# Physics (Theory) 2014-2015 ISC (Commerce) Class 12 Question Paper Solution

Physics (Theory)
Date: March 2015

Question 1 is Compulsory

Answer any 4 Question From Question 2 to Question 7

Answer Any 3 Question From Question 8 to Question 11

Answer Any 3 Question From Question 12 to Question 15

[20]1
[5]1.1 | Choose the corrective alternative (a), (b), (c) or (d) for each of the question given below
[1]1.1.1

A short electric dipole (which consists of two point charges, +q and -q) is placed at the centre 0 and inside a large cube (ABCDEFGH) of length L, as shown in Figure 1. The electric flux, emanating through the cube is:

a) q"/"4piin_9L

b) zero

c) q"/"2piin_0L

d) q"/"3piin_0L

Concept: Electric Dipole
Chapter: [1.01] Electric Charges and Fields
[1]1.1.2

The equivalent resistance between points. a and f of the network shown in Figure 2 is :

a) 24 Ω

b) 110 Ω

c) 140 Ω

d) 200 Ω

Concept: Cells, Emf, Internal Resistance
Chapter: [2] Current Electricity
[1]1.1.3

A moving electron enters ·a uniform and perpendicular magnetic field. Inside thema~netic field, the electron travels along:

a) a straight line.

b) a parabola

c) a circle

d) a hyperbola

Concept: Magnetic Field Intensity Due to a Magnetic Dipole (Bar Magnet) Perpendicular to Its Axis
Chapter: [3.02] Magnetism and Matter
[1]1.1.4

A fish which is at a depth of l2 em .in water (mu = 4/3) is viewed by an observer on the bank of a lake. Its apparent depth as observed: by the observer is:

a) 3 cm

b) 9 cm

c) 12 cm

d) 16 cm

Concept: Refraction
Chapter: [6.01] Ray Optics and Optical Instruments
[1]1.1.5

if E_p and E_k represent potential energy and kinetic energy respectively, of an orbital electron, then, according to B9hr's theory:

a)E_k = -E_p"/"2

b) E_k = -E_p

c) E_k = -2E_p

d) E_k = 2E_p

Concept: Bohr'S Model for Hydrogen Atom
Chapter: [8.01] Atoms
[15]1.2 | Answer all questions given below briefly and to the point
[1]1.2.1

What is meant by the term Quantization of charge?

Concept: Quantisation of Charge
Chapter: [1.01] Electric Charges and Fields
[1]1.2.2

A resistor R is connected to a cell of-emf e and internal resistance r. The potential difference across the resistor R is found to be V. State the relation between e, V, Rand r.

Concept: Cells, Emf, Internal Resistance
Chapter: [2] Current Electricity
[1]1.2.3

Three identical cells each of emf 2V and internal resistance 10 Ω are connected in series to form a battery. The battery is then connected to a parallel combination of two identical resistors, each of resistance 6 Ω. Find the current delivered by the battery.

Concept: Combination of Resistors – Series and Parallel
Chapter: [2] Current Electricity
[1]1.2.4

State how magnetic susceptibility is different for the three types of magnetic materials, i.e. diamagnetic, paramagnetic and ferromagnetic materials

Concept: Permanent Magnets and Electromagnets
Chapter: [3.02] Magnetism and Matter
[1]1.2.5

An emf of 2V is induced in a coil when the current in it is changed from 0A to 10A in 0·40 sec. Find the coefficient of self-inductance of the coil.

Concept: Electromagnetic Induction
Chapter: [4.01] Electromagnetic Induction
[1]1.2.6

How are electric vector  (vec E), magnetic vector (vec B) and velo~ity vector (vec c) oriented in an electromagnetic wave?

Concept: Electromagnetic Waves
Chapter: [5] Electromagnetic Waves
[1]1.2.7

State any two methods by which ordinary light can be polarised

Concept: Polarisation
Chapter: [6.02] Wave Optics
[1]1.2.8

A monochromatic ray of light falls on a regular prism. What is the relation between the angle of incidence and angle of emergence in the case of minimum deviation?

Concept: Refraction of Monochromatic Light
Chapter: [6.02] Wave Optics
[1]1.2.9

What type of lens is used to correct long-sightedness?

Concept: Lenses
Chapter: [6.01] Ray Optics and Optical Instruments
[1]1.2.10

Write two important advantages of reflecting telescope over a refracting telescope.

