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# Question Paper Solutions - Physics (Theory) 2017 - 2018 ISC (Science) Class 12

SubjectPhysics (Theory)
Year2017 - 2018 (March)

Marks: 70
[12]1
[5]1.1 | Choose the correct alternative (a), (b), (c) or (d) for each of the questions
[1]1.1.1

The order of coloured rings in a carbon resistor is red, yellow, blue and silver. The resistance of the
carbon resistor is:

a) 24 x 106 Ω ± 5%

b) 24 x 106 Ω ± 10%

c) 34 x 104 Ω ± 10%

d) 26 x 104 Ω ± 5%

Chapter: [2] Current Electricity
Concept: Temperature Dependence of Resistance
[1]1.1.2

A circular coil carrying a current I has radius R and number of turns N. If all the three, i.e. the current
I, radius R and number of turns N are doubled, then, the magnetic field at its centre becomes:

(a) Double

(b) Half

(c) Four times

(d) One fourth

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

A circular coil carrying a current I has radius R and number of turns N. If all the three, i.e. the current
I, radius R and number of turns N are doubled, then, the magnetic field at its centre becomes:

(a) Double

(b) Half

(c) Four times

(d) One fourth

Chapter: [3.02] Magnetism and Matter
Concept: Magnetic Force
[1]1.1.3

An object is kept on the principal axis of a concave mirror of focal length 10 cm. at a distance of 15
cm from its pole. The image formed by the mirror is:

(a) Virtual and magnified

(b) Virtual and diminished

(c) Real and magnified

(d) Real and diminished

Chapter: [6.01] Ray Optics and Optical Instruments
Concept: Ray Optics - Mirror Formula
[1]1.1.4

Einstein's photoelectric equation is:

a) E_"max" = hlambda - varphi_0

b) E_"max"= (hc)/lambda  varphi_0

c) E_"max" = hv + varphi_0

d) E_"max" = (hv)/lambda + varphi_0

Chapter: [7] Dual Nature of Radiation and Matter
Concept: Einstein’s Equation - Particle Nature of Light
[1]1.1.5

In Bohr’s model of the hydrogen atom, the radius of the first orbit of an electron is r0 . Then, the radius of the third orbit is:

a) r_0/9

b) r_0

c) 3r_0

d) 9r_0

Chapter: [8.01] Atoms
Concept: Bohr'S Model for Hydrogen Atom
[7]1.2 | Answer the following questions briefly and to the point.
[1]1.2.1

In a potentiometer experiment, balancing length is found to be 120 cm for a cell E1 of emf 2V. What will be the balancing length for another cell E2 of emf 1.5V? (No other changes are made in the experiment.)

Chapter: [2] Current Electricity
Concept: Potentiometer
[1]1.2.2

How will you convert a moving coil galvanometer into a voltmeter?

Chapter: [3.01] Moving Charges and Magnetism
Concept: Moving Coil Galvanometer
[1]1.2.3

A moving charged particle q travelling along the positive x-axis enters a uniform magnetic field B.
When will the force acting on q be maximum?

Chapter: [3.01] Moving Charges and Magnetism
Concept: Force on a Moving Charge in Uniform Magnetic and Electric Fields
[1]1.2.4

Why is the core of a transformer laminated?

Chapter: [4.01] Electromagnetic Induction
Concept: Eddy Currents
[1]1.2.5

Ordinary (i.e. unpolarized) light is incident on the surface of a transparent material at the polarising angle. If it is partly reflected and partly refracted, what is the angle between the reflected and the refracted rays?

Chapter: [6.02] Wave Optics
Concept: Brewster's Law
[1]1.2.6

Define coherent sources of light

Chapter: [6.02] Wave Optics
Concept: Coherent and Incoherent Sources and Sustained Interference of Light
[1]1.2.7

Name a material which is used in making control rods in a nuclear reactor.

