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# Board Paper of Class 12-Science 2010 Physics (SET 3) - 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

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? VIEW SOLUTION
• Question 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’? VIEW SOLUTION
• Question 3

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

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• Question 4

Which part of electromagnetic spectrum is used in radar systems?

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• Question 5

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

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• Question 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?

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• Question 7

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

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• Question 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? VIEW SOLUTION
• Question 9

Explain the function of a repeater in a communication system.

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• Question 10

(i) Write two characteristics of a material used for making permanent magnets.

(ii) Why is the core of an electromagnet made of ferromagnetic materials?

OR

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?

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• Question 11

What is the range of frequencies used in satellite communication? What is common between these waves and light waves?

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• Question 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. VIEW SOLUTION
• Question 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.

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• Question 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?

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• Question 15

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

(ii) Give the formula that can be used to determine refractive index of materials of a prism in minimum deviation condition.

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• Question 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?

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• Question 17

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

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• Question 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.

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• Question 19

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

(b) Deduce the expression for the potential energy of a system of two point charges q1 and q2 brought from infinity to the points and respectively in the presence of external electric field .

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• Question 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 l1, l2 and S. VIEW SOLUTION
• Question 21

What is space wave propagation? 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.

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• Question 22

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

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

(iii) 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?

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• Question 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.

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• Question 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.

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• Question 25

Write the expression for the magnetic moment due 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. VIEW SOLUTION
• Question 26

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.

OR

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.

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• Question 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

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• Question 28

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

(b) 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. (c) Identify the logic gate represented by the circuit as shown and write its truth table. OR

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

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

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• Question 29

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

OR

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.

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• Question 30

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.

OR

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.

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