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XII Physics Sample Paper, Set-5
Wednesday,01 February 2012

Instructions:

1. Section-A  Q. no. 1-8 carry 1 mark each.
2. Section-B  Q. no. 9-18 carry 2 marks each.
3. Section-C  Q. no. 19-27 carry 3 marks each.
4. Section-D  Q. no. 28-30 carry 5 marks each.

Section-A

1. Two wires of equal length, one of copper and the other of magnin have the same resistance. Which wire is thicker?

Ans. Magnin as resistivity is more.

2. Mention two causes for the power loss in a transformer.

Ans. Hysteresis loss and flux leakage.

3. Name the principal on which the working of optical is fiber is based.  

Ans. Total internal reflection.

4. What are thermal neutrons?

Ans. Slow moving neutron whose energy is absorbed by moderators in reactor is known as thermal neutrons.

5. What is the antenna ?

Ans. Device used to transmit signal is known as antenna.

6. Give example of β decay.

Ans. 15P32 ----β----> 16S32 + -1e0 + Q

7. Draw the block diagram of basic element of communication system.    

Ans.

8. Complete the nuclear reaction  2He4 + 4Be9+ - - - - - - - - ?

Ans. 2He4 + 4Be9 0n1 + 6e12 + Q

SECTION-B

9. The oscillating electric field of an electromagnetic wave is given by EY=30 sin(2 x 1011t  + 300πx) Vm-1. Find the dirn of propagation of wave and write down the expression for magnetic field ?

Ans. EY = 30 sin(21 x 1011t + 300πx)

Comparing with EY = E0 sin(wt + kx) dirn of propagation is –x direction &

BZ = B0 sin( 2 x 1011t + 300πx)

Where B0 = E0/C = 10-7 T

10. For a given a.c. circuit distinguish between resistance reactance and impedance.

Ans.

Resistance

Reactance

Impedance

 Opposition caused to the flow of current.

 Total non-resistive opposition to the flow of current.

 Total opposition due to resistance as well as reactance.

 Independent on frequency.

 Dependent XL α f
                 Xc α 1/f

 Dependent.
 

11. Calculate current drawn by the primary of a transformer which steps down 200V to 20V to operate a device of resistance 20Ω . Assume the efficiency of transformer to be 80%.

Ans. VP = 200 v       η = 80%

VS = 20 v

R = 20Ω      

As η = 

   

        = 0.08A

12. Derive an expression for the de Broglie Wavelength of an electron moving under potential difference of  V volt.

Ans. Let an electron be accelerated by applying a potential difference V volt . Then

W = qV =

13. Define half life period and decay constant of radioactive substance. Write the S.I. units.

Ans. Half life time – It is the time during which the number of atoms of substance is reduced to half its original value unit is second

Decay time – It is defined as reciprocal of time during which the number of atoms of radioactive substance falls to 37% of its original value

Unit – per second

14. Using the data given below , state as to which of the given lenses will you prefer to use as an eye piece & an objective to design compound microscope.

Lens

Power

Aperture

A

20 D

0.02 m

B

10 D

0.02 m

C

10 D

0.05 m

D

1.0 D

0.1 m

And fo << fe

A → objective and

B → Eyepiece

15. Define the term Resolving power of a telescope.  How does it get affected  on 

(i) Increasing aperture  of objective  lens.
(ii) Increasing focal length of objective.

Ans. Resolving Power – Ability to Resolve the image of two closely placed object is known as Resolving power.

(i) No effect

(ii) Decreases

16. Show that nuclear density of matter in nuclei is independent of mass number A.

Ans. Density 

                 

            

17. When a monochromatic yellow colored light beam is incident an a given photosensitive surface, photo electrons are not ejected to green colored monochromatic beam.
What will happen if the same surface is exposed to (i) violet and (ii) red colored  monochromatic beam of light.

Ans. λv < λy    , λR  > λy

(i) For violet, color, Photo emission will take place  as Energy increases.

(ii) For Red color , No emission of electrons.

18. What are the eddy currents ? Write their two applications.

Ans. An induced current which has undefined path is known as eddy current.

Applications
(1) In induction furnace

(2) Electromagnetic damping in galvanometers.

SECTION-C

19. The oscillating magnetic field in a plane electromagnetic wave is given by BY=8 x 10-6 sin(20 x 1011t + 300πx)T. Calculate the wavelength of e.m. wave. Write down the expression for oscillating electric field.

Ans. As B = Bo sin(wt + bx)
    BY = 8 x 10-6 sin(2 x 1011t + 300πx)
      ω = 2 x 1011rad/s  and
      K = 300π = 2π/λ
      λ = 2π/300 = 1/150m = 0.006m
    EZ = E0 sin(wt + kx)

Where E0 = CBO = 3 x 108 x 8 x 10-6 = 2400N/C
           EZ = 2400 sin(2 x 1011t + 300πx)

20. A rectangular coil of N turns, area A is held in a uniform magnetic field B. If the coil is rotated at a steady angular speed w, deduce an expression for the induced emf in coil at any time.

