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# Physics (Theory) Set A 2017-2018 ISC (Commerce) Class 12 Question Paper Solution

SubjectPhysics (Theory)
Year2017 - 2018 (March)
Physics (Theory)
Set A
2017-2018 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%

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

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

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

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

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

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

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

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

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

Why is the core of a transformer laminated?

Concept: Eddy Currents
Chapter: [4.01] Electromagnetic Induction
[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?

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

Define coherent sources of light

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

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

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

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

Concept: Electrical Resistivity and Conductivity
Chapter: [2] Current Electricity
[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

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

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

Explain Curie’s law for a paramagnetic substance.

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

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

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

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

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

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

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

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

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

State any two difference between the primary rainbow and secondary rainbow

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

State de Broglie hypothesis

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

With reference to the photoelectric effect, define threshold wavelength

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

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

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

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

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

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

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

In a communication system, what is meant by modulation?

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

State any two types of modulation

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

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
[3]14

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

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

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

Concept: Ohm’s Law
Chapter: [2] Current Electricity
[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

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

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

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

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

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

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

Write one balanced equation to show Nuclear fission

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

Write one balanced equation to show Nuclear fusion

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

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

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

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

Concept: Semiconductor Diode
Chapter: [9] Electronic Devices

What is the use of Zener diode?

Concept: Semiconductor Diode
Chapter: [9] Electronic Devices
[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’?

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

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

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

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

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

Concept: Polarisation
Chapter: [6.02] Wave Optics
[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

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

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

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

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

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

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

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## CISCE previous year question papers Class 12 Physics (Theory) with solutions 2017 - 2018

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