Tiêu đề 19. Heating and Chemical Effects of current Hard.pdf ✅

1. A heater coil is cut into two parts of equal length and one of them is used in the heater. The ratio of the heat produced by this half coil to that by the original coil is (a) 2 : 1 (b) 1 : 2 (c) 1 : 4 (d) 4 : 1 2. A constant voltage is applied on a uniform wire, then the heat is produced. The heat so produced will be doubled, if (a) The length and the radius of wire are halved (b) Both length and radius are doubled (c) Only the length is doubled (d) Only the radius is doubled 3. An electric heater of resistance 6 ohm is run for 10 minutes on a 120 volt line. The energy liberated in this period of time is (a) J 3 7.2  10 (b) 14.4 10 J 5  (c) J 4 43.2  10 (d) J 4 28.8  10 4. A 500 watt heating unit is designed to operate from a 115 Volt line. If the line voltage drops to 110 volt, the percentage drop in heat output will be (a) 10.20% (b) 8.1% (c) 8.6% (d) 7.6% 5. The power of a heater is 500 watt at 800o C. What will be its power at 200o C if =  per C −4  4 10 (a) 484 W (b) 672 W (c) 526 W (d) 611 W 6. A heater of 220 V heats a volume of water in 5 minute time. A heater of 110 V heats the same volume of water in (a) 5 minutes (b) 8 minutes (c) 10 minutes (d) 20 minutes 7. Water boils in an electric kettle in 15 minutes after switching on. If the length of the heating wire is decreased to 2/3 of its initial value, then the same amount of water will boil with the same supply voltage in (a) 15 minutes (b) 12 minutes (c) 10 minutes (d) 8 minutes 8. Forty electric bulbs are connected in series across a 220 V supply. After one bulb is fused, the remaining 39 are connected again in series across the same supply. The illumination will be (a) More with 40 bulbs than with 39 (b) More with 39 bulbs than with 40 (c) Equal in both the cases (d) In the ratio of 402 : 392 9. Two bulbs of 100 watt and 200 watt, rated at 220 volts are connected in series. On supplying 220 volts, the consumption of power will be (a) 33 watt (b) 66 watt (c) 100 watt (d) 300 watt 10. The expression for thermo e.m.f. in a thermocouple is given by the relation 20 40 2  E =  − , where  is the temperature difference of two junctions. For this, the neutral temperature will be (a) 1000 C (b) 2000 C (c) 3000 C (d) 4000 C 11. One junction of a certain thermoelectric couple is at a fixed temperature Tr and the other junction is at temperature T. The thermo electromotive force for this is expressed by       = − − ( + ) 2 1 ( ) 0 r T T T r E K T T at temperature T = 0 2 1 T , the thermo electric power will be (a) 2 0 1 KT (b) KT0 (c) 2 2 0 1 KT (d) 2 2 0 1 ( ) K T − Tr 12. Amount of electricity required to pass through the H2O voltameter so as to liberate 11.2 litre of hydrogen will be (a) 1 Faraday (b) Faraday 2 1 (c) 2 Faraday (d) 3 Faraday 13. Amount of electricity required to liberate 16 gm of oxygen is (a) 1 Faraday (b) 2 Faraday (c) Faraday 2 1 (d) 3 Faraday 14. Total surface area of a cathode is 0.05 m2 and 1 A current passes through it for 1 hour. Thickness of nickel deposited on the cathode is (Given that density of nickel = 9 gm/cc and it’s ECE = 3.04  10–4gm/C) (a) 2.4 m (b) 2.4 cm (c) 2.4 m (d) None of these 15. Resistance of a voltameter is 2 , it is connected in series to a battery of 10 V through a resistance of 3 . In a certain time mass deposited on cathode is 1 gm. Now the voltameter and the 3 resistance are connected in parallel with the battery. Increase in the deposited mass on cathode in the same time will be (a) 0 (b) 1.5 gm (c) 2.5 gm (d) 2 gm 16. A solid of mass 2kg is heated and H (Heat given) vs (change in temperature) is plotted. Specific heat of solid is – 45o H (in kJ) (in oC)
(a) 1 J/kg/oC (b) 0.5 J/kg/oC (c) 2 kJ/kg/oC (d) 0.5 kJ/kg/oC 17. A cylinder of radius R made of material of thermal conductivity K1 is surrounded by a cylindrical shell of inner radius R and outer radius 3R made of a material of thermal conductivity K2. The two ends of the combined system are maintained at two different temperature. What is the effective thermal conductivity of the system ? (a) K1 + K2 (b) 9 K1 + 8K2 (c) 1 2 1 2 K K K K + (d) 9 8K1 + K2 18. A and B are two points on a uniform metal ring whose centre is C. The angle ACB = . A and B are maintained at two different constant temperatures. When  = 180o, the rate of total heat flow from A to B is 1.2 W. When  = 90o, this rate will be - (a) 0.6 W (b) 0.9 W (c) 1.6 W (d) 