Ans. As Φ = BAcosθ


Induced emf = e = -dΦ/dt = -d/dl (NBA cosθ)
                         = NBA d/dt (cos ωt)
                      e = NBAω sin ωt
                      e = e0 sin wt   , where e0 = emax = NBAW

21. Given circuit diagram shows a series L.C.R circuit connected to variable frequency 230V source

(i) Determine the source frequency which drives the circuit in resonance
(ii) Obtain the Impedance of circuit & amplitude of at the resonating frequency
(iii) Determine the rms potential drop across the three elements of circuits

Ans.

(i) = 2011-12-07 50rad/s

(ii) Z = R = 40Ω 

(iii) VRrms = 5.75 x 40 = 230 volt

VLrms = 5.75 x 5 x 50 = 1437.5 volt

VCrms  = 1437.5 volt

22. Three light rays red  ( R ) , Green (G) and Blue   (B) are incident on a right  angled  prism a b c at face ‘ab’ .the refractive index of the material of   the material of prism  for Red , Green and Blue  wavelength are 1.39, 1.44, respectively.

Out of three which ray will emerge out of face a c.

Ans.

μR = 1.39
μB = 1.47
μG = 1.44          

as   Sin c =

23. A neutron is absorbed by 36Li nucleus with the subsequent emission of an α-particle. Write the corresponding nuclear reaction. Calculate the energy released in Mev in this reaction

Given mass(3Li)=6.01512126u, M(10n)=1.0086654u, M(2He4)=4.0026044u and M(1H3) = 3.010000u.

Ans. on1 + 3Li61H3 + 2He4 + Q

Δm = (6.01512126 + 1.0086654) – (3.010000 + 4.0026044)

      = 0.01118226 x 931 mev

      = 10.41 mev

24. A convex lens made up of glass  of refractive index 1.5 is dipped in turn in :

(i) Medium A of refractive  index 1.65
(ii) Medium B of refractive index 1.33

Explain giving reasons ; whether it will behave as a converging or diverging lens.

Ans.

(i) If  μm > μg   =>  fm =-ve => Nature will change it will be diverging.

(ii) μm < μ=> fm =increased, Nature will be same.

25. What is meant by sensitivity of a potentiometer? A battery E1 of 4V and variable resistance R are connected in series with wire AB. Length of wire is 1m. When a cell of emf E2 = 1.5V is connected b/w A & C , no current flows through E2. Length of AC is 60 cm.

(I) Find the potentiometer difference b/w A & B

(II) Would the method  work if the battery E1 is replaced by a cell of emf of 1v.

Ans. Sensitivity – sensitivity of a potentiometer is related to potential gradient. Smaller the potential gradient, more sensitive be the potentiometer.

(i) E2 = kl2   

1.5 = k x 60 x 10-2

(ii) No, as current is lowered as emf of driver cell is less then balancing cell.

26. A short bar magnet which is placed with its axis at 30o experiences a torque of 0.016Nm in an external field  of 800G.

(i) Determine the magnetic moment of magnet.

(ii) What is work done by an external force is moving it from its most stable to most unstable position.

Ans. Given θ = 30o ,     = 0.016Nm.
B = 800 x 10-4 T

(i) = M sinθ

/ B sinθ = 0.016/800 x 10-4 x 1/2  = 16/40 =0.4Am2

(ii) Wθ = 0 → θ = 180 = MB(cosθ1 – cosθ2)
                            = 2MB
                            = 2e = 0.032J

27. Circular coil of radius 8cm and 20 turns rotates about its vertical diameter with an angular speed of 50s-1. In a uniform horizontal magnetic field of magnitude 30 x 10-2 T. Find the maximum and average value of emf induced in the coil?

Ans. Given r = 8cm  N = 20
W = 50 rad/s   B = 3 x 10-2T
⇒ n = w/2π

emf(maximum) = NBAW

= 20 x 3 x 10-2 x π(8 x 10-2)2 x 50
= 30π x 64 x 10-4
= 9.42 x 10-5 volt

Average emf = 0

SECTION-D

28. (a) V – I graph for a metallic wire at two different tempratures T1 & T2 is shown. Which of the two temprature T1 & T2 is Higher and why?

(b) Using current conduction in conductor verify ohm’s law.

Ans. (a)  (slope)1  > (slope)2

     

         

             R1 < R2

⇒           T1 < T2 as R α T

(b)

       

        

    

If T is constant as T α 1/ ohm’s law.

29. (a) Explain with the help of suitable nuclear reaction in each of the following case how neutron proton ratio changes during α decay & β decay

(b) Define actively of a radioactive sample give its S.I unit. 

Ans.

(a) α – decay ,  92U23890Th234 + 2He4 + Q

(b) Activity – Activity is the total decay rate of sample of one or more radionuclide.

Unit - Curie

30. State the condition under which total internal reflection occurs. One face prism with a refracting angle of 30o coated with silver. A ray incident on another  face  at angle of 45o is refracted and reflected from silver coated face and retrace its path. Find the refractive index of the material of prism.

Ans. Condition

(i) Light must pass from Denser to rarer medium

(ii)

Since at P light  retraces its path
        




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