Concept: Optical Instruments - Telescope
Chapter: [6.01] Ray Optics and Optical Instruments
[1]1.2.11

State Moseley's law

Concept: Electromagnetic Spectrum
Chapter: [5] Electromagnetic Waves
[1]1.2.12

Wavelengths of the first lines of the Lyman series, Paschen series and Balmer series, in hydrogen spectrum are denoted by lambda_L, lambda_P and lambda_B respectively. Arrange these wavelengths in increasing order.

Concept: Energy Levels
Chapter: [8.01] Atoms
[1]1.2.13

What is the significance of binding energy per nucleon of a nucleus of a radioactive element?

Concept: Mass-energy and Nuclear Binding Energy - Nuclear Binding Energy
Chapter: [8.02] Nuclei
[1]1.2.14

Write one balanced equation to show Nuclear fission

Concept: Nuclear Energy - Nuclear Fusion – Energy Generation in Stars
Chapter: [8.02] Nuclei
[1]1.2.15

Whatis the difference between analogue signal and. digital signal?

Concept: Bandwidth of Signals
Chapter: [10] Communication Systems
[5]2
[4]2.1

Derive an expression for the intensity of electric field at a point in broadside position or on [4)
an equatorial line of an electric dipole.

Concept: Electric Dipole
Chapter: [1.01] Electric Charges and Fields
[1]2.2

Two point charges of 10C each are kept at a distance of 3m in the vacuum. Calculate their electrostatic potential energy.

Concept: Electrical Potential Energy of a System of Two Point Charges and of Electric Dipole in an Electrostatic Field
Chapter: [1.02] Electrostatic Potential, Potential Energy and Capacitance
[5]3
[3]3.1

Four capacitors, C1, C2, C3 andC4 are connected as shown in Figure 3 below. Calculate equivalent capacitance of the circuit between points X . and Y.

Concept: Capacitance of a Parallel Plate Capacitor with and Without Dielectric Medium Between the Plates
Chapter: [1.02] Electrostatic Potential, Potential Energy and Capacitance
[2]3.2

Draw labelled graphs to show how electrical resistance varies with temperature for:

1) a metallic wire.

2) a piece of carbon

Concept: Temperature Dependence of Resistance
Chapter: [2] Current Electricity
[5]4
[2]4.1

Two resistors R1 = 400Ω and R2 = 20 n are connected in parallel to a battery. If heating the power developed in R1 is 25 W. find the heating power developed in R2

Concept: Combination of Resistors – Series and Parallel
Chapter: [2] Current Electricity
[3]4.2

With the help of a labelled diagram, show that the balancing condition of a Wheatstone bridge is

R_1/R_2 = R_3/R_4 where the terms have their usual meaning.

Concept: Wheatstone Bridge
Chapter: [2] Current Electricity
[5]5
[3]5.1

An I0m long uniform metallic wire having a resistance of 20Ω IS used as a  potentiometer wire. This wire is connected in series with another resistance of 480Ω
and a battery of emf 5V having negligible internal resistance. If an unknown emf e is balanced across 6m of the potentiometer wire, calculate

1) the potential gradient across the potentiometer wire

2) the value of the unknown emf e.

Concept: Metre Bridge
Chapter: [2] Current Electricity
[2]5.2
[1]5.2.1

Explain the term hysteresis

Concept: Magnetic Force
Chapter: [3.01] Moving Charges and Magnetism [3.02] Magnetism and Matter
[1]5.2.2

Name three elements of the earth's magnetic field which help in defining earth's magnetic field completely.

Concept: The Earth’s Magnetism
Chapter: [3.02] Magnetism and Matter
[5]6
[3]6.1

Obtain an expression for magnetic flux density B at the centre of a circular coil of radius R, having N turns and carrying a current I

Concept: Magnetic Force
Chapter: [3.01] Moving Charges and Magnetism [3.02] Magnetism and Matter
[2]6.2

A coil of self-inductance 2.5H and resistance 20Ω is connected to a battery of emf 120V having the internal resistance of 5 n. Find:

1) The time constant of the circuit.

2) The current in the circuit in steady state

Concept: Inductance - Mutual Inductance
Chapter: [4.01] Electromagnetic Induction
[5]7
[4]7.1

Figure 4 below shows a capacitor C, an inductor L and a resistor R, connected in series
to an a.c. supply of 220 V

Calculate:

1) The resonant frequency of the given CLR circuit.

2) Current flowing through·the circuit.

3) Average power consumed by the circuit.

Concept: Combination of Capacitors
Chapter: [1.02] Electrostatic Potential, Potential Energy and Capacitance
[1]7.2

In a series LCR circuit, what is the phase difference between VL and VC where VL is the potential difference across the inductor and V c is the potential difference across the capacitor?