Chapter: [8.02] Nuclei
Concept: Atomic Masses and Composition of Nucleus
[2]2

Define current density. Write an expression which connects current density with drift speed

Chapter: [2] Current Electricity
Concept: Electrical Resistivity and Conductivity
[2]3 | Attempt Any One
[2]3.1

A long horizontal wire P carries a current of 50A. It is rigidly fixed. Another wire Q is placed directly above and parallel to P, as shown in Figure 1 below. The weight per unit length of the wire Q is 0.025 Nm-1 and it carries a current of 25A. Find the distance 'r' of the wire Q from the wire P so that the wire Q remains at rest

Chapter: [3.01] Moving Charges and Magnetism
Concept: Force on a Moving Charge in Uniform Magnetic and Electric Fields
[2]3.2

Calculate force per unit length acting on the wire B due to the current flowing in the wire A.(See
Figure 2 below)

Chapter: [2] Current Electricity
Concept: Cells in Series and in Parallel
[2]4
[1]4.1

Explain Curie’s law for a paramagnetic substance.

Chapter: [3.02] Magnetism and Matter
Concept: Magnetic Properties of Materials
[1]4.2

A rectangular coil having 60 turns and area of 0.4m2 is held at right angles to a uniform magnetic field of flux density 5 × 10-5T. Calculate the magnetic flux passing through it.

Chapter: [4.01] Electromagnetic Induction
Concept: Magnetic Flux
[2]5

What is motional emf? State any two factors on which it depends

Chapter: [4.01] Electromagnetic Induction
Concept: Induced Emf and Current
[2]6
[1]6.1

What is the ratio of the speed of gamma rays to that of radio waves in a vacuum?

Chapter: [5] Electromagnetic Waves
Concept: Electromagnetic Waves
[1]6.2

Identify the part of the electromagnetic spectrum which is suitable for radar system used in aircraft navigation.

Chapter: [5] Electromagnetic Waves
Concept: Electromagnetic Spectrum
[2]7

A biconvex lens made of glass (refractive index 1.5) has two spherical surfaces having radii 20 cm and 30 cm. Calculate its focal length.

Chapter: [6.01] Ray Optics and Optical Instruments
Concept: Lenses
[2]8

State any two difference between the primary rainbow and secondary rainbow

Chapter: [6.01] Ray Optics and Optical Instruments
Concept: Dispersion by a Prism
[2]9
[1]9.1

State de Broglie hypothesis

Chapter: [7] Dual Nature of Radiation and Matter
Concept: de-Broglie Relation
[1]9.2

With reference to the photoelectric effect, define threshold wavelength

Chapter: [7] Dual Nature of Radiation and Matter
Concept: Einstein’s Equation - Particle Nature of Light
[2]10

Calculate the minimum wavelength of the spectral line present in Balmer series of hydrogen

Chapter: [8.01] Atoms
Concept: Hydrogen Spectrum
[2]11 | Attempt Any One
[2]11.1

What are means by pair annihilation? Write a balanced equation for the same.

Chapter: [8.01] Atoms
Concept: Energy Levels
[2]11.2

What is meant by the terms half-life of a radioactive substance and binding energy of a nucleus?

Chapter: [8.02] Nuclei
Concept: Mass-energy and Nuclear Binding Energy - Nuclear Binding Energy
[2]12

In a communication system, what is meant by modulation?

Chapter: [10] Communication Systems
Concept: Modulation and Its Necessity

State any two types of modulation

Chapter: [10] Communication Systems
Concept: Modulation and Its Necessity
[3]13

Obtain an expression for an intensity of electric field at a point at the end of position, i.e., the axial position of an electric dipole.

x

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
[3]14

Deduce an expression for equivalent capacitance C when three capacitors C1, C2 and C3 connected in parallel.

Chapter: [1.02] Electrostatic Potential, Potential Energy and Capacitance
Concept: Combination of Capacitors
[3]15 | Attempt Any One
[3]15.1

ε1 and ε2 are two batteries having emf of 34V and 10V respectively and internal resistance of 1Ω and 2Ω respectively. They are connected as shown in the figure below. Using Kirchhoff’s Laws of electrical networks, calculate the currents I1 and I2.

Chapter: [2] Current Electricity
Concept: Kirchhoff’s Rules
[3]15.2

An electrical bulb is marked 200V, 100W. Calculate the electrical resistance of its filament. If five such
bulbs are connected in series to a 200V supply, how much current will flow through them?

