## Sunday, 15 January 2017

### CBSE CLASS 12 PHYSICS REVISION EXAM

1.       When an object is placed between f and 2f of a concave mirror, would the image formed be (i) real or virtual and (ii) diminished or magnified ?
2.       Draw a plot showing the variation of de Broglie wavelength of electron as a function of its K. E.
3.       Why is the frequency of outgoing and incoming signals different in a mobile phone ?
4.       A particle of mass ‘m’ and charge ‘q’ moving with velocity ‘v’ enters the region of uniform magnetic field at right angle to the direction of its motion. How does its kinetic energy get affected ?
5.       Figure shows a current carrying solenoid moving towards a conducting loop. Find the direction of the current induced in the loop.

6.       Distinguish between unpolarised and a linearly polarised light. Describe, with the  help of a diagram, how unpolarised light gets linearly polarised by scattering
7.        Why does white light disperse when passed through a glass prism ? Using lens maker’s formula, show how the focal length of a given lens  depends upon the colour of light incident on it.
8.        Using the graph shown in the figure for stopping potential V/s the incident Frequency of photons, calculate Planck’s constant.

9.       Complete the following nuclear reactions :

OR
If both the number of protons and neutrons in a nuclear reaction is conserved, in  what way is mass converted into
energy (or vice verse) ? Explain giving one example.
10. Using the concept of drift velocity of charge carriers in a conductor, deduce the relationship between current density and resistivity of the conductor.
11. Define the following using suitable diagrams : (i) magnetic declination and (ii) angle of dip. In what direction will  compass needle point when kept at the (i) poles and (ii)equator ?
12. Derive the expression for the magnetic energy stored in a solenoid in terms of magnetic field B, area A and length l  of the solenoid carrying a steady current I.How does this magnetic energy per unit volume compare with the  electrostatic Energy density stored in a parallel plate capacitor ?
13. A circuit containing an 80 mH inductor and a 250 µF capacitor in series connecte  to a 240 V, 100 rad/s supply. The  resistance of the circuit is negligible.(i) Obtain rms value of current.(ii) What is the total average power consumed by  the circuit ?
14. Answer the following questions :
(i) Why is the thin ozone layer on top of the stratosphere crucial for human survival ? Identify to which a part of electromagnetic spectrum does this radiation belong and write one important application of the radiation.
(ii) Why are infrared waves referred to as heat waves ? How are they produced ?
What role do they play in maintaining the earth’s warmth through the greenhouse effect ?
15. Define the term ‘critical angle’ for a pair of media.
A point source of monochromatic light ‘S’ is kept at the centre of the bottom of a cylinder of radius 15.0 cm. The cylinder  contains water (refractive index 4/3) to a height of 7.0 cm. Draw the ray diagram and calculate the area of water surface  through which the light emerges in air.
16. Which two of the following lenses L1, L2 and L3 will you select as objective and  eyepiece for constructing best possible

17. Explain by drawing a suitable diagram that the interference pattern in a double slit is actually a superposition of single
slit diffraction from each slit. Write two basic features which distinguish the interference pattern from those seen in
a coherently illuminated single slit.
18. Distinguish between n-type and p-type semi-conductors on the basis of energy band diagrams. Compare their
conductivities at absolute zero temperature and at room  temperature.
19. (a) Given a block diagram of a generalized communication system.

Identify the boxes ‘X’ and ‘Y’ and write their functions.
(b) Distinguish between “Point to Point” and “Broadcast” modes of communication
20. An electric dipole of dipole moment is placed in a uniform electric field .Obtainthe expression for the torque experienced by the dipole. Identify two pairs of perpendicular vectors in the expression.

21. (a) Two spherical conductors of radii R1 and R2 (R2 > R1) are charged. If they are connected by a conducting wire, find
out the ratio of the surface charge densities on them.
(b) A steady current flows in a metallic conductor of non-uniform cross-section.  Which of these quantities is constant
along the conductor : current, current density, electric field, drift speed ?
22. In the two electric circuits shown in the figure, determine the readings of ideal ammeter (A) and the ideal voltmeter (V).

OR
In the circuit shown in the figure, find the current through each resistor.

