CBSE Board Physics Question Paper -1998
Q. 16. How does the (i) magnifying power and (ii) resolving power of a telescope change on increasing the diameter of its objective ? Give reasons for your answer.
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Q. 17. Explain the laws of photoelectric emission on the basis of Einstein’s photoelectric equation.
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Q. 18. Calculate the half life period of a radioactive substance if its activity drops to l/16th of its initial value in 30 years.
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Q. 19. A potential difference of 2V is applied between points A and B shown in the network drawn in the figure. Calculate (i) equivalent resistance of the net work across points A and B (ii) the magnitude of current flowing in the arms AFCEB and AFDEB.
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Q. 20. The potential difference across the terminals of a battery of e.m.f. 12 V and internal resistance 2Ω drops to 10 V when connected to a silver voltameter. Calculate the silver deposited at the cathode in half an hour. Atomic weight of silver is 107.9 g mole-1. 1 faraday = 96,500 C mole-1
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Q. 21. Derive the formula for the force between two parallel straight conductors carrying the current in opposite direction and write the nature of the force. Hence define an ampere.
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Q. 22. What are coherent sources of light ? Why is no interference pattern observed when two coherent sources are
(i) infinitely close to each other ‘.’ (ii) far apart from each other ?
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Q. 23. A slit of width d is illuminated by light of wavelength 6,500 A. For what values of d will the
(i) first minimum fall at an angle of diffraction of 30° ?
(ii) first maximum fall at an angle of diffraction of 30° ?
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Q. 24. Draw a ray diagram to show the formation of an image of an object placed between f and 2 f of a thin concave lens. Deduce the relation between the object distance, the image distance and the focal length of the leans under this condition .
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Q. 25. Define the terms potential barrier and depletion region for a p-n juction diode. State how the thickness of depletion region will change when the p-n junction diode is (i) forward biased ; (ii) reverse biased.
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Q. 26. With the help of a labelled circuit diagram, explain the use of a transistor as an oscillator.
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Q. 27. Define absolute magnitude of a star. Assuming the diminiest star visible to the naked eye has a magnitude of 6, compare the brightness with planet Venus which has a magnitude of —4.
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Q. 28. Derive an expression for the energy .stored in a parallel plate capacitor with air as t medium between its plates. How does the total energy stored by the capacitor change when t medium of the air is replaced by a medium of dielectric constant K ? Explain.
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Q. 29. By using Bohr’s postulates of atomic model derive mathematical expression for (i) kinetic energy and (ii) potential energy of an electron revolving in an orbit or radius r. How does the potential energy change with the increase of principal quantum number n for the electron and why ?
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Q. 30. Draw the graphs showing the variation of reactance of (a) a capacitor and (b) an inductor with the frequency of an a.c. circuit.
11 kW of electric power can be transmitted to a distant station at (i) 220 V or (ii) 22,000 V. Which of the two modes of transmission should be preferred and why 7 Support your answer with possible calculations.
Prove that an ideal inductor does not dissipate power in an a.c. circuit.
A sinusoidal voltage V = 200 sin 314 t is applied to a resistor of 10Q resistance. Calculate
(i) r.m.s. value of the voltage
(ii) r.m.s- value of the current, and
(iii) power dissipated as heat in watts.
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