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Question Paper Solutions - Physics 2017 - 2018-CBSE 12th-Class 12 CBSE (Central Board of Secondary Education)

Alternate Sets

   

Marks: 70
[1]1

A proton and an electron travelling along parallel paths enter a region of uniform magnetic field, acting perpendicular to their paths. Which of them will move in a circular path with higher frequency?

Chapter: [3.01] Moving Charges and Magnetism
Concept: Motion in Combined Electric and Magnetic Fields - Cyclotron
[1]2

Name the electromagnetic radiations used for (a) water purification, and (b) eye surgery.

Chapter: [14] Electromagnetic Waves
Concept: Electromagnetic Spectrum
[1]3

Draw graphs showing variation of photoelectric current with applied voltage for two incident radiations of equal frequency and different intensities. Mark the graph for the radiation of higher intensity.

Chapter: [7.01] Dual Nature of Radiation and Matter
Concept: Experimental Study of Photoelectric Effect
[1]4

Four nuclei of an element undergo fusion to form a heavier nucleus, with release of energy. Which of the two – the parent or the daughter nucleus – would have higher binding energy per nucleon?

Chapter: [8.02] Nuclei
Concept: Radioactivity - Introduction of Radioactivity
[1]5

Which mode of propagation is used by short wave broadcast serves?

Chapter: [10.01] Communication Systems
Concept: Propagation of Electromagnetic Waves
[2]6

Two electric bulbs P and Q have their resistances in the ratio of 1 : 2. They are connected in series across a battery. Find the ratio of the power dissipation in these bulbs

Chapter: [2.01] Current Electricity
Concept: Combination of Resistors – Series and Parallel
[2]7 | Attempt Any One
[2]7.1

A 10 V cell of negligible internal resistance is connected in parallel across a battery of emf 200 V and internal resistance 38 Ω as shown in the figure. Find the value of current in the circuit.

Chapter: [11] Current Electricity
Concept: Cells, Emf, Internal Resistance
[2]7.2

In a potentiometer arrangement for determining the emf of a cell, the balance point of the cell in open circuit is 350 cm. When a resistance of 9 Ω is used in the external circuit of the cell, the balance point shifts to 300 cm. Determine the internal resistance of the cell.

Chapter: [11] Current Electricity
Concept: Cells, Emf, Internal Resistance
[2]8

Why are infra-red waves often called heat waves ? Explain.

Chapter: [14] Electromagnetic Waves
Concept: Electromagnetic Spectrum

What do you understand by the statement, "Electromagnetic waves transport momentum"?

Chapter: [14] Electromagnetic Waves
Concept: Electromagnetic Spectrum
[2]9

If light of wavelength 412.5 nm is incident on each of the metals given below, which ones will show photoelectric emission and why?

Metal Work Function (eV)
Na 1.92
K 2.15
Ca 3.20
Mo 4.17
Chapter: [7.01] Dual Nature of Radiation and Matter
Concept: Photoelectric Effect and Wave Theory of Light
[2]10

A carrier wave of peak voltage 15 V is used to transmit a message signal. Find the peak voltage of the modulating signal in order to have a modulation index of 60%

Chapter: [10.01] Communication Systems
Concept: Modulation and Its Necessity
[3]11 | Attempt Any One
[3]11.1

Four point charges Q, q, Q and q are placed at the corners of a square of side 'a' as shown in the figure.

Find the

1) resultant electric force on a charge Q, and

2) potential energy of this system.

Chapter: [1.02] Electrostatic Potential and Capacitance
Concept: Potential Energy of a System of Charges
[3]11.2

Three-point charges q, – 4q and 2q are placed at the vertices of an equilateral triangle ABC of side 'l' as shown in the figure. Obtain the expression for the magnitude of the resultant electric force acting on the charge q

Chapter: [10] Electrostatics
Concept: Coulomb’s Law

Find out the amount of the work done to separate the charges at infinite distance.

Chapter: [1.02] Electrostatic Potential and Capacitance
Concept: Potential Energy of a System of Charges
[3]12

Define the term 'conductivity' of a metallic wire. Write its SI unit.

