# Physics Foreign Set 1 2011-2012 Science (English Medium) Class 12 Question Paper Solution

Physics [Foreign Set 1]
Date: March 2012

[1] 1

Why is the potential inside a hollow spherical charged conductor constant and has the same value of as on its surface?

Concept: Electric Field - Introduction of Electric Field
Chapter: [0.02] Electric Charges and Fields
[1] 2

A magnetic needle, free to rotate in a vertical plane, orients itself vertically at a certain place on the Earth. What are the values of (i) Horizontal component of Earth’s magnetic field and (ii) angle of dip at this place?

Concept: The Earth’s Magnetism
Chapter: [0.04] Magnetism and Matter
[1] 3

The closed loop (PQRS) of wire is moved into a uniform magnetic field at right angles to the plane of the paper as shown in the figure, Predict the direction of the induced current in the loop.

Chapter: [0.07] Electromagnetic Induction
[1] 4

Name the electromagnetic waves which (i) maintain the Earth’s warmth and (ii) are used in aircraft navigation.

Concept: The Earth’s Magnetism
Chapter: [0.04] Magnetism and Matter
[1] 5

How does focal length of a lens change when red light incident on it is replaced by violet light? Give reason for your answer.

Concept: Power of a Lens
Chapter: [0.09] Ray Optics and Optical Instruments
[1] 6

Write the relationship between the size of a nucleus and its mass number (A)?

Concept: Mass-energy and Nuclear Binding Energy - Mass - Energy
Chapter: [0.12] Nuclei
[1] 7

Show on a graph the variation of the de Broglie wavelength (λ) associated with an electron, with the square root of accelerating potential (V) ?

Concept: de-Broglie Relation
Chapter: [0.11] Dual Nature of Radiation and Matter
[1] 8

Define dipole moment of an electric dipole. Is it a scalar or a vector?

Concept: Electric Dipole
Chapter: [0.02] Electric Charges and Fields
[2] 9

A conductor of length ‘l’ is connected to a dc source of potential ‘V’. If the length of the conductor is tripled by gradually stretching it, keeping ‘V’ constant, how will (i) drift speed of electrons and (ii) resistance of the conductor be affected? Justify your answer.

Concept: Drift of Electrons and the Origin of Resistivity
Chapter: [0.03] Current Electricity
[2] 10

Two students ‘X’ and ‘Y’ perform an experiment on potentiometer separately using the circuit given below:

Keeping other parameters unchanged, how will the position of the null point be affected if

(i) ‘X’ increases the value of resistance R in the set-up by keeping the key K1 closed and the Key K2 opens?

(ii) ‘Y’ decreases the value of resistance S in the set-up, while the key K2 remains open and they K1 closed?

Justify.

Concept: Potentiometer
Chapter: [0.03] Current Electricity
[2] 11

A particle of charge ‘q’ and mass ‘m’ is moving with velocity .vecV It is subjected to a uniform magnetic field vecB directed perpendicular to its velocity. Show that it describes a circular path. Write the expression for its radius.

Concept: Magnetic Force
Chapter: [0.05] Moving Charges and Magnetism
[2] 12

Calculate the quality factor of a series LCR circuit with L = 2.0 H, C = 2μF and R = 10 Ω. Mention the significance of quality factor in LCR circuit.

Concept: LC Oscillations
Chapter: [0.06] Alternating Current
[2] 13

Explain briefly how electromagnetic waves are produced by an oscillating charge. How is the frequency of the em waves produced related to that of the oscillating charge?

Concept: Electromagnetic Waves
Chapter: [0.08] Electromagnetic Waves
[3] 14

In a given sample, two radioisotopes, A and B, are initially present in the ration of 1 : 4. The half lives of A and B are respectively 100 years and 50 years. Find the time after which the amounts of A and B become equal.

Chapter: [0.12] Nuclei
[2] 15

Figure shows a block diagram of a transmitter identify the boxes ‘X’ and ‘Y’ and write their functions.

