Class - XII Physics Sample Paper- 5

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. What is the work done in moving a test charge q through a distance of 1cm along the equatorial axes of dipole?
Ans. Zero as W_AB = q(V_B - V_A) = 0 as V_A = V_B = 0
2. Define the term electric dipole moment of a dipole. State its S. I. unit.

Ans. The product of magnitude of either the charge and separation between them as known as dipole moment . 

S.I Unit – cm.

3. Write the nuclear decay process for β decay of  .

Ans. ₁₅P³² ------> ₁₀S³² + _-1e⁰ + Q

4. How a wave front  is related with light rays?

Ans. Wave front  is normal to the light rays path.

5. The sequence of bands marked on a carbon resistor are brown black, brown gold.Write the value of resistance?

Ans. 10 x 10¹ ± 5% Ω.

6. You are asked to measure of emf of a cell. Which instruments will you use a high resistance voltmeter or a potentiometer?

Ans. Potentiometer as it does not draw current.

7. Why should the material used making permanent magnet have high coercivity?

Ans. Such as it could not loose its magnetism property.

8. The Vertical components of earth’s magnetic field at a given place is 0.4 x 10^-14 WB/m² and angle of dip is 30^o . what will be horizontal component at that point.?   

Ans. Bv = B sinθ = 4 x 10^-5

B_H = B cosθ = Bv cot θ = 4 x 10^-5 cot30 = 4 x 10^-5 x  T = 6.8 x 10^-5 T.

Section-B

9. Define the terms retentivety & coercivety.

Ans. Retentivety – It is the residual flux density in a substance when the magnetizing field is field is returned to zero.

Coercivety – It is the reverse magnetic field that is required to reduce the magnetic flux density in a substance from its remnant value to zero.

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

Ans.

11. Explain the function of Repeater in communication system.

Ans. To increase the range of the transmission of microwaves; number of in between sets of receiver and transmitters are erected.

    A repeater picks up signal from transmitter amplifies & retransmits.

12. Calculate the shortest wavelength of Lyman series of H-atom spectrum.

Ans. Lyman series equation  

For shortest wavelength 
                                   

13. How did the debroglie equation lead to the quantization condition laid down by Bohr ?

Ans. For an electron in n^th order, 

14. 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 = 

15. Explain the effect of increase of intensity and potential difference on photoelectrons kinetic energy.

Ans. Due  to increase in Intensity , No effect on kinetic energy of Photo electrons as well as on Potential Difference. As due  to increase in Intensity , there  is only an increase in the number of Photons per unit area , and not the energy incident.

16. Calculate the number of photons emitted per second by transmitter of 10 KW power; radio waves of frequency 6×10⁵ Hz.

Ans.

17. 1μF capacitor is connected to 220 V 50Hz source. Find the peak value of voltage across the capacitor.

Ans. C = 10^-6F      as      V_rms = 220 V

       f = 50Hz

Potential difference across the capacitor V_rms = I_rms X_C

18. A bulb of resistor 10? connected to an Inductor of inductance L is in series with an a.c. source marked 100v 50Hz. If phase angle between the voltage and current is π/4 radian. Calculate the value of L.

Ans. as Φ = π/4, R = 10Ω    f = 50Hz

tanΦ = X_L/R ⇒ tanπ/4 = 2πfL/R

⇒ 2πfL = R ⇒ L = R/2πf = 0.3henery

Section-C

19. What are eddy currents ? How are these produced? In what sense are eddy currents considered undesirable in a transformer?

Ans. Eddy currents  - If a plate of a conducting material is kept in a varying magnetic field , induced current are produced . which have all possible random directions. These currents are known as EDDY CURRENTS.

    When a block of conducting material is moved rapidly in a magnetic field cutting across the magnetic field lines, eddy currents are produced .

    As eddy current causes heating of core of transformers. Hence  undesirable.

20. Derive an expression for the impedance of an a.c. circuit with series L.C.R Combination.

Ans.