(i) What qualities/values, in your opinion did Rohit possess ?
(ii) Why CFLs and LEDs are better than traditional incandescent lamps ?
(iii) In what way earthing reduces electricity bill ?

24. (a) Draw a schematic arrangement of Geiger-Marsden experiment showing the scattering of a-particles by a thin foil of  gold. Why is it that most of the a-particles go right through the foil and only a small fraction gets scattered at   large angles ?
Draw the trajectory of the a-particle in the coulomb field of a nucleus. What is the significance of impact parameter and what information can be obtained regarding the size of the nucleus ?
(b) Estimate the distance of closest approach to the nucleus (Z = 80) if a 7.7 MeV a-particle before it comes momentarily to rest and reverses its direction.
OR
(a) Write two important limitations of Rutherford model which could not explain the observed features of atomic spectra.
How were these explained in Bohr’s model of hydrogen atom ? Use the Rydberg formula to calculate the wavelength
of the Hα line.  (Take R = 1.1 X107 m–1).
(b) Using Bohr’s postulates, obtain the expression for the radius of the nth orbit in hydrogen atom.
25. (a) Figure shows the input waveform which is converted by a device ‘X’ into an  output waveform. Name the device and  explain its working using the proper  circuit. Derive the expression for its voltage gain and power gain.

(b) Draw the transfer characteristic of a base biased transistor in CE configuration.Explain clearly which region of the curve
is used in an amplifier.
OR
(a) Explain briefly, with the help of circuit diagram, the working of a full wave rectifier. Draw its input and output waveforms.
(b) Identify the logic gate equivalent to the circuit shown in the figure.Draw the truth table for all possible values of inputs A and B.

26. (a) Use Biot-Savart law to derive the expression for the magnetic field due to a circular coil of radius R having N turns at  a point on the axis at a distance ‘x’  from its centre. Draw the magnetic field lines due to this coil.
b) A current ‘I’ enters a uniform circular loop of radius ‘R’ at point M and flows out at N as shown in the figure.Obtain the net magnetic field at the centre of the loop. (a) Show how Biot-Savart law can be alternatively expressed in the form of Ampere’s   circuital law. Use this law to obtain the expression for the magnetic field inside a  solenoid of length ‘l’, cross-sectional area ‘A’ having ‘N’ closely wound  turn and   carrying a steady current ‘I’. Draw the magnetic field lines of a finite solenoid carrying   current I.
(b) A straight horizontal conducting rod of length 0.45 m and mass 60 g is  suspended by two vertical wires at its ends. A   current of 5.0 A is set up in the rod through the wires.Find the magnitude and direction of the magnetic field which  should be set up in order that the tension in the wire is zero.

## Saturday, 14 January 2017

### CBSE CLASS11 MIDTERM PHYSICS PAPER

1.The wheels of vehicles are provided with mudguards. Why?
2. Define spring constant of a spring?
3. What happens to the P.E of a bubble when it rises in water?
4. A light and a heavy body have same K.E. which of the two have more momentum and why?
5. The position x of a body of mass 2 kg varies with time t as x = (2t2 + 3t – 4) m.Find  Force acting on the
body
6. A body of mass 2kg moving with a speed of 100 m/s hits a wall and rebounds with the same speed. If
the contact time is (1/50)s,Find the force applied on the wall
7. A body dropped from a height H reaches the ground with a speed of 1.m/s2. Calculate the work done
by air-friction.
8. A light string passes over a frictionless pulley. To one of its ends a mass of 6 kg is attached and
to its other end a mass of 10 kg is attached. Find tension in the string .
9. A hunter has a machine gun that can fire 50g bullets with a velocity of 150 ms–1. A 60 kg tiger springs
at him with a velocity of 10 ms–1 How many bullets must the hunter fire into the target so as to stop
him in his track?
10. A bullet of mass 0.01 kg is fired horizontally into a 4 kg wooden block at rest on a horizontal surface.
The coefficient of kinetic friction between the block and the surface is 0.25. The bullets gets
embedded in the block and the combination moves 20m before coming to rest. With what speed
did the bullet strike the block?
11. You push your physics book 1.50 m along a horizontal tabletop with a horizontal push of 2.40 N
while  the opposing force    of friction is 0.600 N. How much work does each of the following forces
do on the   book: (a) your 2.40-N push, (b) the   friction force, (c) the normal force from the tabletop,
and (d) gravity? (e) What is the net work done on the book?
12.  Your job is to lift 30-kg crates a vertical distance of 0.90 m from the ground onto the bed of a
truck.(a) How many crates would you have to load onto the truck in1 minute for the average power
output you use to lift the crates  to equal 0.50 hp? (b) How many crates for an average power output
of 100 W

