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# Physics Foreign Set 3 2013-2014 CBSE (Arts) Class 12 Question Paper Solution

Physics [Foreign Set 3]
Date: March 2014

1

Figure shows the field lines due to a positive point charge. Give the sign of potential energy difference of a small negative charge between the points Q and P. Concept: Potential Energy of a System of Charges
Chapter: [1.01] Electrostatic Potential and Capacitance
2

In both β and β+ decay processes, the mass number of a nucleus remains the same, whereas the atomic number Z increases by one in β decay and decreases by one in β+ decay. Explain giving reason.

Concept: Atomic Spectra
Chapter: [8.02] Atoms
3

Why must electrostatic field at the surface of a charged conductor be normal to the surface at every point? Give reason?

Concept: Electric Field Due to a Point Charge
Chapter: [1.02] Electric Charges and Fields
4

A triangular loop of wire placed at abc is moved completely inside a magnetic field which is directed normal to the plane of the loop away from the reader to a new position a'b'c'. What is the direction of the current induced in the loop? Give reason. Concept: Current Loop as a Magnetic Dipole and Its Magnetic Dipole Moment
Chapter: [3.01] Magnetism and Matter
5

An electron in an atom revolves round the nucleus in an orbit of radius r with frequency v. Write the expression for the magnetic moment of the electron.

Concept: Alpha-particle Scattering and Rutherford’S Nuclear Model of Atom
Chapter: [8.02] Atoms
6

A ray of light falls on a transparent sphere with centre C as shown in the figure. The ray emerges from the sphere parallel to the line AB. Find the angle of refraction at A if the refractive index of the material of the sphere is $\sqrt{3}$. Concept: The Parallel Plate Capacitor
Chapter: [1.01] Electrostatic Potential and Capacitance
7

Arrange the following electromagnetic waves in decreasing order of wavelength:

γ-rays, infrared rays, X-rays and microwaves.
Concept: Electromagnetic Waves
Chapter: [5.01] Electromagnetic Waves
8

Plot a graph showing the variation of photoelectric current with collector plate potential at a given frequency but for two different intensities I1 and I2, where I2 > I1.

Concept: Einstein’S Photoelectric Equation: Energy Quantum of Radiation
Chapter: [7.01] Dual Nature of Radiation and Matter
9

In the circuit shown in the figure, find the total resistance of the circuit and the current in the arm AD. Concept: Ohm’s Law
Chapter: [2.01] Current Electricity
10

Figure shows a ray of light passing through a prism. If the refracted ray QR is parallel to the base BC, show that (i) r1 = r2 = A/2 and (ii) angle of minimum deviation, Dm = 2i − A. Concept: Dispersion by a Prism
Chapter: [6.01] Ray Optics and Optical Instruments
11 | Attempt any one of the following.
11.1

Draw energy band diagrams of an n-type and p-type semiconductor at temperature T > 0 K. Mark the donor and acceptor energy levels with their energies.

Concept: Energy Bands in Conductors, Semiconductors and Insulators
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
11.2

Distinguish between a metal 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
12

Show that the current leads the voltage in phase by π/2 in an AC circuit containing an ideal capacitor ?

Concept: Ac Voltage Applied to a Capacitor
Chapter: [4.01] Alternating Current
13

Two very small identical circular loops, (1) and (2), carrying equal currents I are placed vertically (with respect to the plane of the paper) with their geometrical axes perpendicular to each other as shown in the figure. Find the magnitude and direction of the net magnetic field produced at the point O. Concept: Torque on a Current Loop in Magnetic Field
Chapter: [3.02] Moving Charges and Magnetism
14
14.1

How does oscillating charge produce electromagnetic waves?

Concept: Electromagnetic Waves
Chapter: [5.01] Electromagnetic Waves
14.2

Sketch a schematic diagram depicting oscillating electric and magnetic fields of an em wave propagating along + z-direction ?

Concept: Motion in a Magnetic Field
Chapter: [3.02] Moving Charges and Magnetism
15

Give two points to distinguish between a paramagnetic and a diamagnetic substance ?

