Tiêu đề 14. Transmission of Heat Easy.pdf ✅

1. In which case the thermal conductivity increases from left to right (a) Al, Cu, Ag (b) Ag, Cu, Al (c) Cu, Ag, Al (d) Al, Ag, Cu 2. Which of the following cylindrical rods will conduct most heat, when their ends are maintained at the same steady temperature (a) Length 1 m; radius 1 cm (b) Length 2 m; radius 1 cm (c) Length 2 m; radius 2 cm (d) Length 1 m; radius 2 cm 3. The heat is flowing through two cylindrical rods of same material. The diameters of the rods are in the ratio 1 : 2 and their lengths are in the ratio 2 : 1. If the temperature difference between their ends is the same, the ratio of rate of flow of heat through them will be (a) 1 : 1 (b) 2 : 1 (c) 1 : 4 (d) 1 : 8 4. Two identical square rods of metal are welded end to end as shown in figure (i), 20 calories of heat flows through it in 4 minutes. If the rods are welded as shown in figure (ii), the same amount of heat will flow through the rods in (a) 1 minute (b) 2 minutes (c) 4 minutes (d) 16 minutes 5. For cooking the food, which of the following type of utensil is most suitable (a) High specific heat and low conductivity (b) High specific heat and high conductivity (c) Low specific heat and low conductivity (d) Low specific heat and high conductivity 6. Under steady state, the temperature of a body (a) Increases with time (b) Decreases with time (c) Does not change with time and is same at all the points of the body (d) Does not change with time but is different at different points of the body 7. The coefficient of thermal conductivity depends upon (a) Temperature difference of two surfaces (b) Area of the plate (c) Thickness of the plate (d) Material of the plate 8. When two ends of a rod wrapped with cotton are maintained at different temperatures and after some time every point of the rod attains a constant temperature, then (a) Conduction of heat at different points of the rod stops because the temperature is not increasing (b) Rod is bad conductor of heat (c) Heat is being radiated from each point of the rod (d) Each point of the rod is giving heat to its neighbour at the same rate at which it is receiving heat 9. The length of the two rods made up of the same metal and having the same area of cross-section are 0.6 m and 0.8 m respectively. The temperature between the ends of first rod is C o 90 and C o 60 and that for the other rod is 150 and C o 110 . For which rod the rate of conduction will be greater (a) First (b) Second (c) Same for both (d) None of the above 10. The ratio of thermal conductivity of two rods of different material is 5 : 4. The two rods of same area of cross-section and same thermal resistance will have the lengths in the ratio (a) 4 : 5 (b) 9 : 1 (c) 1 : 9 (d) 5 : 4 11. The thermal conductivity of a material in CGS system is 0.4. In steady state, the rate of flow of heat 10 cal/sec-cm2 , then the thermal gradient will be (a) 10C / cm (b) 12C / cm (c) 25C / cm (d) 20C / cm 12. Two rectangular blocks A and B of different metals have same length and same area of cross-section. They are kept in such a way that their cross-sectional area touch each other. The temperature at one end of A is 100°C and that of B at the other end is 0°C. If the ratio of their thermal conductivity is 1 : 3, then under steady state, the temperature of the junction in contact will be (a) 25°C (b) 50°C (c) 75°C (d) 100°C 13. Two vessels of different materials are similar in size in every respect. The same quantity of ice filled in them gets melted in 20 minutes and 30 minutes. The ratio of their thermal conductivities will be (a) 1.5 (b) 1 (c) 2/3 (d) 4 14. Two rods A and B are of equal lengths. Their ends are kept between the same temperature and their area of cross-sections are A1and A2and thermal conductivities K1 and K2 . The rate of heat transmission in the two rods will be equal, if 0 oC 100oC (i) 100 0 oC oC (ii)
(a) K1A2 = K2A1 (b) K1A1 = K2A2 (c) K1 = K2 (d) K1A1 2 = K2A2 2 15. In variable state, the rate of flow of heat is controlled by (a) Density of material (b) Specific heat (c) Thermal conductivity (d) All the above factors 16. If the ratio of coefficient of thermal conductivity of silver and copper is 10 : 9, then the ratio of the lengths upto which wax will melt in Ingen Hausz experiment will be (a) 6 : 10 (b) 10 : 3 (c) 100 : 81 (d) 81 : 100 17. The thickness of a metallic plate is 0.4 cm. The temperature between its two surfaces is C o 20 . The quantity of heat flowing per second is 50 calories from 2 5cm area. In CGS system, the coefficient of thermal conductivity will be (a) 0.4 (b) 0.6 (c) 0.2 (d) 0.5 18. In Searle's method for finding conductivity of metals, the temperature gradient along the bar (a) Is greater nearer the hot end (b) Is greater nearer to the cold end (c) Is the same at all points along the bar (d) Increases as we go from hot end to cold end 19. The dimensions of thermal resistance are (a) M−1L −2T 3K (b) ML 2T −2K −1 (c) ML 2T −3K (d) ML 2T −2K −2 20. A piece of glass is heated