# Physics 2014-2015 HSC Science (Computer Science) 12th Board Exam Question Paper Solution

Physics
Date & Time: 3rd October 2015, 4:00 pm
Duration: 3h

Section 1
 1 | Attempt any one of the following :
 1.1

State an expression for the moment of intertia of a solid uniform disc, rotating about an axis passing through its centre, perpendicular to its plane. Hence derive an expression for the moment of inertia and radius of gyration:

i. about a tangent in the plane of the disc, and

ii. about a tangent perpendicular to the plane of the disc.

Concept: Theorems of Perpendicular and Parallel Axes
Chapter: [0.01] Rotational Dynamics [0.03] Angular Momentum

In a set, 21 turning forks are arranged in a series of decreasing frequencies. Each tuning fork produces 4 beats per second with the preceding fork. If the first fork is an octave of the last fork, find the frequencies of the first and tenth fork.

Concept: Study of Vibrations of Air Columns
Chapter: [0.06] Superposition of Waves [0.08] Stationary Waves
Or
 1.2

Discuss the composition of two S.H.M.s along the same path having same period. Find the resultant amplitude and intial phase.

Concept: LC Oscillations
Chapter: [0.13] AC Circuits [0.16] Electromagnetic Inductions

A sonometer wire is in unison with a tuning fork of frequency 125 Hz when it is stretched by a weight. When the weight is completely immersed in water, 8 beats are heard per second. Find the specific gravity of the material of the weight.

Concept: Formation of Beats
Chapter: [0.07] Wave Motion
 2 | Select and write the most appropriate answer from the given alternatives for each sub-question :
 2.1

Which of the following substances is ductile?

Glass

High carbon steel

Steel

Copper

Concept: Magnetic Properties of Materials
Chapter: [0.11] Magnetic Materials [0.15] Magnetism
 2.2

Angle of contact for the pair of pure water with clean glass is _______.

acute

obtuse

90o

0o

Concept: Surface Tension
Chapter: [0.02] Mechanical Properties of Fluids [0.06] Surface Tension
 2.3

A seconds pendulum is suspended in an elevator moving with constant speed in downward direction. The periodic time (T) of that pendulum is _______.

less than two seconds

equal to two seconds

greater than two seconds

very much greater than two seconds

Concept: Periodic and Oscillatory Motion
Chapter: [0.04] Oscillations [0.05] Oscillations
 2.4

The equation of a progressive wave is y = 7 sin (4t - 0.02x), where x and y are in cms and time t in seconds. The maximum velocity of a particle is _______.

28 cm/s

32 cm/s

49 cm/s

112 cm/s

Concept: Simple Harmonic Progressive Waves,
Chapter: [0.07] Wave Motion
 2.5

The dimensions of emissive power are

[M1 L-2 T-3 ]

[M1 L2 T-3 ]

[M1 L0 T-3 ]

[M1 L0 T-2 ]

Concept: Heat and Temperature
Chapter: [0.09] Kinetic Theory of Gases and Radiation
 2.6

The pressure (P) of an ideal gas having volume (V) is 2E/3V , then the energy E is _______.

translational kinetic

rotational kinetic

vibrational kinetic

inversely proportional to pressure

Concept: Phase of K.E (Kinetic Energy)
Chapter: [0.04] Oscillations
 2.7

The fundamental frequency of transverse vibration of a stretched string of radius r is proportional to _______.

r-2

r-1

r-1/2

r2

Concept: Formation of Stationary Waves on String
Chapter: [0.08] Stationary Waves
 3 | Attempt any SIX:
 3.1

Draw a neat labelled diagram of conical pendulum. State the expression for its periodic time in terms of length.

Concept: Uniform Circular Motion (UCM)
Chapter: [0.01] Circular Motion
 3.2

A raindrop of diameter 4 mm is about to fall on the ground. Calculate the pressure inside the raindrop. [Surface tension of water T = 0.072 N/m, atmospheric pressure = 1.013 x 105 N/m2 ]

Concept: Surface Tension
Chapter: [0.02] Mechanical Properties of Fluids [0.06] Surface Tension
 3.3

Explain why an astronaut in an orbiting satellite has a feeling of weightlessness.

Concept: Weightlessness
Chapter: [0.02] Gravitation
 3.4

The periodic time of a linear harmonic oscillator is 2π second, with maximum displacement of 1 cm. If the particle starts from extreme position, find the displacement of the particle after π/3  seconds.

Concept: Periodic and Oscillatory Motion
Chapter: [0.04] Oscillations [0.05] Oscillations
 3.5

State and prove : Law of conservation of angular momentum.

