CBSE Physics Previous Question Paper
All India (Comptt.) 1998

 

 

 

1. What is the limit of frequency up to which signals using a sky wave can be transmitted?

2. Draw the graph showing the variation of See beck coefficient with temperature of the hot junction.

3. What is the power factor of an LCR series a.c. circuit at resonance?

 

4. The binding energy per nucleon for two nuclei A, B is given as 4 MeV and 8.5 MeV. Which of the two nuclei is more stable?

 

5. Why is the energy of the electron emitted during beta decay continuous?

 

6. “The magnetic lines of force prefer to pass through iron than air.” Give reason,

 

7. The surface temperatures of two stars are in the ratio 1: 2. What is the ratio of the energy radiated by them per second?

 

8. Two point charges 5 µC and 10 µC are separated by a distance V in air. If an additional charge of - 4 µC is given to each, by what factor does the force between the charges change?

 

9. Explain briefly, with a ray diagram, how a mirage is formed in deserts.

 

10. A wire of 5 ohm resistance is stretched to twice its original length. What will be its (0 new resistivity, and (ii) new resistance?

 

11. The following circuit shows the use of potentiometer to measure the internal resistance of a cell:
(i) When the key K is open, how does the balance point change, if the current from the driver cell decreases?
(ii) When the key K is closed, how does the balance point change if R is increased, keeping the current from the driver cell constant?

 

12. Three identical resistors when connected in series to a d.c. source, dissipate a power X watts. If these resistors are connected in parallel to the same d.c. source, what will be the power dissipation in this case?

 

13. Draw energy band diagrams of (i) n-type, and (ii) /Hype semiconductors.

 

14. A proton, placed in a uniform electric field of magnitude 10³ NC^-1, moves from a point A to B in the direction of electric field. If AB = 0.1 m, calculate the
(i) potential difference between A and B, and (ii) work done in moving the proton from A to B.

 

15. An air-core solenoid is connected to an a.c. source and a bulb. If an iron-core is inserted in the solenoid, how does the brightness of the bulb change? Give reason for your .

 

16. A small light source having uniform luminous intensity of 10 candela is situated at the centre of a spherical surface of radius 1 m. Calculate (0 the luminous flux, and (ii) illuminance on the spherical surface.

 

17. Write, in brief, the method to determine the distance of an inferior planet from the earth.

 

18. A circular coil of 30 turns and radius 8.0 cm carrying a current of 6.0 A is suspended vertically in a uniform horizontal magnetic field of magnitude 1.0 T. The field lines make an angle of 60° with the normal to the coil. Calculate the magnitude of counter torque that must be applied to prevent the coil from turning.

 

19. If the frequency of incident radiation on a photo-cell is doubled for the same intensity, what changes will you observe in
(0 the kinetic energy of photo-electrons emitted Hi) photo-electric current, and (lit) stopping potential?

 

20. Write the expression for magnetic dipole moment for a closed current loop. Give its S.I. unit.
Derive the expression for the torque experienced by a magnetic dipole in a uniform magnetic field.

 

21. A battery of emf, E and internal resistance V, gives a current of 0.4 A with an external resistor of 12 ohm, and a current of 0.25 A with an external resistor of 20 ohm. Calculate (i) internal resistance and (ii) emf of the battery.

 

22. Two long parallel metallic wires carrying electric current in the opposite directions repel each other. Why? Derive the expression for the force experienced in this case.

 

23. When the solar spectrum is observed by means of a spectroscope, a number of dark lines crossing the spectrum are observed. What are these lines called? What is their origin? What information is obtained by studying these lines?

 

24. Sketch the graph showing the variation of intensity of transmitted light on the angle of rotation between a polarizer and analyser. A ray of light is incident at an angle of incidence i^p on the surface of separation between air and a medium of refractive index µ, such that the angle between the reflected and refracted rays is 90°. Obtain the relation between ip and µ.

25. A parallel beam of light is incident on an air-glass interface. Using Huygen’ sprinciple and drawing the sketches of wave fronts, show how the wave is refracted and hence verify law of refraction.

 

26. Write the truth table of a 2-input NOR gate. Explain, using a logic circuit, how a NOR gate can be converted into an AND gate.

 

27. Explain the use oip-n junction diode as a rectifier. Draw the circuit diagram of a full wave rectifier. Explain its working giving its input and output wave forms.

 

28. Derive an expression for the energy stored in a parallel plate capacitor. Hence show that electric field E itself is a source of energy with energy density
½ ε₀ E² Jm^-3

29. (a) State Bohr’s postulates for hydrogen atom.
(b) Draw the energy diagram showing the ground state, and the next few excited states for hydrogen (H2) atom. Mark the transition which corresponds to the emission of spectral lines for the B aimer-series.
© Calculate the wavelength of the first spectral line in this series.

 

30. (a) Write the principle on which an a.c. generator works. Draw its labelled diagram.
(b) An ax. generator consists of a coil of 50 turns and area 2.5 m² rotating at an angular speed of 60 rad s^-1 in a uniform magnetic field B = 0.2 T between the two fixed pole pieces. The resistance of the circuit including that of the coil is 500 ohm.
(i) Calculate the maximum current drawn from the generator.
(ii) What is the flux when the current is zero?
(iii) Would the generator work if the coil were stationary and instead the pole pieces rotated together with the same speed as above? Give reason.

 

Or

 

Explain, with the help of a labelled diagram, the working principle uf a step up transformer.
The primary of a transformer has 200 turns and the secondary has 1000 turns. If the power output from the secondary at 1000 V is 9 kW, calculate (0 the primary voltage, and

(ii) the heat loss in the primary coil if the resistance of primary is 0.2 ohm and the efficiency of the transformer is 90%.