to a high temperature and then allowed to cool. If it cracks, a probable reason for this is the following property of glass (a) Low thermal conductivity (b) High thermal conductivity (c) High specific heat (d) High melting point 21. Two walls of thicknesses d1 and d2 and thermal conductivities k1 and k2 are in contact. In the steady state, if the temperatures at the outer surfaces are T1 and T2 , the temperature at the common wall is (a) 1 2 2 1 1 1 2 2 2 1 k d k d k T d k T d + + (b) 1 2 1 1 2 2 d d k T k d + + (c) 1 2 1 2 1 1 2 2 T T T T k d k d         + + (d) 1 1 2 2 1 1 1 2 2 2 k d k d k d T k d T + + 22. A slab consists of two parallel layers of copper and brass of the same thickness and having thermal conductivities in the ratio 1 : 4. If the free face of brass is at C o 100 and that of copper at C o 0 , the temperature of interface is (a) 80°C (b) 20°C (c) 60°C (d) 40°C 23. The temperature gradient in a rod of 0.5 m long is 80 oC/m. If the temperature of hotter end of the rod is 30 oC, then the temperature of the cooler end is (a) 40 oC (b) −10 oC (c) 10 oC (d) 0 oC 24. On heating one end of a rod, the temperature of whole rod will be uniform when (a) K = 1 (b) K = 0 (c) K = 100 (d) K =  25. Snow is more heat insulating than ice, because (a) Air is filled in porous of snow (b) Ice is more bad conductor than snow (c) Air is filled in porous of ice (d) Density of ice is more 26. Two thin blankets keep more hotness than one blanket of thickness equal to these two. The reason is (a) Their surface area increases (b) A layer of air is formed between these two blankets, which is bad conductor (c) These have more wool (d) They absorb more heat from outside 27. Ice formed over lakes has (a) Very high thermal conductivity and helps in further ice formation (b) Very low conductivity and retards further formation of ice (c) It permits quick convection and retards further formation of ice (d) It is very good radiator 28. Two rods of same length and material transfer a given amount of heat in 12 seconds, when they are joined end to end. But when they are joined lengthwise, then they will transfer same heat in same conditions in (a) 24 s (b) 3 s (c) 1.5 s (d) 48 s 29. Wires A and B have identical lengths and have circular cross- sections. The radius of A is twice the radius of B i. e. rA = 2rB. For a given temperature difference between the two ends, both wires conduct heat at the same rate. The relation between the thermal conductivities is given by (a) KA = 4KB (b) KA = 2KB (c) KA = KB/2 (d) KA = KB/4
30. Two identical plates of different metals are joined to form a single plate whose thickness is double the thickness of each plate. If the coefficients of conductivity of each plate are 2 and 3 respectively, then the conductivity of composite plate will be (a) 5 (b) 2.4 (c) 1.5 (d) 1.2 31. If the radius and length of a copper rod are both doubled, the rate of flow of heat along the rod increases (a) 4 times (b) 2 times (c) 8 times (d) 16 times 32. The coefficients of thermal conductivity of copper, mercury and glass are respectively Kc, Km and Kg such that Kc > Km > Kg. If the same quantity of heat is to flow per second per unit area of each and corresponding temperature gradients are Xc, Xm and Xg, then (a) Xc = Xm = Xg (b) Xc > Xm > Xg (c) Xc < Xm < Xg (d) Xm < Xc < Xg 33. If two metallic plates of equal thicknesses and thermal conductivities K1and K2 are put together face to face and a common plate is constructed, then the equivalent thermal conductivity of this plate will be (a) K1K2 K1+K2 (b) 2K1K2 K1+K2 (c) (K1 2+K2 2 ) 3/2 K1K2 (d) (K1 2+K2 2 ) 3/2 2K1K2 34. The quantity of heat which crosses unit area of a metal plate during conduction depends upon (a) The density of the metal (b) The temperature gradient perpendicular to the area (c) The temperature to which the metal is heated (d) The area of the metal plate 35. The ends of two rods of different materials with their thermal conductivities, radii of cross-sections and lengths all are in the ratio 1 : 2 are maintained at the same temperature difference. If the rate of flow of heat in the larger rod is 4 4 cal/sec , that in the shorter rod in cal/sec will be (a) 1 (b) 2 (c) 8 (d) 16 36. Two spheres of different materials one with double the radius and one-fourth wall thickness of the other, are filled with ice. If the time taken for complete melting ice in the large radius one is 25 minutes and that for smaller one is 16 minutes, the ratio of thermal conductivities of the materials of larger sphere to the smaller sphere is (a) 4 : 5 (b) 5 : 4 (c) 25 : 1 (d) 1 : 25 37. The ratio of the diameters of two metallic rods of the same material is 2 : 1 and their lengths are in the ratio 1 : 4. If the temperature difference between their ends are equal, the rate of flow of heat in them will be in the ratio (a) 2 : 1 (b) 4 : 1 (c) 8 :1 (d) 16 : 1 38. Two cylinders P and Q have the same length and diameter and are made of different materials having thermal conductivities in the ratio 