Concept: Angular Momentum or Moment of Linear Momentum
Chapter: [0.01] Rotational Dynamics
 3.6

A pinhole is made in a hollow sphere of radius 5 cm whose inner wall is at temperature 727oC. Find the power radiated per unit area. [Stefan’s constant σ = 5.7 x 10-8 J/m2 s K4 , emissivity (e) = 0.2]

Concept: Heat and Temperature
Chapter: [0.09] Kinetic Theory of Gases and Radiation
 3.7

Draw a neat labelled diagram showing forces acting on the meniscus of water in a capillary tube.

Concept: Surface Tension
Chapter: [0.02] Mechanical Properties of Fluids [0.06] Surface Tension
 3.8

Compute the temperature at which the r.m.s. speed of nitrogen molecules is 832 m/s. [Universal gas constant, R = 8320 J/k mole K, molecular weight of nitrogen = 28.]

Concept: Heat and Temperature
Chapter: [0.09] Kinetic Theory of Gases and Radiation
 4 |  Attempt any THREE :
 4.1

Discuss the behaviour of wire under increasing load.

Concept: Definition of Stress and Strain
Chapter: [0.05] Elasticity
 4.2

Determine the binding energy of satellite of mass 1000 kg revolving in a circular orbit around the Earth when it is close to the surface of Earth. Hence find kinetic energy and potential energy of the satellite. [Mass of Earth = 6 x 1024 kg, radius of Earth = 6400 km; gravitational constant G = 6.67 x 10-11 Nm2 /kg2 ]

Concept: Binding Energy and Escape Velocity of a Satellite
Chapter: [0.02] Gravitation
 4.3

Show that all harmonics are present on a stretched string between two rigid supports.

Concept: Study of Vibrations in a Finite Medium
Chapter: [0.08] Stationary Waves
 4.4

A stone of mass 100 g attached to a string of length 50 cm is whirled in a vertical circle by giving velocity at lowest point as 7 m/s. Find the velocity at the highest point. [Acceleration due to gravity = 9.8 m/s2 ]

Concept: Acceleration Due to Gravity and Its Variation with Altitude and Depth
Chapter: [0.02] Gravitation
Section II
 5 | Attempt any one of the following :
 5.1

Obtain an expression for average power dissipated in a purely resistive A.C. circult. Define power factor of the circuit and state its value for purely resistive A.C. circult.

Concept: Power in AC Circuit: the Power Factor
Chapter: [0.16] Electromagnetic Inductions

A rectangular coil of a moving coil galvanometer contains 50 turns each having area 12 cm2 . It is suspended in radial magnetic field 0.025 Wb/m2 by a fibre of twist constant 15 x10-10 Nm/degree. Calculate the sensitivity of the moving coil galvanometer.

Concept: Moving Coil Galvanometer
Chapter: [0.14] Magnetic Effects of Electric Current
Or
 5.2

State Bohr’s third postulate for hydrogen (H2) atom. Derive Bohr’s formula for the wave number. Obtain expressions for longest and shortest wavelength of spectral lines in ultraviolet region for hydrogen atom

Concept: Bohr’s Model for Hydrogen Atom
Chapter: [0.18] Atoms, Molecules and Nuclei

The photoelectric current in a photoelectric cell can be reduced to zero by a stopping potential of 1.8 volt. Monochromatic light of wavelength 2200Å is incident on the cathode. Find the maximum kinetic energy of the photoelectrons in joules. [Charge on electron = 1.6 x 10-19 C]

Concept: Photoelectric Effect - Hertz’s Observations
Chapter: [0.17] Electrons and Photons
 6 | Select and write the most appropriate answer from the given alternatives for each sub-question :
 6.1

Which one of the following particles cannot be accelerated by a cyclotron?

(A) Electrons

(B) Protons

(C) Deuterons

(D) α- particles

Concept: Motion in Combined Electric and Magnetic Fields - Cyclotron
Chapter: [0.14] Magnetic Effects of Electric Current
 6.2

In biprism experiment two interfering waves are produced due to division of _______.

amplitude

wavefront

amplitude and wavefront

neither wavefront nor amplitude

Concept: Measurement of Wavelength by Biprism Experiment
Chapter: [0.11] Interference and Diffraction
 6.3

The output of NOR gate is high, when _______.

all inputs are high

all inputs are low

only one of its inputs is high

only one of its inputs is low

Concept: Digital Electronics and Logic Gates
Chapter: [0.19] Semiconductors
 6.4

Light of a certain wavelength has a wave number barv in vacuum. Its wave number in a medium of refractive index n is _______.

n/barv

1/(nbarv)

barv/n

nbarv

Concept: Wavelength of an Electron
Chapter: [0.18] Atoms, Molecules and Nuclei
 6.5

If the radius of a sphere is doubled without changing the charge on it, then electric flux originating from the sphere is ______.

double

half

same

zero

Concept: Inductance - Mutual Inductance
Chapter: [0.12] Electromagnetic Induction [0.16] Electromagnetic Inductions
 6.6

The momentum of a photon of de Broglie wavelength 5000Å is _______.