Concept: Magnetisation and Magnetic Intensity
Chapter: [3.01] Magnetism and Matter
16

Identify the gate equivalent to the circuit shown in the figure. Write its truth table. Concept: Integrated Circuits
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
17

Define the term modulation. Draw a block diagram of a simple modulator for obtaining AM signal ?

Concept: Modulation and Its Necessity
Chapter: [10.01] Communication Systems
18

A circuit is set up by connecting inductance L = 100 mH, resistor R = 100 Ω and a capacitor of reactance 200 Ω in series. An alternating emf of $150\sqrt{2}$  V, 500/π Hz is applies across this series combination. Calculate the power dissipated in the resistor.

Concept: Combination of Capacitors
Chapter: [1.01] Electrostatic Potential and Capacitance
19 | Attempt any one of the following.
19.1
19.1.1

Why is zener diode fabricated by heavily doping both p- and n-sides of the junction?

Concept: Special Purpose P-n Junction Diodes
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
19.1.2

Draw the circuit diagram of zener diode as a voltage regulator and briefly explains its working ?

Concept: Zener Diode as a Voltage Regulator
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
19.2
19.2.1

How is a photodiode fabricated?

Concept: Special Purpose P-n Junction Diodes
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
19.2.2

Briefly explain its working. Draw its V - I characteristics for two different intensities of illumination ?

Concept: Special Purpose P-n Junction Diodes
Chapter: [9.01] Semiconductor Electronics - Materials, Devices and Simple Circuits
20
20.1

Two long straight parallel conductors 'a' and 'b', carrying steady currents Ia and Ib are separated by a distance d. Write the magnitude and direction of the magnetic field produced by the conductor 'a' at the points along the conductor 'b'. If the currents are flowing in the same direction, what is the nature and magnitude of the force between the two conductors?

Concept: Force Between Two Parallel Currents, the Ampere
Chapter: [3.02] Moving Charges and Magnetism
20.2

Show with the help of a diagram how the force between the two conductors would change when the currents in them flow in the opposite directions?

Concept: Motion in a Magnetic Field
Chapter: [3.02] Moving Charges and Magnetism
21
21.1

Describe briefly how the Davisson-Germer experiment demonstrated the wave nature of electrons.

Concept: Wave Nature of Matter
Chapter: [7.01] Dual Nature of Radiation and Matter
21.2

An electron is accelerated from rest through a potential V. Obtain the expression for the de-Broglie wavelength associated with it ?

Concept: de-Broglie Relation
Chapter: [7.01] Dual Nature of Radiation and Matter
22
22.1

A point charge (+Q) is kept in the vicinity of an uncharged conducting plate. Sketch the electric field lines between the charge and the plate?

Concept: Electric Field Lines
Chapter: [1.02] Electric Charges and Fields
22.2

Two infinitely large plane thin parallel sheets having surface charge densities σ1 and σ2 (σ1 > σ2) are shown in the figure. Write the magnitudes and directions of the net fields in the regions marked II and III. Concept: Force Between Two Parallel Currents, the Ampere
Chapter: [3.02] Moving Charges and Magnetism
23

When Puja, a student of 10th class, watched her mother washing clothes in the open, she observed coloured soap bubbles and was curious to know why the soap bubbles appear coloured. In the evening when her father, an engineer by profession, came home, she asked him this question. Her father explained to her the basic phenomenon of physics due to which the soap bubbles appear coloured.
(a) What according to you are the values displayed by Puja and her father?
(b) State the phenomenon of light involved in the formation of coloured soap bubbles.

Concept: Electromagnetic Induction
Chapter: [4.02] Electromagnetic Induction
24

Three concentric metallic shells A, B and C or radii a, b and c (a < b < c) have surface charge densities + σ,  −σ and + σ, respectively as shown in the figure If shells A and C are at the same potential, then obtain the relation between the radii a, b and c.

Concept: Flow of Electric Charges in a Metallic Conductor
Chapter: [2.01] Current Electricity
25

A toroidal solenoid with air core has an average radius of 15 cm, area of cross-section 12 cm2 and has 1200 turns. Calculate the self-inductance of the toroid. Assume the field to be uniform across the cross-section of the toroid.