Concept: Potential Difference and Emf of a Cell
Chapter: [2] Current Electricity
[5]8
[4]8.1

On the basis of Huygens Wave theory of light, show that angle of reflection is equal to the angle of incidence. You must draw a labelled diagram for this derivation

Concept: Proof of Laws of Reflection and Refraction Using Huygen'S Principle
Chapter: [6.02] Wave Optics
[1]8.2

State any one difference between interference of light and diffraction of light

Concept: Interference
Chapter: [6.02] Wave Optics
[5]9
[3]9.1

Laser light of wavelength 630 nm is incident on a pair of slits which are separated by 1.8 mm. If the screen is kept 80 cm away from the two slits, calculate:

1) fringe separation i.e. fringe width.

2) distance of 10th bright fringe from the centre of the interference pattern

Concept: Interference
Chapter: [6.02] Wave Optics
[2]9.2

Show graphically the intensity distribution in Fraunhofer's single slit diffraction experiment. Label the axes.

Concept: Fraunhofer Diffraction Due to a Single Slit
Chapter: [6.02] Wave Optics
[5]10
[3]10.1

A point object O is placed at a distance of 15cm from a convex lens L of focal length 1 Ocm as shown in Figure 5 below. On the other side of the lens, a convex mirror M is placed such that its distance from the lens is equal to the focal length of the lens. The final image formed by this combination is observed to coincide with the object O. Find the focal length of the convex mirror

Concept: Ray Optics - Mirror Formula
Chapter: [6.01] Ray Optics and Optical Instruments
[2]10.2

What is the chromatic aberration? How can it be minimised or eliminated?

Concept: Lenses
Chapter: [6.01] Ray Optics and Optical Instruments
[5]11
[3]11.1

Draw a labelled ray diagram showing the formation of a final image by a compound microscope at least distance of distinct vision

Concept: Optical Instruments - The Microscope
Chapter: [6.01] Ray Optics and Optical Instruments
[2]11.2

With regard to an astronomical telescope of refracting type~ state how you will increase its:

1) magnifying power

2) resolving power

Concept: Optical Instruments - Telescope
Chapter: [6.01] Ray Optics and Optical Instruments
[5]12
[3]12.1

In an experiment of the photoelectric effect, the graph of maximum kinetic energy EK of the emitted photoelectrons versus the frequency v of the incident light is a straight line AB shown in Figure 6 below:

Find:

1) Threshold frequency of the metal

2) The work function of the metal.

3) Stopping potential for the photoelectrons emitted by the light of frequency v = 30 xx 10^14 Hz

Concept: Photoelectric Effect - Hertz’S Observations
Chapter: [7] Dual Nature of Radiation and Matter
[2]12.2

State how de-Broglie wavelength (lambda) of moving particles varies with their linear momentum (p).

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

State any one phenomenon in which moving particles exhibit wave nature.

Concept: Wave Nature of Matter
Chapter: [7] Dual Nature of Radiation and Matter
[5]13
[3]13.1

On the basis of Bohr's theory, derive an expression for the radius of the nth orbit of an electron of the hydrogen atom.

Concept: Bohr'S Model for Hydrogen Atom
Chapter: [8.01] Atoms
[2]13.2

Using the constants given on page 8 ofthis·Paper, find the minimum wavelength of the emitted X rays, when an X-ray tube is operated at 50 kV.

Concept: Continuous and Characteristics X-rays
Chapter: [7] Dual Nature of Radiation and Matter
[5]14
[3]14.1

Define half-life of a radioactive substance

Concept: Mass-energy and Nuclear Binding Energy - Nuclear Binding Energy
Chapter: [8.02] Nuclei

Using the equation N = N_0e^(-lambdat) obtain the relation between half-life (T) and decay constant (lambda) of a radioactive substance.

Chapter: [8.02] Nuclei
[2]14.2

With the help of a suitable example and an equation, explain the term pair production.

Concept: Atomic Masses and Composition of Nucleus
Chapter: [8.02] Nuclei
[5]15
[3]15.1

Draw a labelled diagram of a full wave rectifier. Show how output voltage varies with time if the input voltage is a sinusoidal voltage.

Concept: Semiconductor Diode
Chapter: [9] Electronic Devices
[2]15.2

Give the truth table and circuit symbol for NAND gate

Concept: Digital Electronics and Logic Gates
Chapter: [9] Electronic Devices

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