 100 Ω resistor and 8 H inductor are connected in series with an AC source.
(i) What should be the frequency of the source such that current drawn in the circuit is maximum? What is this frequency called?
(ii) If the peak value of e.m.f. of the source is 200 V, find the maximum current.
(iii) Draw a graph showing variation of amplitude of circuit current with changing frequency of applied voltage in a series LRC circuit for two different values of resistance R1 and R2 (R1 > R2).
(iv) Define the term 'Sharpness of Resonance'. Under what condition, does a circuit become more selective?

Concept: AC Voltage Applied to a Capacitor
Chapter: [0.06] Alternating Current
[5] 24.2 | Attempt any one of the following.
[3] 24.2.1

With the help of a neat and labelled diagram, explain the principle and working of a moving coil galvanometer ?

Concept: Moving Coil Galvanometer
Chapter: [0.05] Moving Charges and Magnetism
[1] 24.2.2

What is the function of uniform radial field and how is it produced?

Concept: Electric Field Due to a Point Charge
Chapter: [0.02] Electric Charges and Fields
[1] 24.2.3

Define current sensitivity of a galvanometer. How is current sensitivity increased?

Concept: Moving Coil Galvanometer
Chapter: [0.05] Moving Charges and Magnetism
[5] 25 | Attempt any one of the following.
[5] 25.1
[2] 25.1.1

In the circuit diagram given below, AB is a uniform wire of resistance 15 Ω and length 1 m. It is connected to a cell E1 of emf 2V and negligible internal resistance and a resistance R. The balance point with another cell E2 of emf 75 mV is found at 30 cm from end A. Calculate the value of R.

Concept: Electric Potential Difference
Chapter: [0.01] Electrostatic Potential and Capacitance
[1] 25.1.2

Why is potentiometer preferred over a voltmeter for comparison of emf. of cells?

Concept: Potential Difference and Emf of a Cell
Chapter: [0.03] Current Electricity
[2] 25.1.3

Draw a circuit diagram to determine internal resistance of a cell in the laboratory?

Concept: Measurement of Internal Resistance of a Cell
Chapter: [0.03] Current Electricity
[5] 25.2
[2] 25.2.1

Plot a graph showing variation of voltage vs the current drawn from the cell. How can one get information from this plot about the emf of the cell and its internal resistance?

Concept: Cells, Emf, Internal Resistance
Chapter: [0.03] Current Electricity
[3] 25.2.2

Two cells of emf E1 and E2 and internal resistances r1 and r2 are connected in parallel. Derive the expression for the (i) emf and (ii) internal resistance of a single equivalent cell which can replace this combination.

Concept: Induced e.m.f. and Induced Current
Chapter: [0.07] Electromagnetic Induction
[5] 26 | Attempt any one of the following.
[5] 26.1

State the essential conditions for diffraction of light ?

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

(i) State the essential conditions for diffraction of light.
(ii) Explain diffraction of light due to a narrow single slit and the formation of pattern of fringes on the screen.
(iii) Find the relation for width of central maximum in terms of wavelength 'λ', width of slit 'a', and separation between slit and screen 'D'.
(iv) If the width of the slit is made double the original width, how does it affect the size and intensity of the central band?

Concept: Diffraction of Light - Seeing the Single Slit Diffraction Pattern
Chapter: [0.1] Wave Optics
[5] 26.2
[2] 26.2.1

Draw a labelled schematic ray diagram of astronomical telescope in normal adjustment.

Concept: Diffraction of Light - Resolving Power of Microscope and Astronomical Telescope
Chapter: [0.1] Wave Optics
[2] 26.2.2

Which two aberrations do objectives of refracting telescope suffer from? How are these overcome in reflecting telescope?

Concept: Diffraction of Light - Resolving Power of Microscope and Astronomical Telescope
Chapter: [0.1] Wave Optics
[1] 26.2.3

How does the resolving power of a telescope change on increasing the aperture of the objective lens? Justify your answer.

Concept: Diffraction of Light - Resolving Power of Microscope and Astronomical Telescope
Chapter: [0.1] Wave Optics

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