# Physics Delhi set 3 2009-2010 CBSE (Science) Class 12 Question Paper Solution

Physics [Delhi set 3]
Date: March 2010

[1] 1

A plot of magnetic flux (Φ) versus current (I) is shown in the figure for two inductors A and Β. Which of the two has larger value of self inductance?

Concept: Inductance - Self-Inductance
Chapter: [4.02] Electromagnetic Induction
[1] 2

Figure shows three point charges +2q, −q and + 3q. Two charges + 2q and −q are enclosed within a surface ‘S’. What is the electric flux due to this configuration through the surface ‘S’?

Concept: Electric Flux
Chapter: [1.02] Electric Charges and Fields
[1] 3

In which orientation, a dipole placed in a uniform electric field is in (i) stable, (ii) unstable equilibrium?

Concept: Electric Dipole
Chapter: [1.02] Electric Charges and Fields
[1] 4

Which part of electromagnetic spectrum is used in radar systems?

Concept: Electromagnetic Spectrum
Chapter: [5.01] Electromagnetic Waves
[1] 5

Calculate the speed of light in a medium whose critical angle is 30° ?

Concept: Total Internal Reflection
Chapter: [6.01] Ray Optics and Optical Instruments
[1] 6

A glass lens of refractive index 1.45 disappears when immersed in a liquid. What is the value of refractive index of the liquid?

Concept: Refraction
Chapter: [6.01] Ray Optics and Optical Instruments
[1] 7

Write the expression for Bohr’s radius in hydrogen atom ?

Concept: Bohr’s Model for Hydrogen Atom
Chapter: [8.02] Atoms
[1] 8

A wire of resistance 8R is bent in the form of a circle. What is the effective resistance between the ends of a diameter AB?

Concept: Electric Current
Chapter: [2.01] Current Electricity
[2] 9

Explain the function of a repeater in a communication system.

Concept: Elements of a Communication System
Chapter: [10.01] Communication Systems
[2] 10 | Attempt any one of the following.
[2] 10.1
[1] 10.1.1

Write two characteristics of a material used for making permanent magnets ?

Concept: Permanent Magnets and Electromagnets
Chapter: [3.01] Magnetism and Matter
[1] 10.1.2

Why is the core of an electromagnet made of ferromagnetic materials?

Concept: Permanent Magnets and Electromagnets
Chapter: [3.01] Magnetism and Matter
[2] 10.2

Draw magnetic field line when a (i) diamagnetic, (ii) paramagnetic substance is placed in an external magnetic field. Which magnetic property distinguishes this behaviour of the field line due to the substances?

Concept: Magnetic Properties of Materials
Chapter: [3.01] Magnetism and Matter
[2] 11

What is the range of frequencies used in satellite communication?

Concept: Satellite Communication
Chapter: [10.01] Communication Systems

What is common between these waves and light waves?

Concept: Satellite Communication
Chapter: [10.01] Communication Systems
[2] 12

A coil Q is connected to low voltage bulb B and placed near another coil P as shown in the figure. Give reasons to explain the following observations:

(a) The bulb ‘B’ lights

(b) Bulb gets dimmer if the coil Q is moved towards left.

Concept: Representation of Ac Current and Voltage by Rotating Vectors - Phasors
Chapter: [4.01] Alternating Current
[2] 13

Find the radius of curvature of the convex surface of a plano-convex lens, whose focal length is 0.3 m and the refractive index of the material of the lens is 1.5.

Concept: Power of a Lens
Chapter: [6.01] Ray Optics and Optical Instruments
[2] 14

An electron is accelerated through a potential difference of 64 volts. What is the de-Broglie wavelength associated with it? To which part of the electromagnetic spectrum does this value of wavelength correspond?

Concept: de-Broglie Relation
Chapter: [7.01] Dual Nature of Radiation and Matter
[2] 15
[1] 15.1

Out of blue and red light which is deviated more by a prism? Give reason.

Concept: Dispersion by a Prism
Chapter: [6.01] Ray Optics and Optical Instruments
[1] 15.2

Give the formula that can be used to determine refractive index of materials of a prism in minimum deviation condition ?

Concept: Dispersion by a Prism
Chapter: [6.01] Ray Optics and Optical Instruments
[2] 16

Plot a graph showing the variation of stopping potential with the frequency of incident radiation for two different photosensitive materials having work functions W1 and W2 (W1 > W2). On what factors does the (i) slope and (ii) intercept of the lines depend?

Concept: Resistivity of Various Materials
Chapter: [2.01] Current Electricity
[2] 17

Draw the circuit diagram of an illuminated photodiode in reverse bias. How is photodiode used to measure light intensity?

Concept: Special Purpose P-n Junction Diodes
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
[2] 18

A heavy nucleus X of mass number 240 and binding energy per nucleon 7.6 MeV is split into two fragments Y and Z of mass numbers 110 and 130. The binding energy of nucleons in Y and Z is 8.5 MeV per nucleon. Calculate the energy Q released per fission in MeV.

Concept: Mass-energy and Nuclear Binding Energy - Mass - Energy
Chapter: [8.01] Nuclei
[3] 19
[1] 19.1

Depict the equipotential surfaces for a system of two identical positive point charges placed a distance(d) apart?

Concept: Equipotential Surfaces
Chapter: [1.01] Electrostatic Potential and Capacitance
[2] 19.2

Deduce the expression for the potential energy of a system of two point charges q1 and qbrought from infinity to the points vecr_1and vecr_2respectively in the presence of external electric field vecE.

Concept: Potential Energy in an External Field - Potential Energy of a System of Two Charges in an External Field
Chapter: [1.01] Electrostatic Potential and Capacitance
[3] 20

In a meter bridge, the null point is found at a distance of l1 cm from A. If now a resistance of X is connected in parallel with S, the null point occurs at l2 cm. Obtain a formula for X in terms of l1l2 and S.

