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# Physics (Theory) Set 1 2018-2019 ISC (Science) Class 12 Question Paper Solution

Physics (Theory) [Set 1]
Date: March 2019

1 . All Question are compulsary in Section A .

2 . All Question are compulsary in Section B (Question No. 4 And 11 have internal Option).

3 .  All Question are compulsary in Section C (Question No. 15 And 16 have internal Option).

4 . In Section D all Question(20-22) have internal Option.

Section A
1
1.a | Choose the correct alternative (a), (b), (c) or (d) for each of the questions given below :
1.a.i

A closed surface in vacuum encloses charges –q and +3q. The total electric flux emerging out of the surface is :

Zero

(2q)/∈_0

(3q)/∈_0

(4q)/∈_0

Concept: Applications of Gauss’s Law
Chapter: [1.01] Electric Charges and Fields
1.a.ii

What is the angle of dip at a place where the horizontal component (B_H) and vertical component (B_V) of earth’s magnetic field are equal :

130°

60°

45°

90°

Concept: The Earth’S Magnetism
Chapter: [3.02] Magnetism and Matter
1.a.iii

A beam of light is incident at the polarizing angle of 35° on a certain glass plate. The refractive index of the glass plate is :

sin 35°

tan 35°

tan 55°

sin 55°

Concept: Polarisation
Chapter: [6.02] Wave Optics
1.a.iv

In a gamma ray emission from nucleus :

only the number of protons change

the number of protons and neutrons, both change

there is no change in the number of protons and the number of neutrons

only the number of neutrons change

Chapter: [8.02] Nuclei
1.a.v

The energy associated with light of which of the following colours is minimum :

violet

red

green

yellow

Concept: Electromagnetic Waves
Chapter:  Electromagnetic Waves
1.b | Answer the following questions briefly and to the point.
1.b.i

Define equipotential surface.

Concept: Equipotential Surfaces
Chapter: [1.02] Electrostatic Potential, Potential Energy and Capacitance
1.b.ii

Calculate the net emf across A and B shown in Figure 1 below : Concept: Electric Current
Chapter:  Current Electricity
1.b.iii

Why are the pole pieces of a horseshoe magnet in a moving coil galvanometer made cylinder in shape?

Concept: Moving Coil Galvanometer
Chapter: [3.01] Moving Charges and Magnetism
1.b.iv

What is the value of power factor for a pure resistor connected to an alternating current source ?

Concept: Power in Ac Circuit: the Power Factor
Chapter: [4.02] Alternating Current
1.b.v

What should be the path difference between two waves reaching a point for obtaining constructive interference in Young’s Double Slit experiment ?

Concept: Interference of Light Waves and Young’S Experiment
Chapter: [6.02] Wave Optics
1.b.vi

Define critical angle for a given medium.

Concept: Total Internal Reflection
Chapter: [6.01] Ray Optics and Optical Instruments
1.b.vii

Name the series in the atomic spectra of the hydrogen atom that falls in the ultra violet region.

Concept: Hydrogen Spectrum
Chapter: [8.01] Atoms
Section B
2

In a potentiometer experiment, the balancing length with a resistance of 2Ω is found to be 100 cm, while that of an unknown resistance is 500 cm. Calculate the value of the unknown resistance.

Concept: Potentiometer
Chapter:  Current Electricity
3

A rectangular loop of area 5m2, has 50 turns and carries a current of 1A. It is hold in a uniform magnetic field of 0.1T, at an angle of 30°. Calculate the torque experienced by the coil.

Concept: Torque on a Magnetic Dipole (Bar Magnet) in a Uniform Magnetic Field
Chapter: [3.02] Magnetism and Matter
4
4.a

An electric current I flows through an infinitely long conductor as shown in Figure 2 (a) below. Write an expression and direction for the magnetic field at point P. Concept: Magnetic Field Due to a Current Element, Biot-Savart Law
Chapter: [3.01] Moving Charges and Magnetism
OR
4.b

An electric current I flows through a circular loop as shown in Figure 2(b) below. Write an expression and direction for the magnetic field at the centre of the loop at point P. Concept: Magnetic Field Due to a Current Element, Biot-Savart Law
Chapter: [3.01] Moving Charges and Magnetism
5

A transformer is used to step up an alternating emf of 200V to 440V. If the primary coil has 1000 turns, calculate the number of turns in the secondary coil.

Concept: Transformers
Chapter: [4.02] Alternating Current
6

This is an example of step-up transformer .

Concept: Electromagnetic Waves
Chapter:  Electromagnetic Waves
7

Write any one use for each of the following mirrors :

(a) Convex

(b) Concave

Concept: Ray Optics - Mirror Formula
Chapter: [6.01] Ray Optics and Optical Instruments
8

The deviation produced for violet, yellow and red lights for crown glass are 3.75°, 3.25° and 2.86° respectively. Calculate the dispersive power of the crown glass.

Concept: Dispersion by a Prism
Chapter: [6.01] Ray Optics and Optical Instruments
9

(a) What is mean by mass defect ?

