KENDRIYA VIDYALAYA SANGATHAN, REGIONAL OFFICE, RAIPURdsUæh; fo|ky; laxBu] ks=h; dk;kZy;] jk;iqj

2 st Pre Board Examination दसरा izh&cksMZ ijhkk Session l= :- 2014-2015 Class dkk :- XII th

ckjgohaSUBJECT – PHYSICS

Time: Three hours Max marks: 70

General Instructions:(i).All Questions are compulsory.(ii).Question numbers 1 to 5 are very short answer questions and carry 1 mark each.(iii) Question numbers 6 to 10 are short answer questions and carry2 marks each.(iv) Question numbers 11 to 22 are also short answer questions and carry3 marks each.(v) Question number 23 is avalue based question and carry4 marks each.(vi) Question numbers 24 to 26 are long answer questions and carry5 marks each.Q.1.A Gaussian surface encloses an electric dipole within it. What is the total flux across sphere?Q.2.What is the ratio of radii of the orbits corresponding first excited state and ground state in a hydrogen

atom?Q.3.Define ionization energy.What is its value for a Hydrogen atom.Q.4. Two wires of equal length, one of copper and the other is manganin have the same resistance which is

the thicker? Q.5 A plane electromagnetic wave travels in vacuum along z-direction. What can you say about the

direction of electric and magnetic field vectors ?Q.6. Draw the V-I characteristics for Gallium arsenide and explain different regions of the graph.

Q.7 The ratio of vertical component to the horizontal component of earth’s magnetic field at a place is 1. What is the angle of dip?

Q.8. A current is induced in coil C1 due to the motion of current carrying coil C2. (a) Write any two ways by which a large deflection can be obtained in the galvanometer G. (b) Suggest an alternative device to demonstrate the induced current in place of a galvanometer.

Q.9. The figure shows variation of stopping potential (V0) with the frequency (v) for the two photosensitive materials M1 & M2 (i) why is the slope same for both the lines (ii) for which material will emitted e- have greater K.E. for the incident radiation of same frequency ? Justify your answer.

V0 M1 M2

v

Q.10. The figure adjoining shows the V-I characteristics of a semiconductor diode.(i) Identify the semiconductor diode used. (ii) Draw the circuit diagram to obtain the given characteristic of this device.

I (mA)

Reverse Forward

I( µA)Q.11. Three identical capacitors C1 C 2 and C3 of capacitance 6 µF each are connected to a 12 V battery as

shown. Find: (i) charge on each capacitor (ii) equivalent capacitance of the network (iii) energy stored in the network of capacitors

Q.12.An electric lamp having coil of negligible inductance connected in series with a capacitor and an AC source is glowing with certain brightness. How does the brightness of the lamp change on reducing the (i) capacitance, and (ii) the frequency? Justify your answer.

Q.13.Two identical circular wires P and Q each of radius R and carrying current ‘I’ are kept in perpendicular planes such that they have a common centre as shown in the figure. Find the magnitude and direction of the net magnetic field at the common centre of the two coils.

Q.14 Derive the expression for force per unit length between two long straight parallel current carrying conductors. Hence define one ampere.Q.15. (i) State the Principle of workup of meter bridge.

(ii) The figure shows experimental set up of a meter bridge. When the two unknown resistances X and Y are inserted, the null point D is obtained 40 cm from the end A. When a resistance of 10 Ω is connected in series with X, the null point shifts by 10 cm Determine the values of the resistances X and Y.

Q.16. A circular copper disc. 10 cm in radius rotates at a speed of 20π rad/s about an axis through its centre and perpendicular to the disc. A uniform magnetic field of 0.2T acts perpendicular to the disc.1) Calculate the potential difference developed between the axis of the disc and the rim.2) What is the induced current if the resistant of the disc is 2Ω?

Q. 17. Arrange the following in ascending order of wavelength X-rays, radio wave, Microwaves, ϒ rays Write one use of each

Q.18 A point source S is placed at a distance of 15 cm from a converging lens of focal length 10 cm on its principle axis. Where should a diverging mirror of focal length 12 cm be placed, so that a real image is formed on the source itself.

Q.19. Describe Youngs Double slit experiment to produce interference pattern due to a monochromatic source of light deduce the expression for the fringe width.

Q.20 What is an unpolarised light ? Explain with the help of suitable ray diagram how an unpolarised light can be polarised by reflection from a transparent medium? Write the expression for Brewsters angle in

terms of refractive index of denser mediumQ.21. An electron is accelerated through a potential difference of 100 volts. What is the de-Broglie

wavelength associated with it? To which part of the electromagnetic spectrum does this value of wavelength correspond? An electron & alpha particle have the same de-Broglie wavelength with them. How are their Kinetic Energy related to each other.

Q.22. The energy level diagram of an element is given below. Identify, by doing necessary calculations, which transition corresponds to the emission of a spectral line of wavelength 102. 7 nm.

Q.23 In a debate competition in a school on the topic effect of communication, Mohan expressed his views that advancements in communication principles and technology has helped mankind to communicate almost instantly in any part of the globe. These advancements, however, have also shown adverse effects. Many school authorities and parents are not in favour of free and unlimited access of these electronic devices to students fearing adverse effects on their emotional and psychological health.

Answer the following questions related to the information in the above passage.i) Name any two communication system which are used for the communication?ii) Write any two adverse effects of misuse of these devices on students?iii) Write 02 values which are Reflected in Mohan’s Behavior.

Q.24. (a) Using Gauss law, derive an expression for the electric field intensity at any point outside a uniformly charged thin spherical shell of radius R and charge density s C/m2. Draw the field lines when the charge density of the sphere is (i) positive, (ii) negative. (b) A uniformly charged conducting sphere of 2. 5 m in diameter has a surface charge density of 100µC/m2. Calculate the (i) charge on the sphere (ii) total electric flux passing through the sphere.

OR(a) Derive an expression for the torque experienced by an electric dipole kept in a uniformly electric field.

(b) Calculate the work done to dissociate the system of three charges placed on the vertices of a triangle as shown. Here q = 1.6x10-10 C.

Q.25. (a) Draw a ray diagrams to show refraction of a ray of monochromatic light passing through a glass prism. Deduce the expression for the refractive index of glass in terms of angle of prism and angle of minimum deviation. (b) State the principle on which working of an optical fibre is based. Write the

necessary conduction for the phenomenon to occure ?OR

(a) Obtain lens makers formula. Here the ray of light propagating from a rarer medium of refractive index (n1) to a denser medium of refractive index (n2), is incident on the convex side of spherical refracting surface of radius of curvature R.

(b) Draw a ray diagram to show the image formation by a concave mirror when the object is kept between its focus and the pole. Using this diagram, derive the magnification formula for the image formed.

Q.26. Draw a simple circuit of a CE transistor amplifier. Explain its working. Show that the voltage gain

AV of the amplifier is given by , where βac is the current gain, RL is the load resistance and riis the input resistance of the transistor. What is the significance of the negative sign in the expression for the voltage gain?

OR (a) Draw the circuit diagram of a full wave rectifier using p-n junction diode. Explain its working and show the output, input waveforms. (b) Show the output waveforms (Y) for the following inputs A and B of (i) OR gate (ii) NAND gate