Transmission of heat Questions and Answers

Q14 Two liquids A and B are at 32 C and 24 C When mixed in equal masses the temperature of the mixture is found to be 28 C Their specific heats are in the ratio of a 3 2 b 2 3 c 1 1 d 4 3

A refrigerator works between 4 C and 30 C It is required to remove 600 calories of heat every second in order to keep the temperature of the refrigerated space constant The power required is Take 1 cal 4 2 joules A 236 5 W C 2 365 W B D Phase I 2016 2365 W 23 65 W

The temperature inside a refrigerator is t C and the room temperature is t C The amount of heat delivered to the room for each joule of electrical energy consumed ideally will be Phase II 2016 t 4 1 1 273 1 A C 1 1 t 273 tit 273 t t B D

Knowledge Sections Q No 9 It is desirable to handle the slip gauges with a cloth or chamois leather in order to A Avoid injury to hands B Protect the surfaces of slip gauges C Insulate them from the heat of the hand D Ensure that the varnish applied on gauges does not come out 10

Two identical objects A and B are at initial temperatures TA and TB TA TB respectively The specific heat capacity of the material of these objects increases with temperature If these two objects are brought in contact then their final equilibrium temperature is T Assuming that there is no heat exchange with the surroundings then XAT TA BT XC T XD T TA TB 2 TA TB 2 TA TB

View In English Two metallic spheres S and 52 are made of the same material and have identical surface finish The mass of St is three times that of S Both the spheres are heated to the same high temperature and placed in he 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 1 3 4 t Answer 4 unattempted 17111413 to Bookmark List 3 3 1 3 16 Right Answer 21 Your Answor Difficulty Lovol low I Topic physics magnetic offect of current and magnatium Poor Stat x Unattempted 416 Correct 441 incorrect 14 3 0 4 329 View V

2 14 gm of nitrogen gas at constant pressure has temperature of 300 K How much heat energy would be required to change its temperature to 350 K at same pressure 1 672 J 3 727 J 2 935 J 4 1234 J

The temperature of a well stirred liquid kept open to a cold surrounding is plotted against time The value of sec 0 is Temperature C 50 45 40 35 30 N 20 0 10 20 30 40 50 60 70 Time s Question Type Single Correct Type 1 1 9 tan 0 2 1 tan 02 3 1 3 tan 0

Two plates of equal areas are placed in contact with each other Their thickness are 2 0 cm and 5 0 cm respectively The temperature of the external surface of the first plate is maintained at 20 C and that of the external surface of the second plate is maintained at 20 C What will be the temperature of the contact surface in steady state if the plates have thermal conductivities in the ratio 2 5 Question Type Single Correct Type 1 0 C 24 C 3 420 29 C

6 Three copper blocks of masses M M and M kg respectively are brought into thermal contact till they reach equilibrium Before contact they were at T T T T T T Assuming there is no heat loss to the surroundings the equilibrium temprature T is s is specific heat of copper 3 a T b T T T T3 3 c T M T M T M3T3 M M M3 M T M T M T3 2 2 3 M M M3 d T M Ts M T s M TS 2 2 M M M3

12 A glass full of hot milk is poured on the table It begins to cool gradually Which of the following is correct a The rate of cooling is constant till milk attains the temperature of the surrounding b The temperature of milk falls off exponentially with time c While cooling there is a flow of heat from milk to the surrounding as well as from surrounding to the milk but the net flow of heat is from milk to the surounding and that is why it cools d All three phenomenon conduction convection and radiation are responsible for the loss of heat from milk to the surroundings

Two rods with the same dimensions have thermal conductivities in the ratio 1 2 They are arranged between heat reservoirs with the same temperature difference in two different configurations A and E The rates of heat flow in A and B are IA and IB respectively The ratio is equal to IA a 1 2 A T BT R K KR IB R 22K T T 2K R b 1 32 c 2 5 RA Series Parallel KB 45 0

A cylindrical rod having temperature T and T2 at its ends The rate of flow of heat is Q cal s If all the linear dimensions are doubled keeping temperature constant then rate of flow of heat Q2 will be 2001 A 4Q B 2Q Q C D lc

Two identical square rods of metal are welded end to end as shown in Fig i 20 calories of heat flows through it in 4 min If the rods are welded as shown in Fig ii the same amount of heat will flow through the rods in R R 0 C a 1 min 0 20 Calone R in 4men i Series 0 C O 100 C b 2 min d 16 min 20 Calone in t 100 C R ii Parallel

CPMT 1978 a Increases with time b Decreases with time c Does not change with time and is same at all the points of the body Does not change with time but is different at different points of the body

The two ends of a rod of length x and a uniform cross sectional area A are kept at two temperatures 8 and 0 0 02 The rate of dQ heat transfer through the rod in a steady dt state is given by 2009 A B e n dQ K 0 0 dt XA dQ dt KX A 0 0 dQ KA 01 0 dt X dQ Kx 01 0 g 0 0 DA x H KADT

