CBSE Class 12

Pre Boards

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Sample Papers

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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.


Magnetic field induction at some point P on wire 2 due to current I1 passing through wire 1 is given by, 


Magnetic field is produced by wire 1 and current carrying wire 2 lies in magnetic field B1.

 The unit length of wire 2 will experience a force, given by 

                 F= B1 I2 x 1 = 

According to Fleming's left-hand rule, the force on wire 2 acts in the plane of paper perpendicular to wire 2, directed towards wire 1. Similarly, wire 1 also experiences the same force towards wire 2. Thus, both the conducting wires attract each other with the same force F.

One ampere can be defined as the amount of current flowing through two parallel conductors, which are in the same direction or opposite directions, placed at a distance of one metre in free space, and both the wires attract or repel each other with a force of 2 x 10-7 per metre of their lengths. 


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.


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. 


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


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? 

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.


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

16. State Brewster's law. 
The value of Brewster's angle for a transparent medium is different for light of different colours. Give reason.

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.


(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.


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.

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.