Chapter: [2] Current Electricity
Concept: Ohm’s Law
[3]16 | Attempt Any One
[3]16.1

For any prism, prove that :

'n' or mu = sin((A + delta_m)/2)/sin(A/2)

where the terms have their usual meaning

Chapter: [6.01] Ray Optics and Optical Instruments
Concept: Dispersion by a Prism
[3]16.2

When two thin lenses are kept in contact, prove that their combined or effective focal length F is
given by :

1/F = 1/f_1  + 1/f_2

where the terms have their usual meaning.

Chapter: [6.01] Ray Optics and Optical Instruments
Concept: Refraction at Spherical Surfaces and by Lenses - Combination of Thin Lenses in Contact
[3]17

In Young’s double slit experiment, show graphically how the intensity of light varies with distance

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

In Fraunhofer diffraction, how is the angular width of the central bright fringe affected when slit separation is increased?

Chapter: [6.02] Wave Optics
Concept: Fraunhofer Diffraction Due to a Single Slit
[3]18

Write one balanced equation to show Nuclear fission

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

Write one balanced equation to show Nuclear fusion

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

Write one balanced equation to show Emission of beta^- (i.e. a negative beta particle)

Chapter: [8.02] Nuclei
Concept: Atomic Masses and Composition of Nucleus
[3]19

With reference to semiconductor devices, define a p-type semiconductor and a Zener diode.

Chapter: [9] Electronic Devices
Concept: Semiconductor Diode

What is the use of Zener diode?

Chapter: [9] Electronic Devices
Concept: Semiconductor Diode
[5]20 | Attempt Any One
[5]20.1

An alternating emf of 220 V is applied to a circuit containing a resistor R having the resistance of 160Ω and a capacitor ‘C’ is series. The current is found to lead the supply voltage by an angle

θ = tan^(-1) ("3/4")

a) Calculate

1) The capacitive reactance

2) The impedance of the circuit

3) Current flowing in the circuit

b) If the frequency of the applied emf is 50 Hz, what is the value of the capacitance of the capacitor ‘C’?

Chapter: [4.02] Alternating Current
Concept: Power in Ac Circuit: the Power Factor
[5]20.2

An A.C. generator generating an emf of ε = 300 sin (100 πt) V is connected to a series combination of 16μ F capacitor, 1 H inductor and 100 Ω resistor.

Calculate :

1) An impedance of the circuit at the given frequency.

2) Resonant frequency f_0

3)Power factor at the resonant frequency f_0.

Chapter: [4.02] Alternating Current
Concept: Power in Ac Circuit: the Power Factor
[5]21 | Attempt Any One
[5]21.1

Draw a labelled ray diagram of an image formed by a refracting telescope with the final image formed at infinity. Derive an expression for its magnifying power with the final image at infinity

Chapter: [6.01] Ray Optics and Optical Instruments
Concept: Optical Instruments - Telescope
[5]21.2

Derive Snell’s law on the basis of Huygen’s wave theory when the light is travelling from a denser to a rarer medium.

Chapter: [6.01] Ray Optics and Optical Instruments
Concept: Snell’s Law

With the help of an experiment, state how will you identify whether a given beam of light is polarised or unpolarized?

Chapter: [6.02] Wave Optics
Concept: Polarisation
[5]22 | Attempt Any One
[5]22.1

The forward characteristic curve of a junction diode is shown in Figure 4 below :

Calculate the resistance of the diode at

(1) V = 0.5 V

(2) I = 60 mA

Chapter: [9] Electronic Devices
Concept: Diode as a Rectifier

Draw separate energy band diagram for conductors, semiconductors and insulators and
label each of them.

Chapter: [9] Electronic Devices
Concept: Energy Bands in Conductors, Semiconductors and Insulators
[5]22.2

The arrangement given below represents a logic gate :

Copy the following truth table in your answer booklet and complete it showing outputs at C
and D.

 A B C D 0 0 1 0 0 1 1 0

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

Draw a labelled diagram of a common emitter amplifier, showing waveforms of the signal voltage and output voltage.

Chapter: [9] Electronic Devices
Concept: Junction Transistor - Transistor as an Amplifier (Ce-configuration)
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