[Planck’s constant = 6.63 x10-34 J.s.]

1.326 x10-28 kg-m/s

7.54 x10-28 kg-m/s

1.326 x10-27 kg-m/s

7.54 x10-27 kg-m/s

Concept: Einstein’s Equation - Particle Nature of Light
Chapter: [0.17] Electrons and Photons
 6.7

Ionosphere mainly consists of _______.

positive ions and electrons

water vapour and smoke

ozone layer

dust particles

Concept: Elements of a Communication System
Chapter: [0.2] Communication Systems
 7 | Attempt any SIX:
 7.1

State any ‘two’ possible sources of errors in meter-bridge experiment. How can they be minimised?

Concept: Metre Bridge
Chapter: [0.13] Current Electricity
 7.2

A potentionmeter wire has resistance of per unit length of 0.1 Ω/m. A cell of e.m.f. 1.5V balances against 300 cm length of the wire. Find the current in the potentiometer wire.

Concept: Potentiometer
Chapter: [0.09] Current Electricity [0.13] Current Electricity
 7.3

Give any ‘two’ points of differences between diamagnetic and ferromagnetic substances.

Concept: Magnetic Properties of Materials
Chapter: [0.11] Magnetic Materials [0.15] Magnetism
 7.4

An iron rod of the area of cross-section 0.1m2 is subjected to a magnetizing field of 1000 A/m. Calculate the magnetic permeability of the iron rod. [Magnetic susceptibility of iron = 59.9, magnetic permeability of vacuum = 4π x 10-7 S. I. unit]

Concept: Magnetic Properties of Materials
Chapter: [0.11] Magnetic Materials [0.15] Magnetism
 7.5

Draw a neat labelled circuit diagram of experimental arrangement for study of photoelectric effect.

Concept: Photoelectric Effect - Hertz’s Observations
Chapter: [0.17] Electrons and Photons
 7.6

A coil of 100 turns, each of area 0.02m2 is kept in a uniform field of induction 3.5 x10-5 T. If the coil rotates with a speed of 6000 r.p.m. about an axis in the plane of the coil and perpendicular to the magnetic induction, calculate peak value of e.m.f. induced in the coil.

Concept: Coil Rotating in Uniform Magnetic Induction
Chapter: [0.16] Electromagnetic Inductions
 7.7

Define modulation and transducer

Concept: Modulation and Its Necessity
Chapter: [0.2] Communication Systems
 7.8

In a biprism experiment, when a convex lens was placed between the biprism and eyepiece at a distance of 30 cm from the slit, the virtual images of the slits are found to be separated by 7 mm. If the distance between the slit and biprism is 10 cm and between the biprism and eyepiece is 80cm, find the linear magnification of the image.

Concept: Measurement of Wavelength by Biprism Experiment
Chapter: [0.11] Interference and Diffraction
 8 | Attempt any THREE:
 8.1

With the help of a neat circuit diagram, explain the working of a photodiode.

Concept: Special Purpose P-n Junction Diodes
Chapter: [0.19] Semiconductors

State its any ‘two’ uses of photodiode.

Concept: Special Purpose P-n Junction Diodes
Chapter: [0.19] Semiconductors
 8.2

A parallel beam of monochromatic light is incident on a glass slab at an angle of incidence 60o. Find the ratio of width of the beam in the glass to that in the air if refractive index of glass is 3/2.

Concept: Interference of Light
Chapter: [0.11] Interference and Diffraction
 8.3

With the help of neat diagram, explain how non-polar dielectric material is polarised in external electric field of increasing intensity. Define polarisation in dielectrics.

Concept: Polarisation
Chapter: [0.1] Wave Theory of Light
 8.4

In a single slit diffraction pattern, the distance between first minima on the right and first minima on the left of central maximum is 4 mm. The screen on which the pattern is displaced, is 2m from the slit and wavelength of light used is 6000Å. Calculate width of the slit and width of the central maximum.

Concept: Fraunhofer Diffraction Due to a Single Slit
Chapter: [0.11] Interference and Diffraction

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