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

Using the concept of free electrons in a conductor, derive the expression for the conductivity of a wire in terms of number density and relaxation time. Hence obtain the relation between current density and the applied electric field E.

Chapter: [2] Current Electricity
Concept: Electrical Resistivity and Conductivity
[3]13

A bar magnet of magnetic moment 6 J/T is aligned at 60° with a uniform external magnetic field of 0·44 T Calculate  

a) the work done in turning the magnet to align its magnetic moment

i) normal to the magnetic field, 

ii) opposite to the magnetic field, and

b)  the torque on the magnet in the final orientation in case (ii).

Chapter: [3.01] Moving Charges and Magnetism
Concept: Magnetic Diapole
[3]14

An iron ring of relative permeability µr has windings of insulated copper wire of n turns per meter. When the current in the windings is I, find the expression for the magnetic field in the ring.

Chapter: [3.01] Moving Charges and Magnetism
Concept: Solenoid and the Toroid - the Toroid

The susceptibility of a magnetic material is 0·9853. Identify the type of magnetic material. Draw the modification of the field pattern on keeping a piece of this material in a uniform magnetic field.

Chapter: [3.02] Magnetism and Matter
Concept: Magnetic Properties of Materials
[3]15

Show using a proper diagram how unpolarised light can be linearly polarised by reflection from a transparent glass surface.

Chapter: [6.02] Wave Optics
Concept: Polarisation

The figure shows a ray of light falling normally on the face AB of an equilateral glass prism having refractive index`3/2`, placed in water of refractive index `4/3`.Will this ray suffer total internal reflection on striking the face AC? Justify your answer.

Chapter: [6.01] Ray Optics and Optical Instruments
Concept: Total Internal Reflection
[3]16

If one of two identical slits producing interference in Young’s experiment is covered with glass, so that the light intensity passing through it is reduced to 50%, find the ratio of the maximum and minimum intensity of the fringe in the interference pattern.

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

What kind of fringes do you expect to observe if white light is used instead of monochromatic light?

Chapter: [6.02] Wave Optics
Concept: Refraction of Monochromatic Light
[3]17

A symmetric biconvex lens of the radius of curvature R and made of glass of refractive index 1.5, is placed on a layer of liquid placed on top of a plane mirror as shown in the figure. An optical needle with its tip on the principal axis of the lens is moved along the axis until its real, inverted image coincides with the needle itself. The distance of the needle from the lens is measured to be x. On removing the liquid layer and repeating the experiment, the distance is found to be y. Obtain the expression for the refractive index of the liquid in terms of x and y

Chapter: [6.01] Ray Optics and Optical Instruments
Concept: Lenses
[3]18

State Bohr's postulate to define stable orbits in the hydrogen atom. How does de Broglie's hypothesis explain the stability of these orbits?

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

A hydrogen atom initially in the ground level absorbs a photon, which excites it to the = 4 level. Determine the wavelength and frequency of the photon.

Chapter: [8.01] Atoms
Concept: Energy Levels
[3]19

Explain the processes of nuclear fission and nuclear fusion by using the plot of binding energy per nucleon (BE/A) versus the mass number A

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

A radioactive isotope has a half-life of years. How long will it take the activity to reduce to a) 3.125%, b) 1% of its original value?

Chapter: [8.02] Nuclei
Concept: Radioactivity - Law of Radioactive Decay
[3]20

A student wants to use two p-n junction diodes to convert alternating current into direct current. Draw the labelled circuit diagram she would use and explain how it works.

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

Give the truth table and circuit symbol for NAND gate

Chapter: [9] Electronic Devices
Concept: Digital Electronics and Logic Gates
[3]21

Draw a typical input and output characteristics of an n-p-n transistor in CE configuration. Show how these characteristics can be used to determine (a) the input resistance (r1), and (b) current amplification factor (β)

Chapter: [19] Semiconductors
Concept: Transistor and Characteristics of a Transistor
[3]22

Give three reasons why modulation of a message signal is necessary for long-distance transmission

Chapter: [20] Communication Systems
Concept: Need for Modulation and Demodulation

Show graphically an audio signal, a carrier wave and an amplitude modulated wave

Chapter: [20] Communication Systems
Concept: Need for Modulation and Demodulation
[4]23

The teachers of Geeta’s school took the students on a study trip to a power generating station, located nearly 200 km away from the city. The teacher explained that electrical energy is transmitted over such a long distance to their city, in the form of alternating current (ac) raised to a high voltage. At the receiving end in the city, the voltage is reduced to operate the devices. As a result, the power loss is reduced. Geeta listened to the teacher and asked questions about how the ac is converted to a higher or lower voltage.

