(a) Explain any two factors that justify the need of modulating

Subject

Physics

Class

CBSE Class 12

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 Multiple Choice QuestionsShort Answer Type

11. Two long, straight, parallel conductors carry steady currents, I1 and I2 , separated by a distance . If the currents are flowing in the same direction, show how the magnetic field set up in one produces an attractive force on the other? Obtain the expression for this force. Hence, define one ampere.
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12. How are em waves produced by oscillating charges?
 
Draw a sketch of linearly polarised em waves propagating in the Z-direction. Indicate the directions of the oscillating electric and magnetic fields.

                                                      OR

Write Maxwell's generalisation of Ampere's circuital law. Show that in the process of charging a capacitor, the current produced within the plates of the capacitor is I = straight epsilon subscript straight o space dϕ subscript straight E over dt, where straight ϕ subscript straight E is the electric flux produced during charging of the capacitor plates. 
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13.

Explain the terms (i) Attenuation and (ii) Demodulation used in Communication System.

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14. Plot a graph showing variation of de Broglie wavelength λ versus fraction numerator 1 over denominator square root of straight V end fraction, where V is the is accelerating potential for two particles A and B, carrying the same charge but different masses m1 and m2 (m1 > m2). Which one of the two represents a particle of smaller mass and why? 
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15. A nucleus with mass number A = 240 and BE/A = 7.6 MeV breaks into two fragments, each of A = 120 with BE/A = 8.5 MeV. Calculate the released energy.

                                               OR

Calculate the energy in the fusion reaction: 

straight H presubscript 1 presuperscript 2 space plus space straight H presubscript 1 presuperscript 2 space rightwards arrow He presubscript 2 presuperscript 3 space plus space straight n space comma

where space straight B. straight E. space of space straight H presubscript 1 presuperscript 2 space equals space 2.23 space MeV

space space space space space space space space space space space space straight B. straight E. space of space He presubscript 2 presuperscript 3 space equals space 7.73 space MeV
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16. State Brewster's law. 
The value of Brewster's angle for a transparent medium is different for light of different colours. Give reason.
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17. Write three characteristic features in photoelectric effect that cannot be explained on the basis of wave theory of light, but can be explained only using Einstein's equation.
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18.

(a) Explain any two factors that justify the need of modulating a low-frequency signal.
 
(b) Write two advantages of frequency modulation over amplitude modulation.


Modulating low-frequency signals are required because: 

i) Size of the antenna: Antenna is required for both transmission and reception of the signal whose size (at least ) should be comparable to the wavelength of the signal so that time variation of the signal can be properly sensed by the antenna. For a low-frequency signal, the wavelength is large. Therefore, a large antenna of such a huge height is practically impossible to construct and operate.

Hence, the need for high-frequency transmission arises. 

ii) Overlapping signals from different transmitters: When an information is transmitted using different transmitters, the signals get overlapped and the information is lost. Hence, high-frequency transmission is required. Each band should be allocated to each frequency range in order to avoid mixing of signals. 

iii) Higher energy: High-frequency signals have high energy and therefore, even after loss due to attenuation, signals can be carried over longer distances. 

b) 

Advantage of frequency modulation over amplitude modulation are: 

i) FM is more efficient than AM because the amplitude of an FM wave is constant, irrespective of its modulation index. Thus, the transmitted power is constant. Also, in AM the transmitted power goes waste in the transmitted carrier.  

ii) FM reception is quite immune to noise in comparison to AM reception. Noise is a form of amplitude variation in the transmitted signal. Thus, using amplitude limiters in the FM receivers, the noise is eliminated.   

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19. (i) Write the functions of three segments of a transistor.

(ii) Draw a circuit diagram for studying the input and output characteristics of a n-p-n transistor in common emitter configuration. Using the circuit, explain how input, output characteristics are obtained.
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20. (a) Calculate the distance of an object of height h from a concave mirror of radius of curvature 20 cm, so as to obtain a real image of magnification 2. Find the location of image also.

(b) Using mirror formula, explain why does a convex mirror always produce a virtual image.
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