A metal rod of length 2m has areas 2A and 100 C cross sectional A as shown in figure The ends are maintained at temperatures 100 C and 70 C The temperature at middle point C is a 80 C b 85 C C 90 C 2 A SH c 0 1 m 1 m d 95 C 700 A

Temperature gradient on a 0 75 m long rod is 80 C m If the temperature of the hot end of the rod is 80 C then the temperature of colder end will be O 20 C O 140 C O 140 C O 20 C Mark

7 hours Ice starts forming in a lake with water at 0 C when atmospheric temperature is 10 C If time taken for 1 cm of ice layer to be formed 7 hours time taken for the thickness of ice to increase from 1 cm to 2 cm is Less than 7 hours More than 7 hours but less than 14 hours Read More 21 hours and 4 out of which only one

The air flow to a four cylinder four stroke engine is 2 15 m min During a test on the engine the following data were recorded Bore 10 5cm stroke 12 5cm engine speed 1200 rpm torque 150 N m fuel consumption 5 5 kg h calorific value of fuel 43124 kJ kg ambient temperature and pressure are 20 C and 1 03 bars Calculate The brake thermal efficiency The brakes mean effective pressure The volumetric efficiency

46 Figure shows a composite rod containing two p The thermal conductivities and the geome dimensions are marked If the ends are maintaine at temperature T and T2 respectively temperature of the interface is 1 3 k T 1 T T 2 2 4T T 2 5 22 K 4 2 4 2 2K 2 8T T 9 T 16T 17

Two spheres of same material but radii in ratio 2 1 are heated to same temperature and left in same surrounding The rate of the cooling will be in the ratio of 01 2 01 4 0 1 8 06 1

Two spheres of same radius R have their densities in the ratio 1 8 and the ratio of their specific heats are 4 1 and ratio of their emisivity are 24 1 Both sphere are placed in same environment and initial temprature of both sphere are same then find the initial ratio of rate of cooling of sphere

An ideal string is wrapped several times on a solid cylinder of mass 4 kg and radius 1 m The pulleys are 10x N m ideal and the surface between block and ground is smooth If the torque acting on the cylinder is 9 then find the value of x www o Block 4 kg Solid cylinder R 8 kg Smooth F 40 N 8

14 Which of the following is incorrect NCERT Pg 305 1 If two systems A and B are in thermal equilibrium with system C separately then A and B will be in thermal equilibrium 2 Zeroth law of thermodynamics defines temperature 3 Temperature does not determine the direction of flow of heat when two bodies are placed in thermal contact 4 Internal energy of an ideal gas depends on the state of the system

The temperature of furnace is 200 C in its spectrum the maximum intensity is obtained at about 400 If the maximum intensity is at 200 Calculate the temperature of the furnace in C 1 400 C 2 673 C 3 946 C 4 None of these 30 00 US Dazu 200 atfor 1 400 C 3 946 C 400ff 2 673 C 4

The viscosity of a gas a decreases b c d with increase in 9 temperature increases temperature is independent of temperature is independent of pressure for very high pressure intensities with increase in

A body is cooling under Newton s law of cooling Temperature difference between body and surrounding falls from 80 C to 40 C in 20 minute Approximate time in which temperature difference between body and surrounding falls from 40 C to 10 is 1 10 minute 3 30 minute 2 20 minute 4 36 minute

Two conductors A and B each of cross section area 5 cm are connected in series Variation of temperature in C along the length in cm is as shown in the figure If thermal conductivity of A is 120 J m sec C The thermal conductivity of B is 40K J m sec C Find the value of K 53 25 cm 45 50 cm

5 A solid conducting sphere of radius R is rotat with constant angular velocity o about diameter If charge Q is uniformly distributed its surface then its magnetic dipole moment is 1 QR 00 2 QR 0 3 QR 4 QR

The surface temp of a small planet having a circular orbit round the sun with time period assuming sun and the planet to be black bodies Take radius of sun R mass of sun M surface temp of the sun Q is A B R 0 2r R r 2 00 C R r 2 0 D R Vr 00

3 A rigid container with thermally insulated walls contains a coil of resistance 100 2 carrying current 1 A Change in internal energy after 5 minute will be a 0 kJ b b 10 kJ 10 kJ c c 20 20 kJ kJ d 30 kJ

Two spheres S and S have radij R and 3R temperature I K and 7 3 K respectively if they are coated with a material of same emissivity rate radiation of Si is E then rate of radiation of S is a E b E 3 C E a d E 1

Two spheres S and S have radii R and 3R temperature T K and K respectively I 3 they are coated with a material of same emissivity rate of radiation of S is E then rate o radiation of S is spheres are of the same material 001 E E 9 D 3 A B E C 1 2 nce