13.How does a perfectly inelastic collision differ from perfectly elastic collision? Two particles of
mass  m1 and m2 having velocities u1 and u2 respectively make a head on collision. Derive the
relation for  their finalvelocities. Discuss the following special cases.
(i) m1 = m2 (ii) m1 >> m2 and U2 = 0  (iii) m1 << m2 and u1 = 0
OR
The blades of a windmill sweep out a circle of area A a) If the wind flows at a velocity v perpendicular
to the circle, what is the mass of the air passing through it in time t?
b)What is the kinetic energy of the air? (c) Assume that the windmill converts 25% of the wind’s
energy into electrical energy, and that A = 30m2 , v = 36km/hr and density of air is 1.2 kg   m–3.         What   is the electrical power produced?
14.A disc revolves with a speed of 33 rev/min and has a radius of 15cmTwo coins are placed at 4
cm    and 14 cm away from the centre of the record. If the coefficient of friction between the coins
and  the record is 0.15, which of the coins will revolve with the record?
OR
Why circular roads are banked? Derive an expression for maximum possible speed with which
angle of banking is needed for safe circular turn

### CBSE CLASS11 TERM TEST PHYSICS PAPER

1. The moment of inertia of two rotating bodies A and B are IA and IB(IA > IB) and their angular momenta
are equal. Which one has a greater kinetic energy?
2. What is the value of torque on the planet due to the gravitational force of sun?
3. Name two factors which determine whether a planet would have atmosphere or not.
4. A satellite does not require any fuel to orbit the earth. Why?
5. Why is it easier to swim in sea water than in the river water?
6. From a uniform disc of radius R, a circular disc of radius R/2 is cut out. The centre of the hole is at R/2
from the   centre of original disc. Locate the centre of gravity of the resultant flat body
OR
If the radius of the earth were to decrease by 1%, keeping its mass same, how will the acceleration due
to gravity change?
7. A metre stick is balanced on a knife edge at its centre. When two coins, each of mass 5 g are put one
on top of the other at the 12.0 cm mark, the stick is found to be balanced at 45.0 cm, what is the mass
of the meter stick?
8. How will you distinguish between a hardboiled egg and a raw egg by spinning it on a table top?
9. Give the physical significance of moment of inertia.
10. Define gravitational potential. Give its SI units.
11. A disc 8.00 cm in radius rotates at a constant rate of 1 200 rev/min about its central axis. Determine
(a) its angular speed, (b) the linear speed at a point 3.00 cm from its center, (c) the radial acceleration
of a point on the rim, and (d) the total distance a point on the rim moves in 2.00 s.
12. A weight of 50.0 N is attached to the free end of a light string wrapped around a reel with a radius of
0.250 m and a mass of 3.00 kg. The reel is a solid disk, free to rotate in a vertical plane about the
horizontal axis passing through its center. The weight is released 6.00 m above the floor.
(a) Determine the tension in the string, the acceleration of the mass, and the speed with which the
weight hits the floor. (b) Find the speed calculated inpart (a), using the principle of conservation of
energy.
13. A cylinder of mass 10.0 kg rolls without slipping on a horizontal surface. At the instant its center of
mass has a speed of 10.0 m/s, determine (a) the translational kinetic energy of its center of mass, (b)
the rotational energy about its center of mass, and (c) its total energy.
14.Define a) Young’s Modulus b) Shear Modulus c) Bulk modulus
15. An airliner arrives at the terminal, and its engines are shut off. The rotor of one of the engines has an
initial clockwise angular speed of 2 000 rad/s. The engine’s rotation slows with an angular
acceleration of magnitude 80.0 rad/s2. (a) Determine the angular speed after10.0 s. (b) How long
does it take for the rotor to comes to rest?

