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AnswersCHAPTER- TRANSMISSION OF HEAT 1. The thermal conductivity of a material in CGS system is 0.4. In steady state, the rate of flow of heat is 10 cal/s-cm2, then the thermal gradient is (in oC/cm) i) 10 ii) 12 iii) 25 iv) 20 2. In Searle’s method for finding conductivity of metals, the temperature gradient along the bar i) Is greater near the hot end ii) Is greater near the cold end iii) Is the same at all points along the bar iv) Increases as we go from hot end to cold end 3. A wall has 2 layers A and B made of different materials. The thickness of both the layers is same. The thermal conductivity of A and B are Ka and Kb such that Ka=3Kb. The temperature across the wall is 20oC. In thermal equilibrium, i) The temp difference across A is 15 oC ii) The temp diff across A is 5 oC iii) The temp diff across A ia 10 oC iv) The rate of transfer of heat through A is much more that through B DOUBT: which of the 2 configurations do we consider for Q3. A or B A B In both the cases, thickness of A and B are the same. In the 1st figure, A & B and are in parallel. In 2nd figure, A & B are in series. 4. Mud houses are cooler in summer and warmer in winter because i) Mud is a superconductor of heat ii) Mud is a good conductor of heat iii) Mud is a bad conductor of heat iv) None of these 5. Consider a compound slab consisting of 2 diff materials having equal thickness and thermal conductivities K & 2K respectively. The equivalent thermal conductivity of the slab is i) Root (2K) ii) 3K iii) 4K/3 iv) 2K/3 6. Assuming the Sun to have a spherical outer radius of r radiating like a black body at temp t oC, What is the power received by a unit surface, (normal to the incident rays) at a distance R from the centre of the Sun.? 7. Assuming the sun to be a spherical body of radius R at a temperature of T K, evaluate the total radiant power incident on Earth, at a distance r from the sun. DOUBT- What is the difference btwn Q6 & Q7? How do I solve both? CHAPTER- SURFACE TENSION Surface tension(ST) 1. The value of ST of a liquid at critical temperature is i) 0 ii) Infinite iii) Btwn 0 and infinite iv) Cannot be determined 2. Oil spreads over the surface of water whereas water does not spread over the surface of oil, due to i) ST of water is very high ii) ST of water is very low iii) Viscosity of oil is high iv) Viscosity of water is high 3. ST of a liquid is found to be influenced by i) Increases with increase in temperature ii) Nature of liquid in contact iii) Presence of soap that increases it iv) Its variation with the concentration of the liquid DOUBT- doesn’t ST only depend on nature of liquid? 4. ST may be defined as i) The work done per unit area in increasing the surface area of a liquid under isothermal condition ii) The work done per unit area in increasing the surface area of a liquid under adiabatic condition iii) The work done per unit area in increasing the surface area of a liquid under both isothermal condition & adiabatic condition iv) Free surface energy per unit volume 5. ST is and atomic/molecular property of i) Atoms in the interior ii) Atoms at surface iii) Atoms which are exterior iv) Atoms at surface bounded entirely by different atoms 6. A liquid does not wet the sides of a solid, if the angle of contact is i) Zero ii) Obtuse iii) Acute iv) 90 DOUBT- the liquid does not wet the sides of the liquid if angle of contact is 90 or obtuse. 7. The value of contact angle for kerosene with solid surface i) 0 ii) 90 iii) 45 iv) 33 8. The radii of 2 soap bubbles are r1 & r2. In isothermal conditions, two meet together in vacuum. Then the radius of the resultant bubble is given by i) R=(r1+r2)/2 ii) R=r1(r1.r2+ r2) iii) (R)2=(r1)2+ (r2)2 iv) R=r1+r2 9. When a large bubble rises from the bottom of a lake to the surface, its radius doubles. If the atm pressure is equal to that of column of water height H, then the depth of th lake is i) H ii) 2H iii) 7H iv) 8H 10. The volume of an air bubble becomes 3 times as it rises from the bottom of a lake to its surface. Assuming atm pressure to be 75 cm of Hg and the density of water to be 1/10 of the density of mercury, the depth of the lake is i) 5m ii) 10m iii) 15m iv) 20m 11. In Jager’s method, at the time of bursting of the bubble i) internal pressure of the bubble is always greater than external pressure ii) internal pressure of the bubble = external pressure iii) internal pressure of the bubble < external pressure iv) internal pressure of the bubble is always slightly greater than external pressure
