Subject

Physics

Class

CBSE Class 12

Pre Boards

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Sample Papers

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 Multiple Choice QuestionsShort Answer Type

11.

A metallic rod of length ‘l’ is rotated with a frequency v with one end hinged at the centre and the other end at the circumference of a circular metallic ring of radius r, about an axis passing through the centre and perpendicular to the plane of the ring. A constant uniform magnetic field B parallel to the axis is present everywhere. Using Lorentz force, explain how emf is induced between the centre and the metallic ring and hence obtain the expression for it. 

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12.

In a series LCR circuit connected to an ac source of variable frequency and voltage straight nu space equals space straight nu subscript straight m sinωt draw a plot showing the variation of current (I) with angular frequency (straight omega) for two different values of resistance R1 and R2 (R1>R2). Write the condition under which the phenomenon of resonance occurs. For which values of the resistance out of the two curves, a sharper resonance is produced? Define Q-factor of the circuit and give its significance. 

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13.

Which of the following waves can be polarized (i) Heat waves (ii) Sound waves?

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14.

Write the relationship between angle of incidence ‘i’, prism ‘A’ and angle of minimum deviation for a triangular prism. 

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15.

The given graph shows the variation of photo-electric current (I) versus applied voltage (V) for two different photosensitive materials and for two different intensities of the incident radiation. Identify the pairs of curves that correspond to different materials but same intensity of incident radiation. 


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16.

(a) Write the necessary conditions for the phenomenon of total internal reflections to occur.

(b) Write the relation between the refractive index and critical angle for a given pair of optical media.


a) Conditions for total internal reflection to take place:

i) Light ray must travel from denser medium into rarer medium.

ii) The angle of incidence in denser medium must be greater than the critical angle for given pair of optical media. 

b) Relation between refractive index and critical angle for a given pair is,

    As per Snell’s law, 

 

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17.

In the given circuit diagram, a voltmeter ‘V’ is connected across a lamp ‘L’. How would (i) the brightness of the lamp and (ii) voltmeter reading ‘V’ be affected, if the value of resistance ‘R’ is decreased? Justify your answer.    


    

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18.

Block diagram of a receiver is shown in the figure:



(a) Identify ‘X’ and ‘Y’.

(b) Write their functions.

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19.

Explain, with the help of a circuit diagram, the working of a photo-diode. Write briefly how it is used to detect the optical signals. 

OR

Mention the important considerations required while fabricating a p-n junction diode to be used as a Light Emitting Diode (LED). What should be the order of band gap of an LED if it is required to emit light in the visible range?              

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20.

Write three important factors which justify the need of modulating a message signal. Show diagrammatically how an amplitude modulate wave is obtained when a modulating signal is superimposed on a carrier wave.    

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