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SP/Phy-12/P.B./19–20

Subject: - Physics (042) 1st Pre-Board Examination 2019–20 Time: - 3 hrs.Standard: - 12th Sc. (A,B,C) Set-1 M. M.: - 70

General Instructions(i) All questions are compulsory(ii) There are 37 questions in total. Question number 1 to 20 is very short answer questions and carrying 1 mark

each. Out of which 1 to 10 are multiple choice questions (one option correct), 11 to 15 are completion type and 16 to 20 very short answer type questions.

(iii) Question number 21 to 27 are short answer questions, carrying 2 marks each.(iv) Question number 28 to 34 are short answer questions, carrying 3 marks each.(v) Question number 35 to 37 are long answer questions, carrying 5 marks each.(vi) There is no overall choice. However, internal choice has been provided .You have to attempt only one of the

choices in such questions.(vii) Use of calculators is not permitted. However you may use long tables, if necessary.(viii) You may use the following physical constants wherever necessary

c = 3 × 108 ms–1 h = 6.6 × 10–34 J-s

e = 1.6 × 10–19 C µ0 = 4 × 10–7 T m A-1

1/40= 9 × 109 N m2C–2 me = 9.1 × 10–31 kg

1 eV = 1.6 × 10–19 J 1 atomic mass unit (u) = 931 MeV/c2

Mass of neutron mn = 1.6 × 10–27 kg Boltzmann's constant K = 1.38 × 10–23 JK–1 Avogadro's

number = 6.023 × 1023 /mol

SECTION - A1. In Conductor, Semiconductor and Insulator, the forbidden energy gap are E1 ,E2 and E3

respectively. Which one is correct (a) E1 <E2 < E3 (b) E1 >E2 = E3 (c) E1 = E2 < E3 (d) E1 >E2 > E3

2. A proton, a neutron, an electron and alpha particle have same kinetic energy, then their De-Broglie wavelengths compare as

(a) λe = λp= λn = λa (b) λe>λp>λn>λa

(c) λa<λp<λn<λe (d) λp = λn & λe>λa

3. In a full wave rectifiers, input ac current has a frequency . The output frequency of current is

(a) /2 (b) (c) 2 (d) None of these4. The optical density of turpentine is higher than that of water while its mass density is lower

shows a layer of turpentine floating over water in a container. For which one of the four rays incident on turpentine in the path shown is correct?

(a) 1 (b) 2 (c) 3 (d) 45. Unpolarised beam of light of intensity Io is incident on a polariser P1. Another polariser P2 is held

parallel to it such that its pass axis is oriented at an angle 600, then what percentage of light will emerge from the system:

(a) 30% (b) 100% (c) 12.5% (d) 37.5% OR

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The colour sequence in a carbon resistor is red, brown, orange and silver. The resistance of the resistor is

(a) (21 103) Ω 10% (b) (23 101) Ω 10%

(c) (21 103) Ω 5% (d) (12 103) Ω 5%

6. Diffraction aspect is easier to notice in case of the sound waves then in case of the light waves because sound waves(a) Have longer wavelength (b) Shorter wavelength (c) Longitudinal wave (d) Transverse waves

ORA charge Q is supplied to a metallic conductor. Which is true?(a) Electric field inside it is same as on the surface. (b) Electric potential inside is zero.(c) Electric potential on the surface is zero (d) Electric potential inside it is constant

7. The coil of a moving coil galvanometer is wound over a metal frame in order to (a) reduce hysteresis (b) increase sensitivity (c) increase moment of inertia (d) provide electromagnetic damping

8. Which of the following is an example for diamagnetic substances? (a) copper (b) nickel (c) aluminum (d) iron

9. What is the S. I. unit of electric flux?

(a) 2mC

N (b) 2mN (c) C

m

N

2(d) 2

2

2

Cm

N

10. In the circuit diagram, calculate the electric current through branch BC:

(a) 4 amp (b) 2 amp

(c) 5 amp (d) 10 amp

11. The value of Brewster Angle depends on the nature of the transparent refracting medium and the ______________ of light used.

OREquipotential surfaces due to long linear charge distribution will be ______________ in shape.

12. There is no change in the ______________ as a charged particle moving in a magnetic field, although magnetic force is acting on it.

13. When a magnetic dipole of moment

M rotates freely about its axis from unstable equilibrium to

stable equilibrium in a magnetic field

B , the rotational kinetic energy gained by it is______________ .

14. S.I. unit of magnetic pole strength is______________.15 One atomic mass unit is defined as ______________ of mass of an atom of 6C12.16. An electron is accelerated through a potential difference V. Write the expression for its final

speed, if it was initially at rest.17 A heating element is marked 210V, 630W. What is the value of the current drawn by the element

when connected to a 105 V DC source?

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18 Figure shows the I-V characteristics of a given device. Name the device and write where it is used.

19 In the Rutherford scattering experiment, the distance of closest approach for an α- particle is 0d .

If α- particle is replaced by a proton, then how much kinetic energy in comparison to α-particle will be required to have the same distance of closest approach 0d .

20. 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?

SECTION - B21. What is the reason to operate photodiodes in reverse bias? A p-n photodiode is fabricated from a

semiconductor with a band gap of range of 2.5 to 2.8eV. Calculate the range of wavelengths of the radiation which can be detected by the photodiode.

22. In accordance with the Bohr’s model, find the quantum number that characterizes the Earth’s

revolution around the Sun in an orbit of radius 11105.1 m with orbital speed 4103 m/s. (Mass

of Earth kg24100.6 )

23. Use Gauss’ law to derive the expression or the electric field E due to a straight uniformly

charged infinite line of charge density C/m.24. How are electromagnetic waves produced? What is the source of the energy carried by a

propagating electromagnetic wave? Identify the electromagnetic radiations used(a) in remote switches of household electronic devices: and (b) as diagnostic tool in medicine.

