## Saturday, 26 July 2014

### CLASS 12 PHYSICS TEST PRACTICE 4

Practice Test Paper 4

Physics (Theory) – XII

Time allowed: 3 hours                                                                                                 Maximum marks: 70  Q. 1. An electrostatic field line cannot be discontinuous. Why? 1 Q. 2. The force experienced by a particle of charge q moving with a velocity in a magnetic field is given by = q( x ). Which pairs out of these vectors are always at right angles to each other? 1 Q. 3. Why does the electric field inside a dielectric decrease when it is placed in an external electric field? 1 Q. 4. Electrons are emitted from a photosensitive surface when it is illuminated by green light but electron emission does not take place by yellow light. Will the electrons be emitted when the surface is illuminated by (a) red light, and (b) blue light? 1 Q. 5. Name the process by which exact reproduction of a document at a distant place can be received. 1 Q. 6. Define the term ‘temperature coefficient of resistivity’. Show graphically the variation of resistivity with temperature for nichrome. 2 Q. 7. Define ‘electric line of force’ and give its two important properties. 2 Q. 8. The image of a candle is formed by a convex lens on a screen. The lower half of the lens is painted black to make it completely opaque. Draw the ray diagram to show the image formation. How will this image be different from the one obtained when the lens is not painted black? 2 Q. 9. Write two characteristic properties to distinguish between diamagnetic and paramagnetic materials. 2 Q. 10. A circular coil of radius 8 cm and 20 turns rotates about its vertical diameter with an angular speed of 50 s-1 in a uniform horizontal magnetic field of magnitude 3x10-2T. Find the maximum and average value of the emf induced in the coil. 2 Q. 11. Write two conditions necessary for total internal reflection to take place. 2 Q. 12. Draw a labelled circuit diagram to show the use of zener diode as a voltage regulator. 2 Q. 13. A galvanometer with a coil of resistance 120 ohm shows full scale deflection for a current of 2.5 mA. How will you convert the galvanometer into an ammeter of range 0 to 7.5 A? Determine the net resistance of the ammeter. When an ammeter is put in a circuit, does it read slightly less or more than the actual current in the original circuit? Justify your answer. 3 Q. 14. Give reasons for the following: (a)    Long distance radio broadcasts use short-wave bands. (b)   The small ozone layer on top of the stratosphere is crucial for human survival. (c)    Satellites are used for long distance TV transmission. 3 Q. 15. Deduce an expression for the capacitance of a parallel plate capacitor with air as the medium between the plates. 3 Q. 16. With the help of a circuit diagram explain in brief the use of a potentiometer for comparison of emf of two cells. 3 Q. 17. For a common emitter transistor amplifier, the audio signal voltage across the collector resistance of 2 kΩ is 2 V. If the current amplification factor of the transistor is 100, calculate (a)    input signal voltage, (b)   base current, and (c)    power gain. Given that the value of the base resistance is 1 kΩ. 3 Q. 18. Using Huygen’s principle, explain the refraction of a plane wavefront at a plane surface. Hence prove Snell’s law. 3 Q. 19. Write the nuclear reactions for the following: (a)    a-deacy of (b)   b- - decay of (c)    b+ - decay of 3 Q. 20. A converging lens has a focal length of 20 cm in air. It is made of material of refractive index 1.6. If it is immersed in a liquid of refractive index 1.3, what will be its new focal length? How does the nature of the lens change if this lens is immersed in a liquid of refractive index 1.8? 3 Q. 21. Write two applications each of (a)    microwaves, (b)   infra-red waves, and (c)    radio waves. 3 Q. 22. Mention the significance of Davisson-Germer experiment. An a-particle and a proton are accelerated from rest through the same potential difference V. Find the ratio of de-Broglie wavelengths associated with them. 3 Q. 23. (a)    Draw a labelled ray diagram to show the formation of an image by a compound microscope. Write the expression for its magnifying power. (b)   How does the resolving power of a compound microscope change, when   (i)                 refractive index of the medium between the object and the objective lens increases, and (ii)               wavelength of the radiation used is increased? 3 Q. 24. Distinguish between analog and digital communication. Write any two modulation techniques employed for the digital data. Describe briefly one of the techniques used. 3 Q. 25. A dielectric slab of thickness ‘t’ is kept in between the plates, each of area ‘A’, of a parallel plate capacitor separated by a distance ‘d’. Derive an expression for the capacitance of this capacitor for t<

### CLASS 12 PHYSICS TEST PRACTICE 3

Practice Test Paper 3

Physics (Theory) – XII

Time allowed: 3 hours                                                                                                 Maximum marks: 70  Q. 1.
An electron and a proton, moving parallel to each other in the same direction with equal momenta, enter into a uniform magnetic field which is at right angles to their velocities. Trace their trajectories in the magnetic field.

1
Q. 2.
Two identical loops, one of copper and another of constantan are removed from a magnetic field within the same time interval. In which loop will the induced current be greater?

1
Q. 3.
The power factor of an a.c. circuit is 0.5. What will be the phase difference between voltage and current in this circuit?

1
Q. 4.

# Does the ‘stopping potential’ in photoelectric emission depend upon

(a)    the intensity of the incident radiation in a photocell?
(b)   The frequency of the incident radiation?

