# Physics Delhi set 1 2008-2009 CBSE (Science) Class 12 Question Paper Solution

Physics [Delhi set 1]
Date: March 2009

[1] 1

Propagation of Electromagnetic Wave?

Concept: Propagation of Electromagnetic Waves
Chapter: [10.01] Communication Systems
[1] 2

Write the following radiations in ascending order with respect to their frequencies:

X-rays, microwaves, UV rays and radio waves.

Concept: Electromagnetic Waves
Chapter: [5.01] Electromagnetic Waves
[1] 3

Magnetic field lines can be entirely confined within the core of a toroid, but not within a straight solenoid. Why?

Concept: Solenoid and the Toroid - the Toroid
Chapter: [3.02] Moving Charges and Magnetism
[1] 4

You are given following three lenses. Which two lenses will you use as an eyepiece and as an objective to construct an astronomical telescope?

 Lens Power (P) Aperture (A) L1 3D 8 cm L2 6D 1 cm L3 10D 1 cm

Concept: Diffraction - Resolving Power of Microscope and Astronomical Telescope
Chapter: [6.02] Wave Optics
[1] 5

If the angle between the pass axis of polarizer and the analyser is 45º, write the ratio of the intensities of original light and the transmitted light after passing through the analyser.

Concept: Plane Polarised Light
Chapter: [6.02] Wave Optics
[1] 6

The figure shows a plot of three curves a, b, c, showing the variation of photocurrent vs collector plate potential for three different intensities I1, I2and I3 having frequencies v1v2 and v3 respectively incident of a photosensitive surface.

Point out the two curves for which the incident radiations have same frequency but different intensities.

Concept: Electric Potential Difference
Chapter: [1.01] Electrostatic Potential and Capacitance
[1] 7

What type of wavefront will emerge from a (i) point source, and (ii) distance light source?

Concept: Refraction at Spherical Surfaces and Lenses - Refraction at Spherical Surfaces
Chapter: [6.01] Ray Optics and Optical Instruments
[1] 8

Two nuclei have mass numbers in the ratio 1: 2. What is the ratio of their nuclear densities?

Concept: Atomic Masses and Composition of Nucleus
Chapter: [8.01] Nuclei
[2] 9

A cell of emf ‘E’ and internal resistance ‘r’ is connected across a variable resistor ‘R’. Plot a graph showing the variation of terminal potential ‘V’ with resistance R. Predict from the graph the condition under which ‘V’ becomes equal to ‘E’.

Concept: Cells, Emf, Internal Resistance
Chapter: [2.01] Current Electricity
[2] 10
[1] 10.1

Can two equi-potential surfaces intersect each other? Give reasons.

Concept: Electric Potential
Chapter: [1.01] Electrostatic Potential and Capacitance
[1] 10.2

Two charges −q and +q are located at points A (0, 0, −a) and B (0, 0, +a) respectively. How much work is done in moving a test charge from point P (7, 0, 0) to Q (−3, 0, 0)?

Concept: Electric Potential
Chapter: [1.01] Electrostatic Potential and Capacitance
[2] 11

By what percentage will the transmission ranges of TV tower be affected when the height of the tower is increased by 21%?

Concept: Bandwidth of Transmission Medium
Chapter: [10.01] Communication Systems
[2] 12

Derive an expression for drift velocity of free electrons in a conductor in terms of relaxation time.

Concept: Drift of Electrons and the Origin of Resistivity
Chapter: [2.01] Current Electricity
[2] 13

How does a charge q oscillating at certain frequency produce electromagnetic waves?

Sketch a schematic diagram depicting electric and magnetic fields for an electromagnetic wave propagating along the Z-direction.

Concept: Electromagnetic Waves
Chapter: [5.01] Electromagnetic Waves
[2] 14

A charge ‘q’ moving along the X- axis with a velocity  vecv is subjected to a uniform magnetic field B along the Z-axis as it crosses the origin O.

(i) Trace its trajectory.

(ii) Does the charge gain kinetic energy as it enters the magnetic field? Justify your answer.

Concept: Magnetic Force
Chapter: [3.02] Moving Charges and Magnetism
[2] 15

The following figure shows the input waveforms (A, B) and the output waveform (Y) of a gate. Identify the gate, write its truth table and draw its logic symbol.

Concept: Digital Electronics and Logic Gates
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
[2] 16

State Biot-Savart law.

A current I flows in a conductor placed perpendicular to the plane of the paper. Indicate the direction of the magnetic field due to a small element d vecl at point P situated at distance vecr from the element as shown in the figure.

Concept: Magnetic Field Due to a Current Element, Biot-Savart Law
Chapter: [3.02] Moving Charges and Magnetism
[2] 17 | Attempt any one of the following.
[2] 17.1

Why are high frequency carrier waves used for transmission?

Concept: Bandwidth of Transmission Medium
Chapter: [10.01] Communication Systems
[2] 17.2

What is meant by term ‘modulation’? Draw a block diagram of a simple modulator for obtaining an AM signal.

Concept: Amplitude Modulation
Chapter: [10.01] Communication Systems
[2] 18

A radioactive nucleus ‘A’ undergoes a series of decays according to the following scheme:

The mass number and atomic number of A are 180 and 72 respectively. What are these numbers for A4?

Chapter: [8.01] Nuclei
[3] 19

A thin conducting spherical shell of radius R has charge Q spread uniformly over its surface. Using Gauss’s law, derive an expression for an electric field at a point outside the shell.

Concept: Gauss’s Law
Chapter: [1.02] Electric Charges and Fields

Draw a graph of electric field E(r) with distance r from the centre of the shell for 0 ≤ r ≤ ∞.

