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# Physics Delhi Set 1 2011-2012 CBSE (Science) Class 12 Question Paper Solution

Physics [Delhi Set 1]
Date: March 2012

1

When electrons drift in a metal from lower to higher potential, does it mean that all the free electrons of the metal are moving in the same direction?

Concept: Drift of Electrons and the Origin of Resistivity
Chapter: [2.01] Current Electricity
2

The horizontal component of the earth’s magnetic field at a place is B and angle of dip is 60°. What is the value of vertical component of earth’s magnetic field at equator?

Concept: The Earth’S Magnetism
Chapter: [3.01] Magnetism and Matter
3

Show on a graph, the variation of resistivity with temperature for a typical semiconductor.

Concept: Semiconductor Diode
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
4

Why should electrostatic field be zero inside a conductor?

Concept: Conductors and Insulators Related to Electric Field
Chapter: [1.01] Electrostatic Potential and Capacitance
5

Name of physical quantity which remains same for microwaves of wavelength 1 mm and UV radiations of 1600 Å in vacuum.

Concept: Electromagnetic Spectrum
Chapter: [5.01] Electromagnetic Waves
6

Under what condition does a biconvex lens of glass having a certain refractive index act as a plane glass sheet when immersed in a liquid?

Concept: Lenses
Chapter: [6.01] Ray Optics and Optical Instruments
7

Predict the directions of induced currents in metal rings 1 and 2 lying in the same plane where current I in the wire is increasing steadily. Concept: Lenz’S Law and Conservation of Energy
Chapter: [4.02] Electromagnetic Induction
8

State de Broglie hypothesis

Concept: de-Broglie Relation
Chapter: [7.01] Dual Nature of Radiation and Matter
9

A ray of light, incident on an equilateral prism (μ_g = sqrt3)moves parallel to the base line of the prism inside it. Find the angle of incidence for this ray.

Concept: Refraction Through a Prism
Chapter: [6.01] Ray Optics and Optical Instruments
10 | Attempt any one of the following.
10.1

Distinguish between ‘Analog and Digital signals’.

Concept: Bandwidth of Signals
Chapter: [10.01] Communication Systems
10.2

Mention the function of any two of the following used

(i) Transducer

(ii) Repeater

(iii) Transmitter

(iv) Bandpass Filter

Concept: Elements of a Communication System
Chapter: [10.01] Communication Systems
11

A cell of emf E and internal resistance r is connected to two external resistance R1 and R2 and a perfect ammeter. The current in the circuit is measured in four different situations:

(i) without any external resistance in the circuit

(ii) with resistance R1 only

(iii) with R1 and R2 in series combination

(iv) with R1 and R2 in parallel combination

The currents measured in the four cases are 0.42 A, 1.05 A, 1.4 A and 4.2 A, but not necessarily in the order. Identify the currents corresponding to the four cases mentioned above.

Concept: Cells, Emf, Internal Resistance
Chapter: [2.01] Current Electricity
12

The susceptibility of a magnetic material is − 2.6 × 10−5. Identify the type of magnetic material and state its two properties.

Concept: Magnetic Properties of Materials
Chapter: [3.01] Magnetism and Matter
13

Two identical circular wires P and Q each of radius R and carrying current ‘I’ are kept in perpendicular planes such that they have a common centre as shown in the figure. Find the magnitude and direction of the net magnetic field at the common centre of the two coils. Concept: Motion in a Magnetic Field
Chapter: [3.02] Moving Charges and Magnetism
14

When an ideal capacitor is charged by a dc battery, no current flows. However, when an ac source is used, the current flows continuously. How does one explain this, based on the concept of displacement current?

Concept: Displacement Current
Chapter: [5.01] Electromagnetic Waves
15

Draw a plot showing the variation of (i) electric field (E) and (ii) electric potential (V) with distance r due to a point charge Q.

Concept: Electric Potential
Chapter: [1.01] Electrostatic Potential and Capacitance
16

Define self-inductance of a coil. Show that magnetic energy required to build up the current I in a coil of self inductance L is given by 1/2 LI^2

Concept: Inductance - Self-Inductance
Chapter: [4.02] Electromagnetic Induction
17

The current in the forward bias is known to be more (~mA) than the current in the reverse bias (~μA). What is the reason, then, to operate the photodiode in reverse bias?

