PREVIOUS QUESTION PAPERS CBSE Question Paper 2007 - Class XII Physics

SET -1

1. In a series LCR circuit, the voltages across an inductor, a capacitor and a resisitor are 30 V, 30V and 60 V respectively. What is the phase difference between the applied voltage and the current in the circuit?


2. Ultraviolet radiations of different frequencies f₁ and f₂ are incident on two photosensitive materials having work functions W₁ and W₂ (W₁ > W₂ respectively. The kinetic energy of the emitted electrons is same in both cases. Which one of the two radiations will be of higher frequency?


3. Define the term "activity" of a radio active nuclide. Write its S.I. unit.


4. An electron is moving along the +ve X axis in the presence of uniform magnetic field along +ve Y axis. What is the direction of force acting on it?


5. What should be the length of dipole antenna for a carrier wave of frequency 6 x 10⁸Hz?


6. Two point charges 4µC and -2µC are separated by a distance of 1 m in air. Calculate at what point on the line joining the two charges is the electric potential zero.


7. A voltage of 30 V is applied across a carbon resistor with first, second and third rings of blue, black and yellow colours respectively. Calculate the value of current, in mA, through the resisitor.


8. A galvanometer has a resistance of 30 Ω. It gives full scale deflection with a current of 2 mA. Calculate the value of the resistance needed to convert it into an ammeter of range 0 - 0.3 A.


9. Calculate the current drawn by the primary of a transformer which steps down 200 V to 20 V to operate a device of resistance 20 Ω. Assume the efficiency of the transformer to be 80%.
   OR
   An a.c. voltage of 100 V, 50 Hz is connected across a 20 Ω resistor and 2 mH inductor in series. Calculate (i) impedence of the circuit. (ii) rms current in the circuit.


10. Define resolving power of a compound microscope. How does the resolving power of a compound microscope change when
    (i) refractive index of the medium between the object and objective lens increases.
    (ii) wavelength of the radiation used is increased.


11. Draw a graph showing the variation of potential energy between a pair of nucleons as a function of their separation. Indicate the regions in which the nuclear force is (i) attractive (ii) repulsive


12. Two semiconducting materials X and Y as shown in in figure are made by doping germanium crystal with indium and arsenic respectively. The two are joined end to end and connected to a battery as shown.
    
    (i)Will the junction be forward biased or reverse biased?
    (ii) Sketch a V-I graph for this arrangement.


13. State Gauss Theorem in electrostatics. Apply this theorem to derive an expression for electric field intensity at a point near an infinitely long straight charged wire.


14. Explain the underlying principle of working of a parallel plate capacitor. If two similar plates, each of area A having surface charge densities +σ and -σ are separated by a distance d in air, write expressions for
    (i) the electric field at points between the two plates.
    (ii) the potential difference between the two plates.
    (iii) the capacitance of the capacitor so formed.


15. For the potentiometer circuit shown in the figure, points X and Y represent the two terminals of an unknown e.m.f. E'. A student observed that when the jockey is moved from the end A to the end B of the potentiometer wire, the deflection in the galvanometer remains in the same direction.
    What may be the two possible faults that could result in this observation?
    If the Galvanometer deflection at the end B is (i) more, (ii) less, than at the end A, which of the two faults listed above, would be there in the circuit?
    Give reasons in support of your answer.
    
    OR
    The given figure below shows a network of resistances R₁, R₂, R₃ and R₄. Usinf Kirchhoff's laws, establish the balance condition for the network.
    


16. out of syllabus


17. Name the following constituent radiations of the electromagnetic spectrum which
    (i) produces intense heating effect.
    (ii) is absorbed by the ozone layer in the atmosphere.
    (iii) is used for studying crystal structure.
    Write one moreapplication for each of these radiations


18. A double convex lens of glass of refractive index 1.6 has its both surfaces of equal radii of curvature of 30 cm each. An object of height 5 cm is placed at a distance of 12.5 cm from the lens. Calculate the size of the image formed.


19. Draw a schematic diagram of the experimental arrangement used by Davisson and Germer to establish the wave nature of electrons. Explain briefly how the de-Broglie relation was experimentally verified in case of electrons.


20. Draw the graph to show the vaiation of binding energy per nucleon with mass number of different atomic nuclei. Calculate the binding energy per nucleon of ⁴⁰Ca₂₀ nucleus. Given mass of ⁴⁰Ca₂₀ nucleus = 39.962589 u ; mass of proton = 1.007825 u; mass of neutron = 1.008665 u and 1 u = 931 MeV/c²


21. Draw the circuit diagram of a common emitter amplifier using n-p-n transisitor. What is the phase difference between the input signaland output voltage? State two reasons why a common emitter amplifier is preferred to a common base amplifier.


22. Explain the formation of energy bands in solids. Draw energy band diagrams for (i) a conductor, and (ii) an intrinsic semiconductor


23. What is modulation? Explain the need of modulating a low frequency information signal. With the help of diagrams, differentiate between PAM and PDM


24. Write the acronym LASER in expanded form. State four reasons for preferring diode LASERS as light sources for optical communiation links.


25. Explain with the help of a diagram, the principle and construction of a cyclotron. Deduce an expression for the cyclotron frrequency and show that it does not depend on the speed of the charged particle.
    OR
    Distinguish the magnetic properties of dia-, para-, and ferro- magnetic substances in terms of (i) susceptibility (ii) magnetic permeability and (iii) coercivity. Give one example of each of these materials.
    Draw the field lines due to an external magnetic field near a (i) diamagnetic (ii) paramagnetic substance


26. Explain the term 'inductive reactance'. Show graphically the variation of inductive reactance with frequency of the applied alternating voltage.
    An a.c. voltage E = E₀sin ωt is applied across a pure inductor of inductance L. Show mathematically that the current flowing through it lags the applied voltage by a phase angle of π/2
    OR
    Explain the term 'capacitive reactance'. Show graphically the variation of capacitive reactance with frequency of the applied alternating voltage.
    An a.c. voltage E = E₀sin ωt is applied across a pure capacitor of capacitance C. Show mathematically that the current flowing through it leads the applied voltage by a phase angle of π/2


27. State the essential condition for diffraction of light to take place.
    Use Huygens' Principle to explain diffraction of light due to a narrow sngle slit and the formation of a pattern of fringes obtained on the screen. Sketch the pattern of fringes formed due to diffraction at a single slit showing variation of itensity with angle θ
    OR
    What are coherent sources of light? Why are coherent sources required to obtain sustained interference pattern?
    State three characteristic features which distinguish the interfernce pattern due to two coeherently illuminated sources as compared to that observed in a diffraction pattern due to a single slit.