    OA = V_R                                          

    OD = V_L

    OE = V_C                                                          

    OC = V_L – V_C

  OB² = OA² + AB²

     V² = V_R² + (V_L - V_C)²

     V² = i² [ R² + (X_L - X_C)²]

          = Total resistance offered = Impedance

21. A double convex lens made of glass if refractive index 1.5 has its both surface of equal radii of curvature of 20 cm each. An a object of 5 cm height is placed at a distance of 10 cm from the lens. Find the position nature & size of image.

Ans.  μ_ga 1.5, R₁ = R₂ =20 cm.

                                    h₁ = 5 cm , u = -10 cm

22. A convex lens made of material of refractive index n₂ is held in reference medium of index n₁. Trace the path of a parallel beam of height passing through the lens

When (i)  n₁ = n₂   (ii) n₁ < n₂    (iii)   n₁   > n₂

Ans. (i) 

(ii)

(iii)

23. What is AM? Represent the process graphically. Write its two limitations and  two advantages.

Ans.

Advantages :-  (i) For speech transmission

                      (ii) Short range distance communication.  

24. Give reason for following:

(i) Long distance radio broadcast use short wave bands.
(ii) Satellites are used for long distance TV transmission. 

Ans. (i) As short wave bands are high frequencies radio wave which is being reflected and transmitted. Hence it is used for long distance communication. It is up to 30MHz

(ii) Frequency more than 40 MHz penetrate the ionosphere and does not come back. Hence space wave used for T.V transmission is used by satellite mode.

25. A10μF capacitor is charged by a 30V d.c and then connected across the uncharged 50μF capacitor. Calculate (i) the final potential difference across the combination and initial & final energies.

Ans.

C₁ = 10μF ,    V₁ = 30μF

C₂ = 50μF ,     V₂ = 0

U_i = ½ C₁V₁² = 1/2 x 10 x 10^-6 x 30 x 30 = 45 x 10^-4 J

U_f = 1/2(C₁₊ C₂) V²com = 1/2 x 60 x 10^-6 x 25 = 7.5 x 10^-4 J

26. A parallel plate capacitor, each with plate area A and separation d, is charged to a potential difference V. The battery used to charge is disconnected, what change will take place if any , in

(i) Charge on plates
(ii) Electric field intensity
(iii) capacitance

Ans. (i) No effect , No source remains connected

(ii) E will decreases due to polarization.

(iii) C will increase as V decrease as due to decrease in electric field.

27. An electric dipole of length 2cm is placed with its axis making an angle of 60^o with respect to a uniform electric field of 10⁵N/C. If it exists a torque of 8 , Calculate

Ans. Given   2L = 2cm, θ = 60^o, E = 10⁵N/C and  = 8

(i)  = PE sin θ

     = q(2L) E sin θ

8 = q x ( 2 x 10^-2) x (10⁵) sin 60

8 = q x 2 x 10³ x  /2

8 x 10^-3 = q = 8mc

(ii) U = - PE cos θ = -(8 x 10^-3) (2 x 10^-2) cos 60

                           = -8 x 10^-5 joule

Section-D

28. With the help of labeled diagram, describe the construction and working of Van de graph generator.

Ans.

This is a device to store charge at very high potential.

Working → Motor drives pulley through insulating belt. There are two metallic brushes placed near the insulating belt. The lower brush is connected to high voltage battery.

    The upper brush is connected with the inside the shell

    Due to discharging action of sharp points, a positively charged electric wind is setup , which sprays positive charge on the belt.

    As the belt moves and reaches the sphere a negative charge is included on the sharp ends and equal +ve charge on further end of the brush. Which will shift immediately to the outer surface of the shell.

29. Sketch a graph b/w ? of incident radiation and stopping potential for a given photosensitive material. What information can be drawn from the value of intercept on the potential axis.

Ans. E = h = Φ_o + eV_o

30. What is interference of light? Write two essential condition for sustained interference fringes.
Draw a graph to show the variation of intensity versus position on screen in YDSE. When (a) both the slits are open ( b) When one slit is closed .What is effect on interference pattern when screen is moved closed  to slit. 

Ans. Interference :- Redistribution of energy carried by two or more than two waves are known as Interference.

Two conditions - 
(i) Source must be coherent

(ii) D must be large.

Also  β = 

Fringe Separation will Increases as β α D