Concept: Gauss’s Law
Chapter: [1.02] Electric Charges and Fields
[3] 20

Three identical capacitors C1, C2 and C3 of capacitance 6 μF each are connected to a 12 V battery as shown.

Find

(i) charge on each capacitor

(ii) equivalent capacitance of the network

(iii) energy stored in the network of capacitors

Concept: Capacitors and Capacitance
Chapter: [1.01] Electrostatic Potential and Capacitance
[3] 21
[2] 21.1

The energy levels of an atom are as shown below. Which of them will result in the transition of a photon of wavelength 275 nm?

Concept: Energy Levels
Chapter: [8.02] Atoms
[1] 21.2

Which transition corresponds to emission of radiation of maximum wavelength?

Concept: Energy Levels
Chapter: [8.02] Atoms
[3] 22

A proton and an alpha particle are accelerated through the same potential. Which one of the two has (i) greater value of de−Broglie wavelength associated with it and (ii) less kinetic energy. Give reasons to justify your answer.

Concept: de-Broglie Relation
Chapter: [7.01] Dual Nature of Radiation and Matter
[3] 23

In a single slit diffraction experiment, when tiny circular obstacle is placed in path of light from a distance source, a bright spot is seen at the centre of the shadow of the obstacle. Explain why?

Concept: Diffraction - The Single Slit
Chapter: [6.02] Wave Optics

State two points of difference between the interference patterns obtained in Young’s double slit experiment and the diffraction pattern due to a single slit.

Concept: Diffraction - The Single Slit
Chapter: [6.02] Wave Optics
[3] 24

Define self inductance. Write its S.I. units.

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

Derive an expression for self inductance of a long solenoid of length l, cross-sectional area A having N number of turns.

Concept: Inductance - Self-Inductance
Chapter: [4.02] Electromagnetic Induction
[3] 25

The figure shows experimental set up of a meter bridge. When the two unknown resistances X and Y are inserted, the null point D is obtained 40 cm from the end A. When a resistance of 10 Ω is connected in series with X, the null point shifts by 10 cm. Find the position of the null point when the 10 Ω resistance is instead connected in series with resistance ‘Y’. Determine the values of the resistances X and Y.

Concept: Metre Bridge
Chapter: [2.01] Current Electricity
[3] 26 | Attempt any one of the following.
[3] 26.1

Derive the expression for force per unit length between two long straight parallel current carrying conductors. Hence define one ampere.

Concept: Force Between Two Parallel Currents, the Ampere
Chapter: [3.02] Moving Charges and Magnetism
[3] 26.2

Explain the principle and working of a cyclotron with the help of a schematic diagram. Write the expression for cyclotron frequency.

Concept: Motion in Combined Electric and Magnetic Fields - Cyclotron
Chapter: [3.02] Moving Charges and Magnetism
[3] 27

Three light rays red (R), green (G) and blue (B) are incident on a right angled prism ‘abc’ at face ‘ab’. The refractive indices of the material of the prism for red, green and blue wavelengths are 1.39, 1.44 and 1.47 respectively. Out of the three which colour ray will emerge out of face ‘ac’? Justify your answer. Trace the path of these rays after passing through face ‘ab’.

Concept: Refraction Through a Prism
Chapter: [6.01] Ray Optics and Optical Instruments
[5] 28 | Attempt any one of the following.
[5] 28.1
[3] 28.1.1

Derive an expression for the average power consumed in a series LCR circuit connected to a.c. source in which the phase difference between the voltage and the current in the circuit is Φ.

Concept: Ac Voltage Applied to a Series Lcr Circuit
Chapter: [4.01] Alternating Current
[2] 28.1.2

Define the quality factor in an a.c. circuit. Why should the quality factor have high value in receiving circuits? Name the factors on which it depends.

Concept: Power in Ac Circuit: the Power Factor
Chapter: [4.01] Alternating Current
[5] 28.2
[2] 28.2.1

Derive the relationship between the peak and the rms value of current in an a.c. circuit.

Concept: Alternating Currents
Chapter: [4.01] Alternating Current
[3] 28.2.2

Describe briefly, with the help of labelled diagram, working of a step-up transformer.

A step-up transformer converts a low voltage into high voltage. Does it not violate the principle of conservation of energy? Explain.

Concept: Transformers
Chapter: [4.01] Alternating Current
[5] 29 | Attempt any one of the following.
[5] 29.1
[3] 29.1.1

Draw a circuit diagram to study the input and output characteristics of an n-p-n transistor in its common emitter configuration. Draw the typical input and output characteristics.

Concept: p-n Junction
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
[2] 29.1.2

Explain, with the help of a circuit diagram, the working of n-p-n transistor as a common emitter amplifier.

Concept: p-n Junction
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
[5] 29.2

How is a zener diode fabricated so as to make it a special purpose diode? Draw I-V characteristics of zener diode and explain the significance of breakdown voltage.

Explain briefly, with the help of a circuit diagram, how a p-n junction diode works as a half wave rectifier.

Concept: p-n Junction
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
[5] 30 | Attempt any one of the following.
[3] 30.1

Trace the rays of light showing the formation of an image due to a point object placed on the axis of a spherical surface separating the two media of refractive indices n1 and n2. Establish the relation between the distances of the object, the image and the radius of curvature from the central point of the spherical surface.

Hence derive the expression of the lens maker’s formula.

Concept: Thin Lens Formula
Chapter: [6.01] Ray Optics and Optical Instruments
[2] 30.2

Draw the labelled ray diagram for the formation of image by a compound microscope.

Derive the expression for the total magnification of a compound microscope. Explain why both the objective and the eyepiece of a compound microscope must have short focal lengths.

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

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