Concept: Metre Bridge
Chapter: [2.01] Current Electricity
[3] 21

What is space wave propagation?

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

Give two examples of communication system which use space wave mode.
A TV tower is 80 m tall. Calculate the maximum distance upto which the signal transmitted from the tower can be received.

Concept: Sinusoidal Waves
Chapter: [10.01] Communication Systems
[3] 22
[1] 22.1

Define ‘activity’ of a radioactive material and write its S.I. units.

Chapter: [8.01] Nuclei
[1] 22.2

Plot a graph showing variation of activity of a given radioactive sample with time.

Chapter: [8.01] Nuclei
[1] 22.3

The sequence of stepwise decay of a radioactive nucleus is

If the atomic number and mass number of D2 are 71 and 176 respectively, what are their corresponding values of D?

Chapter: [8.01] Nuclei
[3] 23

What is an unpolarized light? Explain with the help of suitable ray diagram how an unpolarized light can be polarized by reflection from a transparent medium. Write the expression for Brewster angle in terms of the refractive index of denser medium.

Concept: Plane Polarised Light
Chapter: [6.02] Wave Optics
[3] 24

Write the principle of working of a potentiometer. Describe briefly, with the help of a circuit diagram, how a potentiometer is used to determine the internal resistance of a given cell.

Concept: Potentiometer
Chapter: [2.01] Current Electricity
[3] 25

Write the expression for the magnetic moment vecmdue to a planar square loop of side ‘l’ carrying a steady current I in a vector form.

In the given figure this loop is placed in a horizontal plane near a long straight conductor carrying a steady current I1 at a distance l as shown. Give reason to explain that the loop will experience a net force but no torque. Write the expression for this force acting on the loop.

Concept: Torque on a Current Loop in Magnetic Field
Chapter: [3.02] Moving Charges and Magnetism
[3] 26 | Attempt any one of the following.
[3] 26.1

A long straight wire of a circular cross-section of radius ‘a’ carries a steady current ‘I’. The current is uniformly distributed across the cross-section. Apply Ampere’s circuital law to calculate the magnetic field at a point ‘r’ in the region for (i) r < a and (ii) r > a.

Concept: Ampere’s Circuital Law
Chapter: [3.02] Moving Charges and Magnetism
[3] 26.2

State the underlying principle of working of a moving coil galvanometer. Write two reasons why a galvanometer can not be used as such to measure current in a given circuit. Name any two factors on which the current sensitivity of a galvanometer depends.

Concept: Moving Coil Galvanometer
Chapter: [3.02] Moving Charges and Magnetism
[3] 27

A parallel-plate capacitor is charged to a potential difference V by a dc source. The capacitor is then disconnected from the source. If the distance between the plates is doubled, state with reason how the following change:

(i) electric field between the plates

(ii) capacitance, and

(iii) energy stored in the capacitor

Concept: The Parallel Plate Capacitor
Chapter: [1.01] Electrostatic Potential and Capacitance
[5] 28 | Attempt any one of the following.
[5] 28.1
[2] 28.1.1

Explain the formation of depletion layer and potential barrier in a p−n junction.

Concept: Special Purpose P-n Junction Diodes
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
[2] 28.1.2

In the figure given below the input waveform is converted into the output waveform by a device ‘X’. Name the device and draw its circuit diagram.

Concept: Integrated Circuits
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
[1] 28.1.3

Identify the logic gate represented by the circuit as shown and write its truth table.

Concept: Digital Electronics and Logic Gates
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
[5] 28.2
[2] 28.2.1

With the help of circuit diagram explain the working principle of a transistor amplifier as an oscillator.

Concept: Junction Transistor - Feedback Amplifier and Transistor Oscillator
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
[3] 28.2.2

Distinguish between a conductor, a semiconductor and an insulator on the basis of energy band diagrams.

Concept: Energy Bands in Conductors, Semiconductors and Insulators
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
[5] 29 | Attempt any one of the following.
[5] 29.1

Describe briefly, with the help of a labelled diagram, the basic elements of an A.C. generator.

Concept: Ac Generator
Chapter: [4.02] Electromagnetic Induction

State A.C. generator underlying principle?

Concept: Ac Generator
Chapter: [4.02] Electromagnetic Induction

Show diagrammatically how an alternating emf is generated by a loop of wire rotating in a magnetic field. Write the expression for the instantaneous value of the emf induced in the rotating loop.

Concept: Electromagnetic Induction
Chapter: [4.02] Electromagnetic Induction [4.02] Electromagnetic Induction
[5] 29.2

A series LCR circuit is connected to a source having voltage v = vm sin ωt. Derive the expression for the instantaneous current I and its phase relationship to the applied voltage.

Obtain the condition for resonance to occur. Define ‘power factor’. State the conditions under which it is (i) maximum and (ii) minimum.

Concept: Ac Voltage Applied to a Series Lcr Circuit
Chapter: [4.01] Alternating Current
[5] 30 | Attempt any one of the following.
[5] 30.1

State Huygens’s principle. Show, with the help of a suitable diagram, how this principle is used to obtain the diffraction pattern by a single slit.

Draw a plot of intensity distribution and explain clearly why the secondary maxima becomes weaker with increasing order (n) of the secondary maxima.

Concept: Huygens Principle
Chapter: [6.02] Wave Optics
[5] 30.2

Draw a ray diagram to show the working of a compound microscope. Deduce an expression for the total magnification when the final image is formed at the near point.

In a compound microscope, an object is placed at a distance of 1.5 cm from the objective of focal length 1.25 cm. If the eye piece has a focal length of 5 cm and the final image is formed at the near point, estimate the magnifying power of the microscope.

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

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