Concept: Junction Transistor - Transistor as an Amplifier (Ce-configuration)
Chapter: [0.14] Semiconductor Electronics - Materials, Devices and Simple Circuits
[2] 16

Trace the path of a ray of light passing through a glass prism (ABC) as shown in the figure. If the refractive index of glass is sqrt3, find out the value of the angle of emergence from the prism.

Concept: Snell’s Law
Chapter: [0.09] Ray Optics and Optical Instruments
[2] 17 | Attempt any one of the following.
[2] 17.1

Write two characteristic features to distinguish between n-type and p-type semiconductors ?

Concept: Energy Bands in Conductors, Semiconductors and Insulators
Chapter: [0.14] Semiconductor Electronics - Materials, Devices and Simple Circuits
[2] 17.2

How does a light emitting diode (LED) work? Give two advantages of LED’s over the conventional incandescent lamps.

Concept: Special Purpose P-n Junction Diodes
Chapter: [0.14] Semiconductor Electronics - Materials, Devices and Simple Circuits
[2] 18

A short bar magnet of magnetic moment 0.9 J/T is placed with its axis at 30° to a uniform magnetic field. It experiences a torque of 0.063 J.

(i) Calculate the magnitude of the magnetic field.

(ii) In which orientation will the bar magnet be in stable equilibrium in the magnetic field?

Concept: Force on a Current - Carrying Conductor in a Uniform Magnetic Field
Chapter: [0.05] Moving Charges and Magnetism
[3] 19

State Gauss’s law in electrostatics. A cube which each side ‘a’ is kept is an electric field given by vecE = C × l. (as is shown in the figure where C is a positive dimensional constant. Find out

(i) The electric flux through the cube, and

(ii) The net charge inside the cube.

Concept: Magnetism and Gauss’s Law
Chapter: [0.04] Magnetism and Matter
[3] 20

A capacitor of 200 pF is charged by a 300 V battery. The battery is then disconnected and the charge capacitor is connected to another uncharged capacitor of 100 pF. Calculate the difference between the final energy stored in the combined system and the initial energy stored in the single capacitor.

Concept: Potential Energy in an External Field - Potential Energy of a Single Charge
Chapter: [0.01] Electrostatic Potential and Capacitance
[3] 21

Draw a labelled diagram of a moving coil galvanometer and explain its working. What is the function of radial magnetic field inside the coil?

Concept: Moving Coil Galvanometer
Chapter: [0.05] Moving Charges and Magnetism
[3] 22

Define power of a lens. Write its units. Deduce the relation 1/f =1/f_1  +1/f_2for two thin lenses kept in contact coaxially.

Concept: Power of a Lens
Chapter: [0.09] Ray Optics and Optical Instruments
[3] 23

Write two characteristic features observed is photoelectric effect which supports the photon pictures of electromagnetic radiation ?

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

Draw a graph between the frequency of incident radiation (υ) and the maximum kinetic energy of the electrons emitted from the surface of a photosensitive material state clearly how this graph can be used to determine (i) Planck’s constant and (ii) work function of the material.

Concept: Photoelectric Effect - Hallwachs’ and Lenard’s Observations
Chapter: [0.11] Dual Nature of Radiation and Matter
[3] 24

Define modulation index.

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

Given modulation index physical significance .For an amplitude modulated wave, the maximum amplitude is found to be 10 V while the minimum amplitude is 2 V. Determine the modulation index μ ?

Concept: Modulation and Its Necessity
Chapter: [0.15] Communication Systems
[3] 25

Two cells of emf E1, E2 and internal resistance r1 and r2 respectively are connected in parallel as shown in the figure.

Deduce the expressions for

(1) the equivalent e.m.f of the combination

(2) the equivalent resistance of the combination, and

(3) the potential difference between the point A and B.

Concept: Cells, Emf, Internal Resistance
Chapter: [0.03] Current Electricity
[3] 26

Using Bohr’s postulates for hydrogen atom, show that the total energy (E) of the electron in the stationary states tan be expressed as the sum of kinetic energy (K) and potential energy (U), where K = −2U. Hence deduce the expression for the total energy in the nth energy level of hydrogen atom.

