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# CBSE Physics Sample Paper 2018 Class 12th

Posted On 2018-02-11 Sample Question Paper (PHYSICS)

CLASS-XII (2017-18)

Time Allowed: 3Hours                                                                                                                                                                                   Maximum Marks: 70

General Instructions

1. All questions are compulsory. There are 26 questions in all.

2. This question paper has five sections: Section A, Section B, Section C, Section D and Section E.

3. Section A contains five questions of one mark each, Section B contains five questions of two marks each, Section C contains twelve questions of three marks each, Section D contains one value based question of four marks and Section E contains three questions of five marks each.

4. 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 the three questions of five marks weightage. You have to attempt only one of the choices in such questions.

5. You may use the following values of physical constants wherever necessary.

c = 3 X 108 m/s

h = 6.63 X 10-34 Js

e = 1.6 X 10-19 C

μ o = 4 X 10-7 T m A -1

ε0 = 8.854 X 10-12 C2 N -1 m -2

1/4πε0 = 9 X 109 N m 2 C -2

me = 9.1 X 10-31 kg

mass of neutron = 1.675 X 10-27 kg

mass of proton = 1.673 X 10-27 kg

Avogadro’s number = 6.023 X 1023 per gram mole

Boltzmann constant = 1.38 X 10-23 JK -1

Section – A

1. What are superconductors?

2. What is the power factor of an LCR series circuit at resonance?

3. How are x-rays produced?

4. What should be the length of the dipole antenna for a carrier wave of frequency 3x108 Hz?

5. An electric dipole of dipole moment 20x10-6 cm is enclosed by a closed surface. What is the net flux coming out of the surface?

Section – B

6. A current loop is considered a magnetic dipole, Explain. Also give an expression for its dipole moment.

7. An electric lamp having coil of negligible inductance connected in series with a capacitor and an a.c.source is glowing with certain brigthness. How does the brightness of the lamp change on reducing the (i) capacitance, and (ii) the frequency? Justify your answer. 8. What is the ratio of radii of the orbits corresponding to first excited state and ground state in a hydrogen atom?

OR

Define critical angle. Obtain a relation between the refractive index and critical angle for a pair of media.Does critical angle depend on the colour of light? Explain.

9. What does the term LOS communication mean? Name the types of waves that are used for this communication. Which of the two- height of transmitting antenna and height of receiving antenna can affect the range over which this mode of communication remains effective?

10. Draw V-I graph for ohmic and non-ohmic materials. Give one example for each.

Section – C

11. Write any two distinguishing features between conductors, semiconductors and insulators on the basis of energy band diagrams.

12. 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.

OR

Draw a schematic diagram of the experimental arrangement used by Davisson and Germer to establish the wave nature of electrons. Explain breifly how the de-Broglie relation was experimentally verified in case of electrons.

13. A TV transmission tower antenna is at a height of 20 cm. How much service area can it cover if the receiving antenna is (i) at ground level, (ii) at a height of 25 m? Calculate the percentage increase in area covered in case (ii) relative to case (i).

14. How does the angle of dip vary as one moves from the equator towards the North Pole? If the horizontal component of the earth’s magnetic field at a place where the angle of dip is 60° is 0.4x10-4 T. Calculate the vertical component and the resultant magnetic field of the earth at that point?

15. An electron and a photon each have a wavelength of 1.50nm. Find (i) their momenta, (ii) the energy of the photon and (iii) kinetic energy of the electron.

16. Obtain the equivalent capacitance of the network in fig. Shown below. For a 300 V supply, determine the charge and voltage across each capacitor. 17. (i) Define activity7 of a radioactive material and write its S.I.unit.

(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 for D?

18. Using Huygen’s principle draw a diagram showing how a palne wave gets refracted when it is incident on the surface separating a rarer medium from a denser medium. Hence verify Snell’s laws of refraction.

19. A converging lens has a focal length of 23 cm in air. It is made of a material of refractive index 1.5. It is immersed in a liquid of refractive index 1.2. Calculate its new focal length.

20. Compare the properties of para dia and ferromagnetic substances.

OR

Define electronvolt and atomic mass unit. Calculate the energy in joule equivalent to the mass of one proton.

21. A long straight wire of a circular cross-section of radius V carries a steady current T. The current is uniformly distributed across the cross-section. Apply Amperes circuital law to calculate the magnetic field at a point V in the region for (i) r< a (ii) r> a.

22. What are eddy currents? Write any two applications of eddy currents.

Section – D

23. Aman’s Father bought a new flat in a multistory One day he went to visit the building and flat.There, he found that there is no provision for preventing the damage caused by lighting. He told about it to his father. They meet to the secretary of the building society and explained them about lightning strike.Aman told them the method to protect building from lightning.

• What values were displayed by Aman?
• Explain the method to protect building from lightning.

Section – E

24. Three light rays red ( R ) , Green (G) and Blue (B) are incident on a right angled prism ‘abc’ at face ‘ab’. The refractive indices of the material of the prism for Red , Green and Blue wavelength are 1.39, 1.44, and 1.47 respectively. Out of three which ray will emerge out of face ‘ac’? Justify your answer. Trace the path of these rays after passing through face ‘ab’. OR

(a) Derive the relationship between the peak and the rms value of current in an a.c. circuit.
(b) Describe briefly, with the help of labelled diagram, working of a step-up transformer. A step-up transformer converts a low voltage into high voltage. Does it not     violate the principle of conservation of energy? Explain.

25. How is a zener diode fabricated so as to make it a special purpose diode? Draw I-V characteristics of zener. diode and explain the significance of breakdown voltage. Explain briefly, with the help of a circuit diagram, how a p-n junction diode works as a half wave rectifier.

OR

Define the terms depletion layer and barrier potential for a p-n junction diode. How does an increase in the doping concentration affect the width of the depletion region? Draw the circuit of a full wave rectifier. Explain its working.

26. Trace the rays of light showing the formation of an image due to a point object placed on the axis of a spherical surface separating the two media of refractive indices n1 and n2. Establish the relation between the distances of the object, the image and the radius of curvature from the central point of the spherical
surface.

Hence derive the expression of the lens markers formula.

OR

Draw the labelled ray diagram for the formation of image by a compound microscope. Derive the expression for the total magnification of a compound microscope. Explain why both the objective and the eyepiece of a compound microscope must have short focal lengths.