2 : 3. These two cylinders are combined to make a cylinder. One end of P is kept at 100°C and another end of Q at 0°C. The temperature at the interface of P and Q is (a) C o 30 (b) C o 40 (c) C o 50 (d) C o 60 39. Two identical rods of copper and iron are coated with wax uniformly. When one end of each is kept at temperature of boiling water, the length upto which wax melts are 8.4cm and 4.2cm respectively. If thermal conductivity of copper is 0.92, then thermal conductivity of iron is (a) 0.23 (b) 0.46 (c) 0.115 (d) 0.69 40. Mud houses are cooler in summer and warmer in winter because (a) Mud is superconductor of heat (b) Mud is good conductor of heat (c) Mud is bad conductor of heat (d) None of these 41. The temperature of hot and cold end of a 20cm long rod in thermal steady state are at 100 oC and 20 oC respectively. Temperature at the centre of the rod is (a) 50 oC (b) 60 oC (c) 40 oC (d) 30 oC 42. Two bars of thermal conductivities K and 3K and lengths 1cm and 2cm respectively have equal cross-sectional area, they are joined lengths wise as shown in the figure. If the temperature at the ends of this composite bar is 0 oC and K 2/l respectively (see figure), then the temperature φof the interface is (a) 50 oC (b) 100 3 oC (c) 60 oC (d) 200 3 oC K1 K2 0 oC 100oC 1 cm 2 cm K 3K 
43. A heat flux of 4000 J /s is to be passed through a copper rod of length 10 cm and area of cross-section 2 100 cm . The thermal conductivity of copper is W m C o 400 / . The two ends of this rod must be kept at a temperature difference of (a) C o 1 (b) C o 10 (c) C o 100 (d) C o 1000 44. On a cold morning, a metal surface will feel colder to touch than a wooden surface because (a) Metal has high specific heat (b) Metal has high thermal conductivity (c) Metal has low specific heat (d) Metal has low thermal conductivity 45. In order that the heat flows from one part of a solid to another part, what is required (a) Uniform density (b) Density gradient (c) Temperature gradient (d) Uniform temperature 46. At a common temperature, a block of wood and a block of metal feel equally cold or hot. The temperatures of block of wood and block of metal are (a) Equal to temperature of the body (b) Less than the temperature of the body (c) Greater than temperature of the body (d) Either (b) or (c) 47. According to the experiment of Ingen Hausz the relation between the thermal conductivity of a metal rod is K and the length of the rod whenever the wax melts is (a) K/l = constant (b) K / l 2 = constant (c) 2 K / l = constant (d) Kl = constant 48. Temperature of water at the surface of lake is −20 oC. Then temperature of water just below the lower surface of ice layer is (a) −4 oC (b) 0 oC (c) 4 oC (d) −20 oC 49. One end of a metal rod of length 1.0 m and area of cross section 100cm2 is maintained at 100 oC. If the other end of the rod is maintained at 0 oC, the quantity of heat transmitted through the rod per minute is (Coefficient of thermal conductivity of material of rod =100W/m-K) (a) 3 × 10 3 J (b) 6 × 10 3 J (c) 9 × 10 3 J (d) 12 × 10 3 J 50. The coefficient of thermal conductivity of copper is nine times that of steel. In the composite cylindrical bar shown in the figure. What will be the temperature at the junction of copper and steel (a) C o 75 (b) C o 67 (c) C o 33 (d) C o 25 51. The lengths and radii of two rods made of same material are in the ratios 1 : 2 and 2 : 3 respectively. If the temperature difference between the ends for the two rods be the same, then in the steady state, the amount of heat flowing per second through them will be in the ratio (a) 1: 3 (b) 4 : 3 (c) 8 : 9 (d) 3 : 2 52. A slab consists of two parallel layers of two different materials of same thickness having thermal conductivities K1 and K2. The equivalent conductivity of the combination is (a) K1 + K2 (b) 2 K1 + K2 (c) 1 2 1 2 2 K K K K + (d) 1 2 1 2 2K K K + K 53. There are two identical vessels filled with equal amounts of ice. The vessels are of different metals., If the ice melts in the two vessels in 20 and 35 minutes respectively, the ratio of the coefficients of thermal conductivity of the two metals is (a) 4 : 7 (b) 7 : 4 (c) 16 :49 (d) 49 : 16 54. Surface of the lake is at 2°C. Find the temperature of the bottom of the lake (a) 2 oC (b) 3 oC (c) 4 oC (d) 1 oC 55. The heat is flowing through a rod of length 50 cm and area of cross-section 5cm2 . Its ends are respectively at 25 oC and 125 oC. The coefficient of thermal conductivity of the material of the rod is 0.092 kcal/m×s× oC. The temperature gradient in the rod is (a) 2 oC/cm (b) 2 oC/m (c) 20 oC/cm (d) 20 oC/m 56. In the Ingen Hauz’s experiment the wax melts up to lengths 10 and 25 cm on two identical rods of different materials. The ratio of thermal conductivities of the two materials is (a) 1 : 6.25 (b) 6.25 : 1 (c) 1 : 2.5 (d) 1 : 2.5 57. Heat current is maximum in which of the following (rods are of identical dimension) (a) (b) (c) (d) Copper Steel 0 100oC oC 18 cm 6 cm Copper Copper Steel Steel Copper Steel