Ice starts forming in a lake with water at 0 C when atmospheric temperature is 10 C If time taken for 1 cm of ice layer to be formed is 7 hours time taken for the thickness of ice to increase from 1 cm to 2 cm is 7 hours Less than 7 hours More than 7 hours but less than 14 hours 21 houro

3 Radiation Judo 4 26 The temperature gradient in a rod of 0 5 m long is 80 C m If the temperature of hotter end of the rod is 30 C then the temperature of the colder end is 1 40 C 3 10 C Ov 2 4 10 C 0 C tasuxe conductivity of two rods of different material is 5 4 The two rods of same area of cross in the ratio

38 A block of metal is heated to a temperature much higher than the room temperature and allowed to cool in a room free from air currents Which of the following curves correctly represents the cooling T Temperature of block T 1 3 T time time 2 4 T T time time

1 The outer faces of a rectangular slab made of equal thickness of iron and brass are maintained at 100 C and 0 C respectively The temperature at the interface is Thermal conductivity of iron and brass are 0 2 and 0 3 respectively 2 40 C 4 70 C 1 100 C 3 50 C

24 Two circular discs A and B having same mass with equal radii and emissivities heated to same temperature and are cooled under indentical conditions The inference drawn from their colling curves is R is rate of cooling q is surrounding temperature A B 0 00 1 A and B have same specific heats 2 Specific heat of A is less than B Specific heat of B is less than A

Two rods one semi circular and other straight of same material and of same cross sectional area are joined as shown in the figure The points A and B are maintained at different temperature Find the ratio of the heat transferred through a cross section of a semi circular rod to the heat transferred through a cross section of the straight rod in a given time A 2 Semi circular rod Straight rod Question Type Single Correct Type B

15 Certain amount of an ideal gas are contained in a closed vessel The vessel is moving with a constant velocity v The molecular mass of gas is M The rise in temperature of the gas when the vessel is suddenly stopped is y Cp Cy a c My 2R 1 My 2R y 1 b d Mv y 1 2R 2R y 1

14 gm of nitrogen gas at constant pressure has temperature of 300 K How much heat energy would be required to change its temperature to 350 K at same pressure 672 J O 935 J 0727 J O 1234 J

energy take The refrigerator in a kitchen shown in Fig 7 7 receives electrical input power of 150 to drive the system and it rejects 400 W to the kitchen air Find the rate of out of the cold space and the COP of the refrigerator Kitchen air Tamb Ref 2H 400 W QL W 150 W No

A human body has a surface area of approximately 1m The normal body temperature is 10K above the surrounding room temperature To Take the room temperature to he To 300K For To 300K the value of oT 460W m where o is the Stefan Boltzmann constant Which of the following options is are correct This question has multiple correct options If the surrounding temperature reduces by a small amount AT To then to maintain the same body temperature the same living human being needs A to radiate AW 40 TATo more energy per unit time B C D If the body temperature rise significantly then the peak in the spectrum of electromagnetic radiation emitted by the body would shift to longer wavelengths Reducing the exposed surface area of the body e g by curling up allows humans to maintain the same body temperature while reducing the energy lost by radiation The amount of energy radiated by the body in 1 second is close to 60 Joules

8 A cube a cylinder of height equal to its diameter and a sphere all solid are made of a metal and are of identical mass If all the three shapes having initial temperature 50 C are put in a large reservoir of water at 10 C Which of the above three shapes cools the fastest A The cylinder B The sphere C The cube D All three at the same rate

A star with surface temperature 20 000K emits energy at a rate of 2 74 10 W Calculate the peak intensity wavelength and radius of the star a 1 63 10 m 1 25 106 m b 1 45 107 m 1 55 10 m c 1 49x10 8 m 1 01 10 m d 1 59 106 m 1 38x106 m

3 Three rods lengths 21 1 1 made of the same material and having the same area of cross section are joined as shown in figure The end points A B and C are maintained at constant temperatures 100 C 50 C and 0 C respectively Assuming that there is no loss of heat from the surface of the rods find the temperature that the junction P ultimately reaches A 50 C B 40 C C 30 C 21 B D 20 C 54 An AC voltage source of frequency 50 Hz and amplitude vo is turned on at time 0 A second

de In Newton s law of cooling KAO the dt proportionality constant k is k and k for two substances A and B having mass m and m area A and A specific heat s and s emissivity e and e respectively it is given that A C C surrounding 33 Then 1 m 2 2 3 while m temperature remains constant

Out of the metal balls of same diameter one is solid and other is hollow Both are heated to the same temperature at 300 C and then allowed to cool in the same surroundings then rate of loss of heat will be 1 More for hollow sphere 2 More for solid sphere 3 Same for both 4 None of the above

A black metal foil is warmed by radiation from a small sphere at temperature T and at a distance d It is found that the power received by the foil is P If both the temperature and distance are doubled the power received by the foil will be A 16 P B 4 P D P C 2 P ns B