Subject

Physics

Class

CBSE Class 12

Pre Boards

Practice to excel and get familiar with the paper pattern and the type of questions. Check you answers with answer keys provided.

Sample Papers

Download the PDF Sample Papers Free for off line practice and view the Solutions online.
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 Multiple Choice QuestionsShort Answer Type

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11.

A circular coil of N turns and radius R carries a current I. It is unwound and rewound to make another coil of radius R/2, current I remaining the same. Calculate the ratio of the magnetic moments of the new coil and the original coil.


Length of the wire remains same,

straight N subscript 1 space straight x space 2 πR space equals space straight N subscript 2 space straight x space space 2 straight pi space straight R over 2

therefore space straight N subscript 2 space equals space 2 space straight N  

Magnetic moment of a coil, m = NAI

For the coil of radius R, magnetic moment, 

straight m subscript 1 space equals space straight N subscript 1 IA subscript 1 space equals space straight N subscript 1 Iπ space straight R squared
Magnetic moment for coil of radius R/2,

straight m subscript 2 space equals space straight N subscript 2 space IA subscript 2 space equals space fraction numerator 2 straight N subscript 1 IπR squared over denominator 4 end fraction equals fraction numerator straight N subscript 1 IπR squared over denominator 2 end fraction

Therefore, 

straight m subscript 2 over straight m subscript 1 space equals space 1 colon thin space 2

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12.

Deduce the expression for the electrostatic energy stored in a capacitor of capacitance ‘C’ and having charge ‘Q’.

How will the (i) energy stored and (ii) the electric field inside the capacitor be affected when it is completely filled with a dielectric material of dielectric constant ‘K’? 
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13.

Calculate the value of the resistance R in the circuit shown in the figure so that the current in the circuit is 0.2 A. What would be the potential difference between points B and E? 


 

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14.

A series LCR circuit is connected to an ac source. Using the phasor diagram, derive the expression for the impedance of the circuit. Plot a graph to show the variation of current with frequency of the source, explaining the nature of its variation.

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15.

Define relaxation time of the free electrons drifting in a conductor. How is it related to the drift velocity of free electrons? Use this relation to deduce the expression for the electrical resistivity of the material.            

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16.

How does the angular separation between fringes in single-slit diffraction experiment change when the distance of separation between the slit and screen is doubled? 

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17.

For the same value of angle incidence, the angles of refraction in three media A, B and C are 15°, 25° and 35° respectively. In which medium would the velocity of light be minimum? 

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18.

A proton and an electron have same kinetic energy. Which one has greater de-Broglie wavelength and why?

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19.

An object AB is kept in front of a concave mirror as shown in the figure.

(i) Complete the ray diagram showing the image formation of the object.

(ii) How will the position and intensity of the image be affected if the lower half of the mirror’s reflecting surface is painted black?        
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20.

Draw a labelled ray diagram of a reflecting telescope. Mention its two advantages over the refracting telescope

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