Saturday, 14 January 2017

cbse class12 preboard physics paper

1. At what angle of incidence should a beam of light strike a glass slab of refractive index √3,
   such that the reflected and refracted rays are perpendicular to each other?
2. A glass rod rubbed with silk acquires a charge of 1.6X10-12C .What is the charge on
   silk?
3. What is the velocity of a charged particle moving in between the equal electric and
    magnetic fields applied in opposite directions?
4. Name the physical quantity which is measured in weber/A
5. If the length of a cylindrical metallic conductor is doubled, how does the resistivity
   of conductor change?
6. If the output of OR gate is given as an input of NOT gate. Name the gate so formed. Write
   the logic symbol and the truth table of this gate.
7. Draw a neat and labeled diagram to show the image formation in a compound microscope.
   Write the expression for magnifying power,when final image is formed at near point.
8. The magnetic flux through a coil perpendicular to its plane and directed into paper
    is varying according to the relation Ф = 5t2 + 10t + 5 milli weber. Calculate the e.m.f   
    induced in the loop at t = 5 sec.
                                                                 OR
   A galvanometer coil has a resistance of 12 Ω and It shows full scale deflection for a
   current of 3 mA.How will you convert the galvanometer into voltmeter of range 0 to
   18 volt?
9. Name the device which is used to transform a.c. supply from one level of voltage to
   another. State the principle based on which it works.
10. A radioactive isotope has a half-life of T years.How long will it takefor the activity
     to reduce to 3.125 % of its initial value.
11. A potential of E = 50 sin (200 πt + π/4) is applied across a resistor of 10 Ω resistance.   
      Find 1. rms voltage. 2. Frequency of a.c 3. rms value of current.
12. Show by a ray diagram, the image formation of a point object by a thin double convex
      lens having radii of curvature R1 and R2. Hence the derive the relation
        1/f = (μ- 1) (1/R1 – 1/R2) where ‘f ‘is the focal length and ‘μ’ is the refractive index
of material of the lens with respect to air.

13. A potential difference of 2 V is applied between the points A and B shown in network drawn below.
Calculate the equivalent resistance of the network between the points A and B and
the magnitudes of currents flowing in the arms AFCEB and AFDEB.

14. Define the term ‘drift velocity’ of electrons in a conductor. Give it’s SI unit. Show
that the resistance of a conductor is given by R = m l/ne2Aτ using the concept of
drift velocity.



15. a) What are coherent sources? Can two individual monochromatic source of light
behave as coherent sources?
b)What is the effect on the fringe width observed in a young’s double slit
   experiment, when
a. Screen is moved away from the plane of the slits.
b. Separation between the slits is increased.
c. whole apparatus is immersed in water.
                                                                          
16. A proton and a deuteron are accelerated through the same accelerating potential.
     Which one of the two has (a) greater value of de-Broglie wavelength associated
     with it, and (b) less momentum?

17. Explain how an audio signal is extracted from the modulating wave with a neat
     diagram. What is demodulation?
                                                        OR
    Define i) carrier signal ii) amplitude modulated signal iii) attenuation
18. Name the term that is found missing in ampere’s circuital law according to Maxwell.
     Derive the expression for the same and hence write the modified form of
     ampere’s circuital law.
19. Define “half-life period” of a radioactive element. Derive the relation T1/2 = 0.693 /λ.
      where λ is decay constant.
20. a) In case of a photosensitive surface, state the role of threshold wavelength of
          incident photon for photo electric effect.
b) Plot a graph showing the variation of photo current vs collector potential for three
   different intensities I1 > I2 > I3and having same threshold frequency for a given
   metallic cathode.
c) Write the Einstein’s photo electric equation.
21. Distinguish between dia, para and ferro magnetic materials (any three points for
     each)
22. State ‘Bohr’s angular momentum quantization condition’. Derive an expression
     forthe radius of nth orbit of an electron revolving around the nucleus of a hydrogen
     atom.
23. Yogesh was using a galvanometer in the practical class. Unfortunately it fell from
      his hand and broke.His friend’s advised him not to tell the teacher. He was upset. But he
     decided to tell the teacher. Teacher listened to him patiently on knowing that the act
     was not intentional, but just an accident, and used the opportunity to show the internal
     structure of galvanometer to the whole class.
    (i) What are the values displayed by yogesh.
    (ii) Explain the principle based on which moving coil galvanometer works.
    (iii) What is the significance of figure of merit of moving coil galvanometer?

24. a) A parallel plate capacitor is charged to a potential difference ‘V’ by a d.c.
         source. The capacitor is then disconnected from the source. If the distance between the
          plates is doubled, state with reason how the following will change?
          a. electric field between the plates.
          b. capacitance
          c. energy stored in the capacitor.

   b) Three capacitors are connected as shown in figure given below. Calculate the

equivalent capacitance between the points X and Y. ( Given that C1 = C2 = C3 = 2μF.)

25. a) A concave lens made of material of refractive index ‘n2’ is kept in a reference
medium of refractive index ‘n1’ . Trace the path of parallel beam oflight passing
through the lens when
a. n1= n2     b. n12    c. n1> n2
b) The far point of a myopic person is 80 cm in front of the eye. What is the focal
power of the lens required to enable him to see very distant objects clearly?
OR
a) Deduce Snell’s law of refraction using Huygens wave theory of light with a neat
diagram.
b) Define “resolving power” of a compound microscope. On what factors does it
depend?
26. a) Explain the formation of depletion layer and potential barrier in a p-n junction diode.
b) What is called Photo diode? Three photo diodes D1, D2 and D3 are made of   
    semiconductors having band gaps of 2.5 eV, 2 eV and 3 eV respectively. Which one
    of them will be able to detect the wavelength of 600 nm? `
                                                                        OR
a) With the help of a neat and labeled circuit diagram, explain how an n-p-n transistor can be
   used as an amplifier in common emitter configuration.
b) Write an expression for voltage gain. Explain how the input and output voltages
    change by the phase difference of 1800.
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