1) Name the device used to change the alternating voltage to a higher or lower value. State one cause for power dissipation in this device.

2) Explain with an example, how power loss is reduced if the energy is transmitted over long distances as an alternating current rather than a direct current.

3) Write two values each shown by the teachers and Geeta.

Chapter: [4.02] Alternating Current
Concept: Transformers
[5]24 | Attempt Any One
[5]24.1

Define electric flux.

Chapter: [1.01] Electric Charges and Fields
Concept: Electric Flux

A point charge q is at a distance of d/2 directly above the centre of a square of side d, as shown the figure. Use Gauss' law to obtain the expression for the electric flux through the square.

Chapter: [12] Electrostatics
Concept: Applications of Gauss’s Law

If the point charge is now moved to a distance 'd' from the centre of the square and the side of the square is doubled, explain how the electric flux will be affected.

Chapter: [12] Electrostatics
Concept: Applications of Gauss’s Law
[5]24.2

Use Gauss' law to derive the expression for the electric field `(vecE)`due to a straight uniformly charged infinite line of charge density λ C/m.

Chapter: [12] Electrostatics
Concept: Applications of Gauss’s Law

Draw a graph to show the variation of E with perpendicular distance r from the line of charge.

Chapter: [12] Electrostatics
Concept: Applications of Gauss’s Law

Find the work done in bringing a charge q from perpendicular distance r1 to r2 (r2 > r1)

Chapter: [12] Electrostatics
Concept: Applications of Gauss’s Law
[5]25 | Attempt Any One
[5]25.1

State the principle of an ac generator and explain its working with the help of a labelled diagram. Obtain the expression for the emf induced in a coil. having N turns each of cross-sectional area A, rotating with a constant angular speed 'ω' in a magnetic field `vecB` directed perpendicular to the axis of rotation.

Chapter: [4.01] Electromagnetic Induction
Concept: Ac Generator

An aeroplane is flying horizontally from west to east with a velocity of 900 km/hour. Calculate the potential difference developed between the ends of its wings having a span of 20 m. The horizontal component of the Earth's magnetic field is 5 × 10–4 T and the angle of dip is 30°.

Chapter: [4.01] Electromagnetic Induction
Concept: Induced Emf and Current
[5]25.2

A device X is connected across an ac source of voltage V = V0 sin ωt. The current through X is given as 

`I = I_0 sin (omega t + pi/2 )`

1) Identify the device X and write the expression for its reactance.

2) Draw graphs showing the variation of voltage and current with time over one cycle of ac, for X.

3) How does the reactance of the device X vary with the frequency of the ac? Show this variation graphically.

4) Draw the phasor diagram for the device X.

Chapter: [4.02] Alternating Current
Concept: Peak and Rms Value of Alternating Current Or Voltage
[5]26
[5]26.1

Draw a ray diagram to show image formation when the concave mirror produces a real, inverted and magnified image of the object.

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

Obtain the mirror formula and write the expression for the linear magnification.

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

Write two important advantages of reflecting telescope over a refracting telescope.

Chapter: [6.01] Ray Optics and Optical Instruments
Concept: Optical Instruments - Telescope
[5]26.2

Define a wavefront

Chapter: [6.02] Wave Optics
Concept: Huygens Principle

Using Huygens’ principle, verify the laws of reflection at a plane surface.

Chapter: [6.02] Wave Optics
Concept: Huygens Principle

Answer the following questions:

In a single slit diffraction experiment, the width of the slit is made double the original width. How does this affect the size and intensity of the central diffraction band?

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

Answer the following questions:

When a tiny circular obstacle is placed in the path of light from a distant source, a bright spot is seen at the centre of the shadow of the obstacle. Explain why?

Chapter: [6.02] Wave Optics
Concept: Coherent and Incoherent Addition of Waves
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