Concept: Special Purpose P-n Junction Diodes
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
18

A metallic rod of ‘L’ length is rotated with angular frequency of ‘ω’ with one end hinged at the centre and the other end at the circumference of a circular metallic ring of radius L, about an axis passing through the centre and perpendicular to the plane of the ring. A constant and uniform magnetic field B parallel to the axis is presents everywhere. Deduce the expression for the emf between the centre and the metallic ring.

Concept: Induced Emf and Current
Chapter: [4.02] Electromagnetic Induction
19

The figure shows a series LCR circuit with L = 10.0 H, C = 40 μF, R = 60 Ω connected to a variable frequency 240 V source, calculate

(i) the angular frequency of the source which drives the circuit at resonance,

(ii) the current at the resonating frequency,

(iii) the rms potential drop across the inductor at resonance. Concept: Ac Voltage Applied to a Series Lcr Circuit
Chapter: [4.01] Alternating Current
20

A rectangular loop of wire of size 4 cm × 10 cm carries a steady current of 2 A. A straight long wire carrying 5 A current is kept near the loop as shown. If the loop and the wire are coplanar, find (i) the torque acting on the loop and

(ii) the magnitude and direction of the force on the loop due to the current carrying wire.

Concept: Torque on Current Loop, Magnetic Dipole - Torque on a Rectangular Current Loop in a Uniform Magnetic Field
Chapter: [3.02] Moving Charges and Magnetism
21
21.1

Using Bohr’s second postulate of quantization of orbital angular momentum show that the circumference of the electron in the nth orbital state in hydrogen atom is n times the de Broglie wavelength associated with it.

Concept: De Broglie’S Explanation of Bohr’S Second Postulate of Quantisation
Chapter: [8.02] Atoms
21.2

The electron in hydrogen atom is initially in the third excited state. What is the maximum number of spectral lines which can be emitted when it finally moves to the ground state?

Concept: Bohr'S Model for Hydrogen Atom
Chapter: [8.02] Atoms
22 | Attempt any one of the following.
22.1

In the figure a long uniform potentiometer wire AB is having a constant potential gradient along its length. The null points for the two primary cells of emfs ε1 and ε2 connected in the manner shown are obtained at a distance of 120 cm and 300 cm from the end A. Find (i) ε1/ ε2 and (ii) position of null point for the cell ε1.

How is the sensitivity of a potentiometer increased? Concept: Potentiometer
Chapter: [2.01] Current Electricity
22.2

Using Kirchhoff’s rules determine the value of unknown resistance R in the circuit so that no current flows through 4 Ω resistance. Also find the potential difference between A and D. Concept: Kirchhoff’s Rules
Chapter: [2.01] Current Electricity
23
23.1

What characteristic property of nuclear force explains the constancy of binding energy per nucleon (BE/A) in the range of mass number ‘A’ lying 30 < A < 170?

Concept: Mass-energy and Nuclear Binding Energy - Nuclear Binding Energy
Chapter: [8.01] Nuclei
23.2

Show that the density of nucleus over a wide range of nuclei is constant-independent of mass number A.

Concept: Atomic Masses and Composition of Nucleus
Chapter: [8.01] Nuclei
24

Write any two factors which justify the need for modulating a signal ?

Concept: Bandwidth of Signals
Chapter: [10.01] Communication Systems

Draw a diagram showing an amplitude modulated wave by superposing a modulating signal over a sinusoidal carrier wave.

Concept: Bandwidth of Signals
Chapter: [10.01] Communication Systems
25

Write Einstein’s photoelectric equation?

Concept: Einstein’S Photoelectric Equation: Energy Quantum of Radiation
Chapter: [7.01] Dual Nature of Radiation and Matter

State clearly how photoelectric equation is obtained using the photon pictu.re of electromagnetic radiation.