1.8 W 19. The ends of the two rods of different materials with their lengths, diameters of cross-section and thermal conductivities all in the ratio 1:2 are maintained at the same temperature difference. The rate of flow of heat in the shorter rod is 1 cal s–1 . What is the rate of flow of heat in the larger rod ? (a) 1 cal s–1 (b) 4 cal s–1 (c) 8 cal s–1 (d) 16 cal s– 1 20. A rod of length  and cross section area A has a variable thermal conductivity given by K =  T, where  is a positive constant and T is temperature in Kelvin. Two ends of the rod are maintained at temperatures T1 and T2 (T1> T2). Heat current flowing through the rod will be - (a) 3 A (T T ) 2 2 2  1 − (b)  A (T T ) 2 2 2  1 + (c) 3 A (T T ) 2 2 2  1 + (d) 2 A (T T ) 2 2 2  1 − 21. A cylinder of radius R made of a material of thermal conductivity K1 is surrounded by a cylindrical shell of inner radius R and outer radius 2R made of a material of thermal conductivity K2. The two ends of the combined system are maintained at two different temperatures. There is no loss of heat across the cylindrical surface and the system is in steady state. The effective thermal conductivity of the system is – (a) K1 + K2 (b) (K1 + 3K2)/4 (c) K1K2/(K1 + K2) (d) (3K1 + K2)/4 22. Six identical conducting rods are joined as shown in figure. Points A and D are maintained at 2000C and 200C respectively. The temperature of junction B will be – A B C D (a) 1200C (b) 1000C (c) 1400C (d) 800C 23. 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 8.4 cm and 4.2 cm, 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 24. Two vessels of different materials are similar in size in every respect. The same quantity of ice filled in them gets melted in 20 min and 35 min, respectively. The ratio of coefficients of thermal conduction of the metals is - (a) 4 : 7 (b) 7 : 4 (c) 25 : 16 (d) 16 : 25 25. Two solid spheres, of radii R1 and R2 are made of the same material and have similar surfaces. The spheres are raised to the same temperature and then allowed to cool under identical conditions. Assuming spheres to be perfect conductors of heat, their initial rates of loss of heat are - (a) 2 2 2 R1 /R (b) R1/R2 (c) R2/R1 (d) 2 1 2 R2 /R 26. The ratio of thermal capacities of two spheres A and B, if their diameters are in the ratio 1 : 2, densities in the ratio 2 : 1, and the specific heat in the ratio of 1 : 3, will be - (a) 1 : 6 (b) 1 : 12 (c) 1 : 3 (d) 1 : 4 27. In a steady state of thermal conduction, temperature of the ends A and B of a 20 cm long rod are 1000C and 00C respectively. What will be the temperature of the rod at a point at a distance of 6 cm from the end A of the rod ? (a) –300C (b) 700C (c) 50C (d) None of these 28. The ends of a metal bar of constant cross-sectional area are maintained at temperatures T1 and T2 which are both higher than the temperature of the surroundings. If the bar is unlagged, which one of the following sketches best represents the variation of temperature with distance along the bar ? (a) distance T1 T2 O (b) distance T1 T2 O (c) distance T1 T2 O (d) distance T1 T2 O 29. Two identical rods with different thermal conductivities K1 and K2 and different temperatures are first placed along length and then along area, then the ratio of rates of heat flow in both cases is - (a) 2 1 2 1 2 (K K ) 4K K + (b) 2 1 K K
(c) 1 2 1 2 K – K K + K (d) None of these 30. Heat current is maximum in which of the following ? (rods are of identical dimension) (a) Cu (b) Steel Cu (c) Cu Steel (d) Steel 31. Two plates of same thickness, of coefficients of thermal conductivity K1 and K2 and areas of cross-section A1 and A2, are connected as shown ; the common coefficient of thermal conductivity K will be – K1 A1 K2 A2 Q Q (a) K1A1 + K2A2 (b) 1 2 1 1 2 2 A A K A K A + + (c) 1 2 1 2 2 1 A A K A K A + + (d) 2 2 1 1 K A K A 32. A polished surface - (a) Reflect radiation (b) Emit radiation (c) Absorb radiation (d) All 33. The rate of emission of radiation of a black body of temperature 270C is E1. If its temperature is increased to 3270C, the rate of emission of radiation is E2. The relation between E1& E2 is - (a) E2 = 24 E1 (b) E2 = 16 E1 (c) E2 = 8 E1 (d) E2 = 4 E1 34. An electric kettle has two heating coils. When one of the coils is connected to ac source the water in the kettle boils in 10 minutes. When the other coil is used the water boils in 40 minutes. If both the coils are connected in parallel, the