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

SubjectPhysics (Theory)
Year2014 - 2015 (March)

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

Marks: 70
[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

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

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

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

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

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

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

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

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

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

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

Chapter: [4.01] Electromagnetic Induction
Concept: 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?

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

State any two methods by which ordinary light can be polarised

Chapter: [6.02] Wave Optics
Concept: Polarisation
[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?

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

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

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

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

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

State Moseley's law

Chapter: [5] Electromagnetic Waves
Concept: Electromagnetic Spectrum
[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.

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

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

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

Write one balanced equation to show Nuclear fission

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

Whatis the difference between analogue signal and. digital signal?

Chapter: [10] Communication Systems
Concept: Bandwidth of Signals
[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.

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

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

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

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

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

1) a metallic wire.

2) a piece of carbon

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

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

Chapter: [2] Current Electricity
Concept: Wheatstone Bridge
[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.

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

Explain the term hysteresis

Chapter: [3.01] Moving Charges and Magnetism
Concept: Magnetic Force

Explain the term hysteresis

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

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

Chapter: [3.02] Magnetism and Matter
Concept: The Earth’S Magnetism
[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

Chapter: [3.01] Moving Charges and Magnetism
Concept: Magnetic Force

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

Chapter: [3.02] Magnetism and Matter
Concept: Magnetic Force
[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

Chapter: [4.01] Electromagnetic Induction
Concept: Inductance - Mutual Inductance
[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.

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

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

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

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

Chapter: [6.02] Wave Optics
Concept: Interference
[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

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

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

Chapter: [6.02] Wave Optics
Concept: Fraunhofer Diffraction Due to a Single Slit
[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

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

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

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

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

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

1) magnifying power

2) resolving power

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

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

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

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

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

Chapter: [7] Dual Nature of Radiation and Matter
Concept: Wave Nature of 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.

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

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

Define half-life of a radioactive substance

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

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.

Chapter: [8.02] Nuclei
Concept: Atomic Masses and Composition of Nucleus
[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.

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

Give the truth table and circuit symbol for NAND gate

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