Transmission of heat Questions and Answers

the layer A is a 50K b 12 5K c 25 K d 60 K 24 Which of the following statements is are false about mode of heat transfer a In radiation heat is transfered from one medium to another without affecting the intervening medium b Radiation and convection are possible in vaccum while conduction requires material medium Conduction is possible in solids while convection occurs in liquids and gases c d All are correct 25 In a vertical U tube containing a liquid the two arms are maintained at 9 30 12 sec as sho they ar ii the heat in a 2 Consis mater conduc conduc a

Boiling 3 A sphere made of aluminium alloy 2024 with a diameter of 20 mm and a uniform temperature of 500 C is suddenly immersed in a saturated water bath maintained at atmospheric pressure The surface of the sphere has an emissivity of 0 25 Calculate the total heat transfer coefficient for the initial condition What fraction of the total coefficient is contributed by radiation Answers h 180 W m K fraction 6 7

Heat is removed from a molten metal of mass 2 kg at a constant rate of 10 kW till it is completely solidified The cooling curve is shown in the figure 1100 1000 0s 1023K Temperature K 900 800 10s 873 K 700 600 500 0 0 20s 873 K testbok 30 s 600 K 10 20 30 40 Time S Assuming uniform temperature throughout the volume of the metal during solidification the latent host of fusion of the motal in

Four rods of same material with different radii r and length 1 are used to connect two heat reservoirs at different temperatures In which of following case heat conduction is fastest 1 1 1 r cm 1 cm 2 r 3cm 1 9cm 3 r 4cm 1 8cm 4 r 1cm 11cm

Three rods of the same dimensions have thermal conductivities 3k 2k and k They are arranged as shown with their ends at 100 C 50 C and 0 C The temperature of their junction is 50 C 100 CD 3k O 75 C 200 O 40 C 2k k

onsider a compound slab consisting of two different material having ual thickness and thermal conductivities k and 3k respectively as own in figure The equivalent thermal conductivity of the slab is e e 3k k 3k 2

A refrigerator converts 100 g of water at 25 C into ice at 10 C in one hour and 50 minutes The quantity of heat removed per minute is specific heat of ice 0 5 cal g C latent heat of fusion 80 cal g 50 cal O 100 cal O 200 cal O 75 cal

139 A black metal foil is warmed by radiation from a small sphere at temperature T and at a distance d where surrounding temperature is T It is found that the power received by the foil is P If both the temperature and the distance are doubled the power received by the foil will be Assume T Tol 1 16P 2 4P

4 The temperatures on the four surfaces of a very long square prism with dimensions w H 1 meter are given in the figure Find the temperature distribution T x y inside the bar at steady state 4 44 H lm oc 60 sin oc W lm 40sin T X

Two metallic spheres S and S are made of the same material and have identical surface finish The mass of S is three times that of S Both the spheres are heated to the same high temperature and placed in the same room having lower temperature but are thermally insulated from each other The ratio of initial rate of cooling of S to that of S Question Type Single Correct Type 2 113 1 3

83 If a carved black utensil is heated to high temperature and then brought in dark then 1 Both utensil and its carving will shine 2 Only carving will shine 3 Only utensil will shine 4 None of the utensil and carving will shine

In Newton s law of cooling k40 the dt proportionality constant k is k and k for two substances A and B having mass m and m2 area A and A2 specific heat s and s2 emissivity e and e2 respectively it is given that 2 while 5 remains constant Then e e A B 1 C m2 surrounding temperature 3 9 A A 2 1 111 3

s I A container with 1 kg of water in it is kept in sunlight which causes the water to get warmer than the surroundings The average energy per unit time per unit area received due to the sunlight is 700 Wm and it is absorbed by the water over an effective area of 0 05 m Assuming that the heat loss from the water to the surroundings is governed by Newton s law of cooling the difference in C in the temperature of water and the surroundings after a long time will be Ignore effect of the container and take constant for Newton s law of cooling 0 001 s Heat capacity of water 4200 J kg K 8 33 P 700 x 0 05 35 watts P Pr 35 eGA4T0 AT 4eGAT K mC 4eGATO 0 001 x mC 4 2 from equation 1 AT 3 0 0001 35 50 25 6 3 8 33 1

15 f the temperature of the sun becomes twice its present temperature then Radiated energy would be predominantly in infrared Radiated energy would be predominantly in ultraviolet Radiated energy would be predominantly in X ray region Radiated energy would become twice the present radiated energy

Toluene liquid of volume 300 cm at 0 C is contained in a beaker and another quantiy of toluene of volume 110 cm at 100 C is in another beaker The combined volume is 410 cm Determine the total volume of the mixture of the toluene liquids when they are mixed together Given the coefficient of volume expansion a 0 001 C and all forms of heat losses can be ignored

far from the hotter end Also calculate the temperature gradient Three conducting rods of same material and cross section are shown in figure Temperature of A D and C are maintained at 20 C 90 C and 0 C Find the ratio of length BD and BC if there is no heat flow in AB A B 20 C 90 C D C 0 C