time taken by the same quantity of water to boil will be (a) 4 min (b) 25 min (c) 15 min (d) 8 min 35. In the circuit shown in figure, the heat produced in 5 ohm resistance is 10 calories per second. The heat produced in 4 ohm resistance is (a) 1 cal/sec (b) 2 cal/sec (c) 3 cal/sec (d) 4 cal/sec 36. Two heater wires of equal length are first connected in series and then in parallel. The ratio of heat produced in the two cases is (a) 2 : 1 (b) 1 : 2 (c) 4 : 1 (d) 1 : 4 37. A thermo couple develops 200 V between 0oC and 100oC. If it develops 64 V and 76 V respectively between (0oC – 32oC) and (32oC – 70oC) then what will be the thermo emf it develops between 70oC and 100oC (a) 65 V (b) 60 V (c) 55 V (d) 50 V 38. A thermo couple is formed by two metals X and Y metal X comes earlier to Y in Seebeck series. If temperature of hot junction increases beyond the temperature of inversion. Then direction of current in thermocouple will so (a) X to Y through cold junction (b) X to Y through hot junction (c) Y to X through cold junction (d) Both (b) and (c) 39. Peltier co-efficient of a thermo couple is 2 nano volts. How much heat is developed at a junction if 2.5 amp current flows for 2 minute (a) 6 ergs (b) 6  10–7 ergs (c) 16 ergs (d) 6  10–3 erg 40. A current of 16 ampere flows through molten NaCl for 10 minute. The amount of metallic sodium that appears at the negative electrode would be (a) 0.23 gm (b) 1.15 gm (c) 2.3 gm (d) 11.5 gm 41. The diameter of a rod is given by d = d0 (1 + ax) where 'a' is a constant and x is distance from one end. If thermal conductivity of material is K. Then the thermal resistance of the rod if its length is is - (a) 2 K d0 1  (b) K d (a 1) 4 2  0  +  (c) 2 2 0 K d (a 1) 2   +  (d) K d (a 1) 2 2  0  +  42. Aslab 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 100oC and that of copper at 0 0C, the temperature of interface is - (a) 800C (b) 200C (c) 600C (d) 400C 4 6 5 i2 i1 i Line (2) Line (1)
43. One end of a thermally insulated rod is kept at a temperature T1 and the other at T2. The rod is composed of two sections of length 1 and 2 and thermal conductivities K1and K2 respectively. The temperature at the interface of the two section is – T1 1 2 K1 K2 T2 (a) (K K ) (K T K T ) 1 1 2 2 1 1 1 2 2 2     + + (b) (K K ) (K T K T ) 1 1 2 2 2 2 1 1 1 2     + + (c) (K K ) (K T K T ) 2 1 1 2 2 1 1 1 2 2     + + (d) (K K ) (K T K T ) 1 2 2 1 1 2 1 2 1 2     + + 44. A composite cylinder is made of two different materials A and B of thermal conductivities KA and KB. The dimensions of the cylinder are as shown in the figure. The thermal resistance of the cylinder between the two end faces is - r 2r B  A (a) r (K 3K ) A B 2  +  (b)         +  A B 2 K 3 K 1 r  (c)         +  A B 2 K 4 K 1 r  (d) r (K 4K ) A B 2  +  45. A parallel-sided slab is made of two different materials. The upper half of the slab is made of material X, of thermal conductivity  ; the lower half is made of material Y, of thermal conductivity 2. In the steady state, the left hand face of the composite slab is at a higher, uniform temperature than the right-hand face, and the flow of heat through the slab is parallel to its shortest sides. What fraction of the total heat flow through the slab passes through material X ? X Y (a) 1/4 (b) 1/3 (c) 1/2 (d) 2/3 46. Consider the two insulating sheets with thermal resistances R1 and R2 as shown in figure. The temperature  is – R2 R1 1 2  (a) ( )(R R ) R R 1 2 1 2 1 2 1 2  +  +   (b) 1 2 1 1 2 2 R R R R +  +  (c) 2 2 2 1 1 2 1 2 R R ( )R R +  +  (d) 1 2 1 2 2 1 R R R R +  +  47. A cylinder of radius R, made of a material of thermal conductivity k1, is surrounded by a cylindrical shell of inner radius R and outer radius 2R. The shell is made of a material of thermal conductivity k2. The two ends of the combined system are maintained at two different temperatures. There is no loss of heat across the cylindrical surface and the system is in steady state. The effective thermal conductivity of the system is - (a) k1+ k2 (b) 1 2 1 2 k k k k + (c) 4 k1 + 3k2 (d) 4 3k1 + k2 48. On a cold winter day, the temperature of atmosphere is –ToC. The cylindrical diagram shown is made of insulating material and it contains water at 0oC. If L is latent heat of fusion of ice, & is density of ice and K is thermal conductivity of ice, the time taken for total mass of water to freeze is – 2R Water H (a) H KT L 2  (b) 2KT LH2  (c) KT LH2  (d) LKT H 2 