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# Board Paper of Class 12-Science 2004 Physics (SET 1) - Solutions

General Instructions:
(i) All questions are compulsory.
(ii) There are 30 questions in total. Question Nos. 1 to 8 are very short answer type questions and carry one mark each.
(iii) Question Nos. 9 to 18 carry two marks each, question 19 to 27 carry three marks each and question 28 to 30 carry five marks each.
(iii) There is no overall choice. However, an internal choice has been provided in one question of two marks; one question of three marks and all three questions of five marks each. You have to attempt only one of the choice in such questions.
(iv) Use of calculators is not permitted.

• Question 1

In the diagram below is shown a circular loop carrying current I. Show the direction of the magnetic field with the help of lines of force. VIEW SOLUTION

• Question 2

Name the type of modulation scheme preferred for digital communication.

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• Question 3

Write the nuclear decay process for β-decay of .

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• Question 4

What is the name given to that part of electromagnetic spectrum which is used for taking photographs of earth under foggy conditions from great heights?

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• Question 5

Heavy water is often used as a moderator in thermal nuclear reactors’. Give reason.

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• Question 6

The graph shows the variation of voltage ‘V’ across the plates of two capacitors A and B versus increase of charge ‘Q’ stored on them. Which of the two capacitors has higher capacitance? Give reason for your answer. VIEW SOLUTION

• Question 9

Explain with the help of diagram the terms:

(i) Magnetic declination and

(ii) Angle of dip at a given place

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• Question 10

Two circular coils, one of radius r and the other of radius R, are placed coaxially with their centres coinciding. For R >> r, obtain an expression for the mutual inductance of the arrangement.

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• Question 11

Find the wavelength of electromagnetic waves of frequency 5 × 1019 Hz in free space. Give its two applications.

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• Question 12

Draw the graph showing the variation of binding energy per nucleon with mass number. Give the reason for decrease of binding energy per nucleon for nuclei with high mass numbers.

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• Question 13

With the help of a diagram, show the biasing of a light emitting diode (LED). Give its two advantages over conventional incandescent lamps.

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• Question 14

A compound microscope with an objective of 1.0 cm focal length and an eye-piece of 2.0 cm focal length has a tube length of 20 cm. Calculate the magnifying power of the microscope, if the final image is formed at the near point of the eye.

Or

The magnifying power of an astronomical telescope in the normal adjustment position is 100. The distance between the objective and the eyepiece is 101 cm. Calculate the focal lengths of the objective and of the eye-piece.

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• Question 15

State Gauss’ theorem in electrostatics. Using this theorem, derive the expression for the electric field intensity at any points outside a uniformly charged spherical shell.

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• Question 16

Find the total energy stored in the capacitors in the given network. VIEW SOLUTION

• Question 18

The diagram shows a piece of pure semiconductor S in series with a variable resistor R and a source of constant voltage V. Would you increase or decrease the value of R to keep the reading of ammeter (A) constant, when semiconductor S is heated? Give reason. VIEW SOLUTION

• Question 19

State two conditions to obtain sustained interference of light. In Young’s double slit experiment, using light of wavelength 400 nm, interference fringes of width ‘X’ are obtained. The wavelength of light is increased to 600 nm and the separation between the slits is halved. If one wants the observed fringe width on the screen to be the same in the two cases, find the ratio of the distance between the screen and the plane of the interfering sources in the two arrangements.

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• Question 20

Give the mass number and atomic number of elements on the right-hand side of the decay process. The graph shows how the activity of a sample of radon-220 changes with time. Use the graph to determine its half life. Calculate the value of decay constant of radon-220. VIEW SOLUTION

• Question 21

A circular coil of N turns and radius R is kept normal to a magnetic field, given by B = B0 cosωt. Deduce an expression for e.m.f. induced in this coil. State the rule which helps to detect the direction of induced current.

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• Question 22

In a series RC circuit, R = 30 Ω, C = 0.25 μF, V = 100 V and ω = 10,000 radian per second. Find the current in the circuit and calculate the voltage across the resistor and the capacitor.

Is the algebraic sum of these voltages more than the source voltage? If yes, resolve the paradox.

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• Question 23

Define the term ‘work function’ of a metal. The threshold frequency of a metal is f0. When the light of frequency 2f0 is incident on the metal plate, the maximum velocity of electrons emitted is V1. When the frequency of the incident radiation is increased to 5f0, the maximum velocity of electrons emitted is V2. Find the ratio of V1 to V2.

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• Question 24

Draw the circuit diagram of a common-emitter amplifier using an n-p-n transistor. Draw the input and output wave forms of the signal. Write the expression for its voltage gain.

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• Question 25

Write the symbol and truth table of an AND gate. Explain how this gate is realized in practice by using two diodes.

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• Question 26

With the help of relevant diagrams, explain the following terms:

(i) Pulse-position modulation (PPM)

(ii) Pulse-duration modulation (PDM)

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• Question 27

Name the device used for data transmission from one computer to another. Justify the name. Using this device, draw the block diagram for data communication and explain it briefly.

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• Question 28

Derive an expression for the torque acting on a loop of N turns, area A, carrying current i, when held in a uniform magnetic field.

With the help of circuit, show how a moving coil galvanometer can be converted into an ammeter of given range. Write the necessary mathematical formula.

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• Question 29

With the help of a ray diagram, show the formation of image of a point object by refraction of light at a spherical surface separating two media of refractive indices n1 and n2 (n2 > n1) respectively. Using this diagram, derive the relation Write the sign conventions used.

Or

Out of syllabus

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• Question 30

Deduce the condition for balance in a wheat-stone bridge; describe the method to determine the specific resistance of a wire in the laboratory. Draw the circuit diagram and write the formula used. Write any two important precautions you would observe while performing the experiment.

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