1
Q. 5.
What is the function of ‘cladding’ in a typical optical fibre?

1
Q. 6.
(a)    Draw the graphs showing variation of inductive reactance and capacitive reactance with frequency of applied a.c. source.

(b)   Can the voltage drop across the inductor or the capacitor in a series LCR circuit be greater than the applied voltage of the a.c. source? Justify your answer.

2
Q. 7.
Define electric field intensity. Write its S.I. unit. Write the magnitude and direction of electric field intensity due to an electric dipole of length 2a at the mid-point of the line joining the two charges.

2
Q. 8.
State the underlying principle of a transformer. Write two causes of power loss in it.

2
Q. 9.
Two dielectric slabs of dielectric constant K1 and K2 are filled in between the two plates, each of area A, of the parallel plate capacitor as shown in the figure. Find the net capacitance of the capacitor.  d 2
Q. 10.
Explain the term remote sensing in communication. Name the satellite used for this purpose.

2
Q. 11.
With the help of diagrams distinguish between forward biasing and reverse biasing of a p-n junction diode.

2
Q. 12.
Monochromatic light is refracted from air into glass of refractive index n. Find the ratio of wavelengths of the incident and refracted light.

2
Q. 13.
An electric dipole is held in a uniform electric field. (a) using suitable diagram, show that it does not undergo any translatory motion, and (b) derive an expression for the torque acting on it and specify its direction.

3
Q. 14.
Explain, with the help of diagram, the principle and working of an a.c. generator. Write the expression for the e.m.f. generated in the coil in terms of its speed of rotation.

3
Q. 15.
Define the term ‘temperature coefficient of resistivity’. Write its SI unit. Plot a graph showing the variation of resistivity of copper with temperature.

3
Q. 16.
Draw a labelled diagram of a moving coil galvanometer. Explain its working principle. Can this galvanometer be used to detect an a.c. in a circuit? Give reason.

3
Q. 17.
A battery of emf 10 V and internal resistance 3Ω is connected to a resistor R.

(a)    If the current in the circuit is 0.5 A, calculate the value of R.
(b)   What is the terminal voltage of the battery when the circuit is closed?

3
Q. 18.
Draw a labelled diagram of experimental setup of Rutherford’s alpha particle scattering experiment. Write two important inferences drawn from this experiment.

3
Q. 19.
With the help of a labeled diagram explain in brief the working of a cyclotron.

3
Q. 20.
Calculate the binding energy per nucleon (in MeV) for and . Comment on the difference of these binding energies and its significance in relation to a-decay of the nuclei. Given: mass of = 1.00783 u, mass of = 1.00867 u, mass of = 3.01664 u, mass of = 4.00387 u.

3
Q. 21.
A ground receiver station is receiving a signal at (a) 5 MHz and (b) 100 MHz, transmitted from a ground transmitter at a height of 300 m located at a distance of 100 km. Identify whether it is coming via space wave or sky wave propagation or satellite transponder. (Given the value of radius of the earth is 6400 km and maximum electron density, Nmax = 1012 m-3)

3
Q. 22.
(a)    Draw the energy level diagram showing the emission of b-particles followed by g-rays by a .
(b)   Plot the distribution of kinetic energy of b-particles and state why the energy spectrum is continuous.

3
Q. 23.
Name the constituent radiation of electromagnetic spectrum which
(a)    is used in satellite communication.
(b)   is used for studying crystal structure.
(c)    is similar to the radiations emitted during decay of radioactive nuclei.
(d)   has its wavelength range between 390 nm and 770 nm.
(e)    is absorbed from sunlight by ozone layer.
(f)    produces intense heating effect.

3
Q. 24.
Explain, with the help of a circuit diagram, how the thickness of depleteion layer in a p-n junction diode changes when it is forward biased. In the following circuits which one of the two diodes is forward biased and which is reverse biased?   -10V    (a)  (b)  -10 V
3
Q. 25.
Draw a level diagram of a Van de Graaff generator. Give its principle and explain its working.

Or

Define capacitance of a capacitor. Give its S.I. unit. Prove that the total electrostatic energy stored in a parallel plate capacitor is CV2 and hence find the expression for the energy density of the capacitor.

5
Q. 26.
(a)    Define the term ‘impedance of series LCR circuit’. Derive a mathematical expression for it using Phasor diagram.

(b)   Obtain the resonant frequency of a series LCR circuit with L = 2.0 H, C = 32 mF and R = 10 Ω

Or

(a)    Distinguish between the terms resistance, reactance and impedance of an a.c. circuit.
(b)   A 100 mF capacitor in series with a 40 Ω resistance is connected to a 100 V, 60 Hz supply. Calculate (i) the reactance, (ii) impedance, and (iii) maximum current in the circuit.

5
Q. 27.
Using Huygen’s principle, draw a diagram to show propagation of a wavefront originating from a monochromatic point source.
Describe diffraction of light due to a single slit. Explain formation of a pattern of fringes obtained on the screen and plot showing variation of intensity with angle q in single slit diffraction.

Or

What is meant by a linearly polarized light? Which type of waves can be polarized? Briefly explain a method for producing polarized light.
Two polaroids are placed at 90o to each other and the intensity of transmitted light is zero. What will be the intensity of transmitted light when one more polaroid is placed between these two bisecting the angle between them? Take intensity of unpolarised light as I.

5