16.Explain how hydraulic lift works

17.A satellite of the Earth has a mass of 100 kg and is at an altitude of 2.00 x106 m. (a) What is the
potential energy of the satellite–Earth system? (b) What is the magnitude of the gravitational force
exerted by the Earth on the satellite? (c) What force does the satellite exert on the Earth

18.The manual of a car instructs the owner to inflate the tyres to a pressure of 200 k pa. (a) What is the
recommended gauge pressure? (b) What is the recommended absolute pressure (c) If, after the
required inflation of the tyres, the car is driven to a mountain peak where the atmospheric pressure is
10% below that at sea level, what will the tyre gauge read?
19. Prove that the elastic potential energy per unit volume is equal to1/2×stress × strain.
OR
Which is more elastic steel or rubber. Explain.

20. The mass of planet Jupiter is 1.9 × 1027 kg and that of the sun is 1.99 × 1030kg. The mean distance of
Jupiter from the Sun is 7.8 × 1011m. Calculate gravitational force which sun exerts on Jupiter, and the
speed of Jupiter.

21.Explain if the ice on the polar caps of the earth melts, how will it affect the duration of the day?

22. Obtain an expression for the work done by a torque. Hence write the expression for power.
23.Kamal helps others.once,hand of his friend got wounded. He went to a doctor and an injection of
tetanus is given to his friend.
(i)                  What values do you appreciate in Kamal?
(ii)                The nurse filled two syringes of different bores. Diameter of small syringe is 2cm and of larger piston is 3 cm.Which syringe will hurt  more to his friend if the same force is applied on both
24.a) Prove that the rate of change of angular momentum of a system of particles about a reference
point is equal to the net torque acting on the system.
b)Derive the relation between angular momentum and moment of Inertia for rigid body rotating
OR
Obtain an expression for linear acceleration of a cylinder rolling down on inclined plane and hence
find condition for cylinder to roll down inclined plane without slipping
25.a) Define escape velocity obtain an expression for escape velocity of a body from the surface of
earth?
b) Derive expression for the orbital velocity of a satellite and its time period. What is a geostatinary
satellite?. Obtain the expression for the height of the geostationary satellite.
OR
Explain Kepler”s Planetary motion for a) Law of orbits b) Law of areas c) Law of Periods and  Prove
law  of Periods
26. Draw and discuss stress versus strain graph, explaining clearly the terms elastic limit, permanent
set, proportionality limit ,Elastic region, Plastic region and Fracture point
OR

a) State Pascal’s law and prove it. b) Show how pressure varies with depth.

### cbse class12 preboard physics paper

1. At what angle of incidence should a beam of light strike a glass slab of refractive index √3,
such that the reflected and refracted rays are perpendicular to each other?
2. A glass rod rubbed with silk acquires a charge of 1.6X10-12C .What is the charge on
silk?
3. What is the velocity of a charged particle moving in between the equal electric and
magnetic fields applied in opposite directions?
4. Name the physical quantity which is measured in weber/A
5. If the length of a cylindrical metallic conductor is doubled, how does the resistivity
of conductor change?
6. If the output of OR gate is given as an input of NOT gate. Name the gate so formed. Write
the logic symbol and the truth table of this gate.
7. Draw a neat and labeled diagram to show the image formation in a compound microscope.
Write the expression for magnifying power,when final image is formed at near point.
8. The magnetic flux through a coil perpendicular to its plane and directed into paper
is varying according to the relation Ф = 5t2 + 10t + 5 milli weber. Calculate the e.m.f
induced in the loop at t = 5 sec.
OR
A galvanometer coil has a resistance of 12 Ω and It shows full scale deflection for a
current of 3 mA.How will you convert the galvanometer into voltmeter of range 0 to
18 volt?
9. Name the device which is used to transform a.c. supply from one level of voltage to
another. State the principle based on which it works.
10. A radioactive isotope has a half-life of T years.How long will it takefor the activity
to reduce to 3.125 % of its initial value.
11. A potential of E = 50 sin (200 πt + π/4) is applied across a resistor of 10 Ω resistance.
Find 1. rms voltage. 2. Frequency of a.c 3. rms value of current.
12. Show by a ray diagram, the image formation of a point object by a thin double convex
lens having radii of curvature R1 and R2. Hence the derive the relation
1/f = (μ- 1) (1/R1 – 1/R2) where ‘f ‘is the focal length and ‘μ’ is the refractive index
of material of the lens with respect to air.