Concept: Inductance - Self-Inductance
Chapter: [4.02] Electromagnetic Induction
26

Explain, with the help of a suitable diagram, the space wave mode of propagation. Give two examples in communication systems where this mode is used. What is the frequency range of these waves? Give reason for using this range of frequency.

Concept: Elements of a Communication System
Chapter: [10.01] Communication Systems
27
27.1

Distinguish between unpolarised and linearly polarised light.

Concept: Plane Polarised Light
Chapter: [6.02] Wave Optics
27.2

A partially plane polarised beam of light is passed through a polaroid. Show graphically the variation of the transmitted light intensity with angle of rotation of the polaroid ?

Concept: Plane Polarised Light
Chapter: [6.02] Wave Optics
27.3

Explain with the help of a diagram how sunlight is polarised by scattering through atmospheric particles ?

Concept: Plane Polarised Light
Chapter: [6.02] Wave Optics
28 | Attempt any one of the following.
28.1
28.1.1

Draw a labelled ray diagram of an astronomical telescope to show the image formation of a distant object. Write the main considerations required in selecting the objective and eyepiece lenses in order to have large magnifying power and high resolution of the telescope.

Concept: Optical Instruments - Telescope
Chapter: [6.01] Ray Optics and Optical Instruments
28.1.2

A compound microscope has an objective of focal length 1.25 cm and eyepiece of focal length 5 cm. A small object is kept at 2.5 cm from the objective. If the final image formed is at infinity, find the distance between the objective and the eyepiece ?

Concept: Optical Instruments - The Microscope
Chapter: [6.01] Ray Optics and Optical Instruments
28.2
28.2.1

Write three characteristic features to distinguish between the interference fringes in Young's double slit experiment and the diffraction pattern obtained due to a narrow single slit.

Concept: Interference of Light Waves and Young’S Experiment
Chapter: [6.02] Wave Optics
28.2.2

A parallel beam of light of wavelength 500 nm falls on a narrow slit and the resulting diffraction pattern is observed on a screen 1 m away. It is observed that the first minimum is a distance of 2.5 mm away from the centre. Find the width of the slit.

Concept: Interference of Light Waves and Young’S Experiment
Chapter: [6.02] Wave Optics
29 | Attempt any one of the following.
29.1
29.1.1

Using Bohr's postulates, derive the expression for the total energy of the electron in the stationary states of the hydrogen atom ?

Concept: Bohr'S Model for Hydrogen Atom
Chapter: [8.02] Atoms
29.1.2

Using Rydberg formula, calculate the wavelengths of the spectral lines of the first member of the Lyman series and of the Balmer series.

Concept: Hydrogen Spectrum
Chapter: [8.02] Atoms
29.2
29.2.1

Define the terms (i) half-life (T1/2) and (ii) average life (τ). Find out their relationships with the decay constant (λ).

Concept: Mass-energy and Nuclear Binding Energy - Nuclear Binding Energy
Chapter: [8.01] Nuclei
29.2.2

A radioactive nucleus has a decay constant λ = 0.3465 (day)–1. How long would it take the nucleus to decay to 75% of its initial amount?

Chapter: [8.01] Nuclei
30 | Attempt any one of the following.
30.1
30.1.1

State the principle of a potentiometer. Define potential gradient. Obtain an expression for potential gradient in terms of resistivity of the potentiometer wire.

Concept: Potentiometer
Chapter: [2.01] Current Electricity
30.1.2

Figure shows a long potentiometer wire AB having a constant potential gradient. The null points for the two primary cells of emfs ε1 and ε2 connected in the manner shown are obtained at a distance of l1 = 120 cm and l2 = 300 cm from the end A. Determine (i) ε12 and (ii) position of null point for the cell ε1 only. Concept: Potentiometer
Chapter: [2.01] Current Electricity
30.2
30.2.1

Define the term 'drift velocity' of charge carriers in a conductor. Obtain the expression for the current density in terms of relaxation time ?

Concept: Electric Current
Chapter: [2.01] Current Electricity
30.2.2

A 100 V battery is connected to the electric network as shown. If the power consumed in the 2 Ω resistor is 200 W, determine the power dissipated in the 5 Ω resistor. Concept: Electric Current
Chapter: [2.01] Current Electricity

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