(b) What conclusion is drawn from Rutherford’s scattering experiment of alpha-particles ?

Concept: Mass-Energy Relation and Mass Defect
Chapter: [8.02] Nuclei
10

Define the following with reference to photoelectric effect :

(a) Threshold frequency (f_0)

(b) Stopping potential (V_s)

Concept: Photoelectric Effect - Hallwachs’ and Lenard’S Observations
Chapter:  Dual Nature of Radiation and Matter
11
11.a

The half-life of radium is 1550 years. Calculate its disintegration constant (lambda) .

Chapter: [8.02] Nuclei
OR
11.b

Copy and complete the following table for a radioactive element whose half-life is 10 minutes. Assume that you have 30g of this element at t = 0. Chapter: [8.02] Nuclei
12

Define frequency modulation and state any one advantage of frequency modulation (FM) over amplitude modulation (AM).

Concept: Electromagnetic Waves
Chapter:  Electromagnetic Waves
Section C
13

Obtain an expression for electric potential ‘V’ at a point in an end-on position i.e. axial position of the electric dipole.

Concept: Electric Potential
Chapter: [1.02] Electrostatic Potential, Potential Energy and Capacitance
14

Three capacitors of capacitance C_1 = 3muf , C_2 = 6muf , C_3 = 10muf , are connected to a 10V battery as shown in figure 3 below : Calculate :

(a) Equivalent capacitance.

(b) Electrostatic potential energy stored in the system

Concept: Combination of Capacitors
Chapter: [1.02] Electrostatic Potential, Potential Energy and Capacitance
15
15.a

Obtain the balancing  condition for the Wheatstone bridge arrangements as shown in Figure 4 below: Concept: Wheatstone Bridge
Chapter:  Current Electricity
OR
15.b

Draw a labelled circuit diagram of a potentiometer to measure the internal resistance ‘r’ of a cell. Write the working formula (derivation is not required).

Concept: Potentiometer
Chapter:  Current Electricity
16
16.a

A ray of light is incident on a prism whose refractive index is 1.52 at an angle of 40°. If the angle of emergence is 60°, calculate the angle of the prism.

Concept: Dispersion by a Prism
Chapter: [6.01] Ray Optics and Optical Instruments
OR
16.b

Calculate the focal length of a convex lens whose radii of curvature of two surfaces is 10 cm and 15 cm respectively and its refractive index is 1.5.

Concept: Lenses
Chapter: [6.01] Ray Optics and Optical Instruments
17

Derive the law of reflection using Huygen’s Wave Theory.

Concept: Huygens Principle
Chapter: [6.02] Wave Optics
18

State any two Bohr’s postulates and write the energy value of the ground state of the hydrogen atom.

Concept: Bohr'S Model for Hydrogen Atom
Chapter: [8.01] Atoms
19

With reference to semi-conductors answer the following :

(i) What is the change in the resistance of the semi-conductor with increase in temperature ?

(ii) Name the majority charge carriers in n-type semi-conductor.

(iii) What is meant by doping ?

Concept: Semiconductor Diode
Chapter:  Electronic Devices
Section D
20
20.a

(i) An alternating emf of 200 V, 50 Hz is applied to an L-R ciruit, having a resistance R of 10 Ω and an  inductance L of 0.05H connected in series. Calculate :

(1) Impedance

(2) Current flowing in the circuit

(ii) Draw a labelled graph showing the variation of inductive reactance (X_L) verses frequency (f) .

Concept: Reactance and Impedance
Chapter: [4.02] Alternating Current
OR
20.b

(i) An a.c. source of emf ε = 200 sin omegat is connected to a resistor of 50 Ω . calculate :

(1) Average current ("I"_("avg"))

(2) Root mean square (rms) value of emf

(ii) State any two characteristics of resonance in an LCR series circuit.

Concept: Ac Voltage Applied to a Series Lcr Circuit
Chapter: [4.02] Alternating Current
21
21.a

Draw a neat labelled ray diagram showing the formation of an image at the least distance of distinct vision D by a simple microscope. When the final image is at D, derive an expression for its magnifying power at D.

Concept: Optical Instruments - The Microscope
Chapter: [6.01] Ray Optics and Optical Instruments
OR
21.b

Draw a neat labelled diagram of Young’s Double Slit experiment. Show that beta = (lambdaD)/d , where the terms have their usual meanings (either for bright or dark fringe).

Concept: Interference of Light Waves and Young’S Experiment
Chapter: [6.02] Wave Optics
22
22.a

(i) Draw a labelled circuit diagram of a half wave rectifier and give its output waveform.

(ii) Draw a symbol for NOR gate and write its its truth table.

Concept: Materials, Devices and Simple Circuits
Chapter:  Electronic Devices
OR
22.b

(i) Draw a neat circuit diagram to study the input and output characteristics of a common emitter transistor.

(ii) Draw the symbol for AND gate and write its truth table.

Concept: Materials, Devices and Simple Circuits
Chapter:  Electronic Devices

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