13. A potential difference of 2 V is applied between the points A and B shown in network drawn below.
Calculate the equivalent resistance of the network between the points A and B and
the magnitudes of currents flowing in the arms AFCEB and AFDEB.

14. Define the term ‘drift velocity’ of electrons in a conductor. Give it’s SI unit. Show
that the resistance of a conductor is given by R = m l/ne2Aτ using the concept of
drift velocity.

15. a) What are coherent sources? Can two individual monochromatic source of light
behave as coherent sources?
b)What is the effect on the fringe width observed in a young’s double slit
experiment, when
a. Screen is moved away from the plane of the slits.
b. Separation between the slits is increased.
c. whole apparatus is immersed in water.

16. A proton and a deuteron are accelerated through the same accelerating potential.
Which one of the two has (a) greater value of de-Broglie wavelength associated
with it, and (b) less momentum?

17. Explain how an audio signal is extracted from the modulating wave with a neat
diagram. What is demodulation?
OR
Define i) carrier signal ii) amplitude modulated signal iii) attenuation
18. Name the term that is found missing in ampere’s circuital law according to Maxwell.
Derive the expression for the same and hence write the modified form of
ampere’s circuital law.
19. Define “half-life period” of a radioactive element. Derive the relation T1/2 = 0.693 /λ.
where λ is decay constant.
20. a) In case of a photosensitive surface, state the role of threshold wavelength of
incident photon for photo electric effect.
b) Plot a graph showing the variation of photo current vs collector potential for three
different intensities I1 > I2 > I3and having same threshold frequency for a given
metallic cathode.
c) Write the Einstein’s photo electric equation.
21. Distinguish between dia, para and ferro magnetic materials (any three points for
each)
22. State ‘Bohr’s angular momentum quantization condition’. Derive an expression
forthe radius of nth orbit of an electron revolving around the nucleus of a hydrogen
atom.
23. Yogesh was using a galvanometer in the practical class. Unfortunately it fell from
his hand and broke.His friend’s advised him not to tell the teacher. He was upset. But he
decided to tell the teacher. Teacher listened to him patiently on knowing that the act
was not intentional, but just an accident, and used the opportunity to show the internal
structure of galvanometer to the whole class.
(i) What are the values displayed by yogesh.
(ii) Explain the principle based on which moving coil galvanometer works.
(iii) What is the significance of figure of merit of moving coil galvanometer?

24. a) A parallel plate capacitor is charged to a potential difference ‘V’ by a d.c.
source. The capacitor is then disconnected from the source. If the distance between the
plates is doubled, state with reason how the following will change?
a. electric field between the plates.
b. capacitance
c. energy stored in the capacitor.

b) Three capacitors are connected as shown in figure given below. Calculate the

equivalent capacitance between the points X and Y. ( Given that C1 = C2 = C3 = 2μF.)

25. a) A concave lens made of material of refractive index ‘n2’ is kept in a reference
medium of refractive index ‘n1’ . Trace the path of parallel beam oflight passing
through the lens when
a. n1= n2     b. n12    c. n1> n2
b) The far point of a myopic person is 80 cm in front of the eye. What is the focal
power of the lens required to enable him to see very distant objects clearly?
OR
a) Deduce Snell’s law of refraction using Huygens wave theory of light with a neat
diagram.
b) Define “resolving power” of a compound microscope. On what factors does it
depend?
26. a) Explain the formation of depletion layer and potential barrier in a p-n junction diode.
b) What is called Photo diode? Three photo diodes D1, D2 and D3 are made of
semiconductors having band gaps of 2.5 eV, 2 eV and 3 eV respectively. Which one
of them will be able to detect the wavelength of 600 nm? `
OR
a) With the help of a neat and labeled circuit diagram, explain how an n-p-n transistor can be
used as an amplifier in common emitter configuration.
b) Write an expression for voltage gain. Explain how the input and output voltages
change by the phase difference of 1800.
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