Concept: Bohr’s Model for Hydrogen Atom
Chapter: [0.13] Atoms
[3] 27
[3] 27.1

Define a wavefront

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

Use Huygens’ geometrical construction to show the propagation of plane wavefront a rarer medium (1) to a denser medium (2) undergoing refraction.

Hence derive Snell’s law of refraction.

Concept: Snell’s Law
Chapter: [0.09] Ray Optics and Optical Instruments
[3] 27.2
[2] 27.2.1

Use Huygens’ geometrical construction to show the behavior of a plane wavefront.

(i) Passing through a biconvex lens;

(ii) Reflecting by a concave mirror

Concept: Concave Mirror
Chapter: [0.09] Ray Optics and Optical Instruments
[1] 27.2.2

When monochromatic light is incident on a surface separating two media, why does the refracted light have the same frequency as that of the incident light?

Concept: Refraction of Monochromatic Light
Chapter: [0.1] Wave Optics
[5] 28 | Attempt any one of the following.
[5] 28.1
[3] 28.1.1

What is the effect on the interference fringes to a Young’s double slit experiment when

(i) the separation between the two slits is decreased?

(ii) the width of a source slit is increased?

(iii) the monochromatic source is replaced by a source of white light?

Concept: Interference of Light Waves and Young’s Experiment
Chapter: [0.1] Wave Optics
[2] 28.1.2

The intensity at the central maxima in Young’s double slit experimental set-up is I0. Show that the intensity at a point where the path difference is λ/3 is I0/4.

Concept: Interference of Light Waves and Young’s Experiment
Chapter: [0.1] Wave Optics
[5] 28.2
[3] 28.2.1

Obtain the conditions for the bright and dark fringes in diffraction pattern due to a single narrow slit illuminated by a monochromatic source.

Explain clearly why the secondary maxima go on becoming weaker with increasing.

Concept: Refraction of Monochromatic Light
Chapter: [0.1] Wave Optics
[2] 28.2.2

When the width of the slit is made double, how would this affect the size and intensity of the central diffraction band? Justify.

Concept: Width of Central Maximum
Chapter: [0.1] Wave Optics
[5] 29 | Attempt any one of the following.
[5] 29.1
[3] 29.1.1

Sate the principle on which AC generator works. Draw a labeled diagram and explain its working?

Concept: A.C. Generator
Chapter: [0.07] Electromagnetic Induction
[2] 29.1.2

A conducting rod held horizontally along East- West direction is dropped from rest from a certain height near the Earth’s surface. Why should there be an induced emf across the end of the rod? Draw a plot showing the instantaneous variation of emf as a function of time from the instant it begins to fall.

Concept: The Earth’s Magnetism
Chapter: [0.04] Magnetism and Matter
[5] 29.2
[3] 29.2.1

State the principle of a step-up transformer. Explain, with the help of a labeled diagram, its working ?

Concept: Transformers
Chapter: [0.06] Alternating Current
[2] 29.2.2

Describe briefly and two energy losses, giving the reasons for their occurrence in actual transformers ?

Concept: Transformers
Chapter: [0.06] Alternating Current
[5] 30 | Attempt any on of the following.
[5] 30.1
[3] 30.1.1

Draw the circuit for studying the input and output characteristics of and transistor in CE configuration. Show, how, from the output characteristics, the information about the current amplification factor (βac) can obtained.

Concept: Transistor Action
Chapter: [0.14] Semiconductor Electronics - Materials, Devices and Simple Circuits
[2] 30.1.2

Draw a plot of the transfer characteristics (V0 versus Vi) for a base-biased transistor in CE configuration. Show for which regions in the plot, the transistor can operate as a switch ?

Concept: Transistor Action
Chapter: [0.14] Semiconductor Electronics - Materials, Devices and Simple Circuits
[5] 30.2

Why is a zener diode considered as a special purpose semiconductor diode?

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

Draw the I − V characteristics of zener diode and explain briefly how reverse current suddenly increase at the breakdown voltage.

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

Describe briefly with the help of a circuit diagram how a zener diode works to obtain a constant dc voltage from the unregulated dc output of a rectifier ?

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

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