Chapter: [7.01] Dual Nature of Radiation and Matter

Write the three salient features observed in photoelectric effect which can be explained using this equation.

Chapter: [7.01] Dual Nature of Radiation and Matter
26
26.1

Why are coherent sources necessary to produce a sustained interference pattern?

Concept: Coherent and Incoherent Sources and Sustained Interference of Light
Chapter: [6.02] Wave Optics
26.2

In Young’s double slit experiment using monochromatic light of wavelength λ, the intensity of light at a point on the screen where path difference is λ, is K units. Find out the intensity of light at a point where path difference is λ/3.

Concept: Interference of Light Waves and Young’S Experiment
Chapter: [6.02] Wave Optics
27

Use Huygens’s principle to explain the formation of diffraction pattern due to a single slit illuminated by a monochromatic source of light.

Concept: Huygens Principle
Chapter: [6.02] Wave Optics

When the width of the slit is made double the original width, how would this affect the size and intensity of the central diffraction band?

Concept: Huygens Principle
Chapter: [6.02] Wave Optics
28 | Attempt any one of the following.
28.1

Explain the principle of a device that can build up high voltages of the order of a few million volts.

Concept: Electric Potential Difference
Chapter: [1.01] Electrostatic Potential and Capacitance

Draw a schematic diagram and explain the working of Van de Graff generator device.

Concept: Electric Potential Difference
Chapter: [1.01] Electrostatic Potential and Capacitance

Is there any restriction on the upper limit of the high voltage set up in Van de Graff generator machine? Explain.

Concept: Electric Potential Difference
Chapter: [1.01] Electrostatic Potential and Capacitance
28.2
28.2.1

Define Electric Flux. Write its SI unit.

Concept: Electric Flux
Chapter: [1.02] Electric Charges and Fields
28.2.2

Using Gauss’s law, prove that the electric field at a point due to a uniformly charged infinite plane sheet is independent of the distance from it.

Concept: Uniformly Charged Infinite Plane Sheet and Uniformly Charged Thin Spherical Shell (Field Inside and Outside)
Chapter: [1.02] Electric Charges and Fields
28.2.3

How is the field directed if (i) the sheet is positively charged, (ii) negatively charged?

Concept: Uniformly Charged Infinite Plane Sheet and Uniformly Charged Thin Spherical Shell (Field Inside and Outside)
Chapter: [1.02] Electric Charges and Fields
29 | Attempt any one of the following.
29.1

Define magnifying power of a telescope. Write its expression.

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

A small telescope has an objective lens of focal length 150 cm and an eye piece of focal length 5 cm. If this telescope is used to view a 100 m high tower 3 km away, find the height of the final image when it is formed 25 cm away from the eye piece.

Concept: Optical Instruments - Telescope
Chapter: [6.01] Ray Optics and Optical Instruments
29.2

How is the working of a telescope different from that of a microscope?

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

The focal lengths of the objective and eyepiece of a microscope are 1.25 cm and 5 cm respectively. Find the position of the object relative to the objective in order to obtain an angular magnification of 30 in normal adjustment.

Concept: Optical Instruments - Telescope
Chapter: [6.01] Ray Optics and Optical Instruments
30 | Attempt any one of the following.
30.1

Draw a simple circuit of a CE transistor amplifier. Explain its working ?

Concept: Junction Transistor - Basic Transistor Circuit Configurations and Transistor Characteristics
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits

Show that the voltage gain, AV, of the amplifier is given by A_v = (beta_(ac) R_1)/r_iwhere βac is the current gain, RL is the load resistance and ri is the input resistance of the transistor. What is the significance of the negative sign in the expression for the voltage gain?

Concept: Junction Transistor - Basic Transistor Circuit Configurations and Transistor Characteristics
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
30.2
30.2.1

Draw the circuit diagram of a full wave rectifier using p-n junction diode.

Explain its working and show the output, input waveforms.

Concept: Special Purpose P-n Junction Diodes
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
30.2.2

Show the output waveforms (Y) for the following inputs A and B of (i) OR gate (ii) NAND gate ? Concept: Special Purpose P-n Junction Diodes
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits

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