ORA light beam travelling in the x-direction is described by the electric field:

15270 sin 2 10 /y

xE t volt m

c

.

(a) Write expression for magnetic field.(b) Identify the type of radiation and write one possible use of it.

25. A galvanometer having 30 divisions has a current sensitivity of 20 µA/division. It has a resistance of 20 . How will you convert it into an ammeter measuring upto 1 ampere? Also calculate the resistance of this ammeter.

26. Define the following and write their appropriate unit(a) Intensity of magnetisation (b) Magnetic susceptibility

OR

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A domain in ferromagnetic iron is in the form of a cube of side length 1µm. Estimate the number of iron atoms in the domain and the maximum possible dipole moment and magnetization of the domain. The molecular mass of iron is 55 g /mole and its density is 7.9 g/cm³. Assume that each iron atom has a dipole moment of .1027.9 224 mA

27. Determine the equivalent resistance of given network of resistors across terminal A and B. Each resistor has 1 resistance.

SECTION - C28. (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, draw its circuit diagram and write its working principle.

29. 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. With the help of Einstein equation for photoelectric emission show that stopping potential depends on incident frequency and independent of intensity.

30. (a) Define the terms decay constant and write its SI units. (b) Two nuclei P, Q have equal number of atoms at 0t . Their half –lives are 3 hours and 9

hours respectively. Compare their rates of disintegration after 18 hours from the start.

31. (a) Deduce the relation

2sin

2sin

A

A

µ

m

for a given prism having of refracting angle A, angle

of minimum deviation m and refracting index µ.

(b) A ray of light is incident normally on the face AB of a right- angled glass prism of refractive index 5.1ga µ . The prism is partly immersed in a liquid of unknown refractive index. Find the value of refractive index of the liquid so that the ray grazes along the face BC after refraction through the prism.

32. (a) 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 obstacle. Explain why.

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

OR

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(a) State Kirchhoff’s laws and also mention conservation laws on which these are based.

(b) In the circuit shown in the figure, the galvanometer ‘G’ gives zero deflection. If the batteries A and B have negligible internal resistance, find the value of the resistor R.

33. (a) Describe a simple experiment (or activity) to show that the polarity of emf induced in a coil is always such that it tends to produce a current which opposes the change of magnetic flux that produces it.

(b) The current flowing through an inductor of self inductance L is linearly increasing with time (e.g. tI 4 ). Plot a graph showing the variation of: (i) Magnetic flux versus the current.(ii) Magnetic potential energy stored versus the current.

34. Figure shows the circuit diagram of a potentiometer for determining the emf ’ of a cell of negligible internal resistance.

(a) What is the purpose of using high resistance 2R ? (b) How does the position of balance point J change when the resistance 1R is increased? (c) Why cannot the balance point be obtained,

(i) when the emf is greater than 2 V, and (ii) when the key K is closed?

ORA resistance of RΩ draws current from a potentiometer as shown in the figure. The potentiometer has a total resistance 0R . A voltage V is supplied to the potentiometer. Derive an expression for the voltage across R, when the sliding contact is in the middle of potentiometer.

SECTION - D35. (a) Draw a ray diagram to show image formation when the concave mirror produces a real,

inverted and magnified image of the object. (b) Obtain the mirror formula and write the expression for the linear magnification.(c) Explain two advantages of a reflecting telescope over a refracting telescope.

OR

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(a) Draw a ray diagram depicting the formation of the image by an astronomical telescope in normal adjustment.

(b) You are given the following three lenses. Which two lenses will you use as an eyepiece and as an objective to construct an astronomical telescope? Give reason. And also calculate magnifying power of this telescope in normal adjustment. Lenses Power (D) Aperture (cm) 1L 3 8 2L 6 1 3L 10 1

36. (a) A capacitor (C) and resistor (R) are connected in series with an ac source of voltage of frequency 50 Hz. The potential difference across C and R are respectively 120 V, 90 V, and the current in the circuit is 3A. Calculate (i) the impedance of the circuit (ii) the value of the inductance, which when connected in series with C and R will make the power factor of the circuit unity.

(b) In any ac circuit, is the applied instantaneous voltage equal to the algebraic sum of the instantaneous voltages across the series elements of the circuit? Is the same true for rms voltage?

ORThe figure shows a series LCR circuit connected to a variable frequency 230 V source.

(a) Determine the source frequency which drives the circuit in resonance. (b) Calculate the impedance of the circuit and amplitude of current at resonance. (c) Show that potential drop across LC combination is zero at resonating frequency.

37. (a) Distinguish, with the help of a suitable diagram, the difference in the behaviour of a conductor and a dielectric placed in an external electric field. How does polarised dielectric modify the original external field?

(b) A capacitor of capacitance C is charged fully by connecting it to a battery of emf E. It is then disconnected from the battery. If the separation between the plates of the capacitor is now doubled, how will the following change? (i) charge stored by the capacitor. (ii) field strength between the plates.(iii) energy stored by the capacitor. Justify your answer in each case.

OR(a) Explain why, for any charge configuration, the equipotential surface through a point is

normal to the electric field at that point. Draw a sketch of equipotential surfaces due to a single charge q , depicting the electric field lines due to the charge.

(b) Obtain an expression for the work done to dissociate the system of three charges placed at the vertices of an equilateral triangle of side ‘a’ as shown below.

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