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This section includes InterviewSolutions, each offering curated multiple-choice questions to sharpen your knowledge and support exam preparation. Choose a topic below to get started.
| 101. |
A container of negligible heat capacity contains `1kg` of water. It is connected by a steel rod of length `10m` and area of cross-section `10cm^(2)` to a large steam chamber which is maintained at `100^(@)C`. If initial temperature of water is `0^(@)C`, find the time after which it beomes `50^(@)C`. (Neglect heat capacity of steel rod and assume no loss of heat to surroundings) (use table `3.1`, take specific heat of water `=4180 J//kg.^(@)C)` |
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Answer» Let temperature of water at time t be `T`, then thermal current at time t, `I = ((100 -T)/(R ))` This increases the temperature of water form `T to T +dT` `rArr i = (dH)/(dt) = ms (dT)/(dt)` `rArr (100-T)/(R ) = ms(dT)/(dt) rArr overset(50)underset(0)int (dT)/(100-T) = overset(t)underset(0)int (dT)/(Rms)` `rArr -ln ((1)/(2)) = (t)/(Rms)` or `t = Rms ln2 sec` `= (L)/(KA) ms ln2 sec` `= ((10m)(1kg)(4180J//kg-^(@)C))/(46(w//m^(@)C)xx(10xx10^(-4)m^(2)))ln2` `= (418)/(46) (0.69) xx 10^(5) = 6.27 xx 10^(5) sec = 174.16 hours` |
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| 102. |
A thick cylindrical shell made of material of thermal conductivity k has inner and outer radii r and R respectively and its length is L. When the curved surface of the cylinder are lagged (i.e., given insulation cover) and one end is maintained at temperature `T_(1)` and the other end is maintained at `T_(2)(lt T_(1))`, the heat current along the length of the cylinder is H. In another experiment the two ends are lagged and the inner wall and outer wall are maintained at `T_(1)` and `T_(2)` respectively. Find the radial heat flow in this case. |
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Answer» Correct Answer - `(2L^(2)H)/(R^(2)-r^(2)ln((R)/(r)))` |
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| 103. |
A rod of negligible heat capacity has length 20cm, area of cross section `1.0cm^(2)` and thermal conductivity `200Wm^(-1)`^(@)C^(-1)` . The temperature of one end is maintained at `0^(@)C` and that of the other end is showly and linearly varied from `0^(@)C` to `60^(@)C` in 10 minutes. Assuming no loss of heat through the sides, find the total heat transmitted through the rod in these 10 minutes. |
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Answer» Correct Answer - A `(d theta)/(dt)= 60/(10xx60)=0.10^(@)C//sec` `(d theta)/(dt)=(KA)/d(theta_1-theta_2)` `=(KA0.1)/d+(KA0.2)/d+….+(KA60)/d` ltbrge(KA)/d(0.1+0.2+….+60)` `(KA)/d (600/2) (2xx0.1+599xx0.1)` `[a+2a+…na=(n)/(2){2a+(n-1)a}]` `=(200xx1xx10^(-4))/(20xx10^(-2))xx300xx(0.2+59.9)` `=(200xx10^(-2)xx300(60.1))/20` `=3xx10xx60.1` `=1803W=1800W.` |
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| 104. |
Two identical vessels, made of different materials having conductivites `K_(1)` and `K_(2)` are completely filled with ice at `0^(@)C`. Due to temperature of surrounding, the ice in the two vessels melts in `25min` and `20min` respectively. The ratio of `K_(1)` and `K_(2)` isA. `5//4`B. `4//5`C. `16//25`D. `(4//5)/(sqrt((5//4)))` |
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Answer» `((dH)/(dt_(1)))t_(1)=((dH)/(dt_(2)))t_(2)=theta=mL_(f)` `implies(((dH)/(dt))_(1))/(((dH)/(dt_(2))))=(t_(2))/(t_(1))=(4)/(5)implies(k_(1))/(k_(2))=(4)/(5)` |
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| 105. |
For a perfectly black body, its abosrpitve power isA. 1B. 0.5C. 0D. infinity |
| Answer» Correct Answer - A | |
| 106. |
The process in which rate of transfer of heat maximum is .A. ConductionB. convectionC. radiationD. In all these, heat is transferred with the same velocity |
| Answer» Correct Answer - C | |
| 107. |
In summer, a mild wind is often found on the shore of a clam river. This is caused due toA. difference in thermal conductivity of water and soilB. convection currentsC. conduction between air and the soilD. radiation from the soil |
| Answer» Correct Answer - B | |
| 108. |
If a liquid is heated in weightlessness, the heat is tramsmitted thruoghA. conductionB. convectinC. rediationD. neither, because the liquid connot be heated in weightlessness |
| Answer» Correct Answer - A | |
| 109. |
When fluids are heated from the bottom, convection currents are prodcued becauseA. molecular motion of fluid becomes alignedB. molecular collisions take place within the fluidC. heated fluid becomes more dense than the cold fluid above itD. heated fluid becomes less dense than the cold fluid above it |
| Answer» Correct Answer - D | |
| 110. |
The heat is flowing through two cylinderical 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 rates of flow of heat through them will beA. `1 : 4`B. `4 : 1`C. `1 : 8`D. `8 : 1` |
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Answer» Correct Answer - C (3) `(r_(1))/(r_(2)) = (1)/(2), (l_(1))/(l_(2)) = (2)/(1)` `(i_(1))/(i_(2)) = (R_(2))/(R_(1)) = (l_(2))/(l_(1)). (r_(1)^(2))/(r_(2)^(2)) = (1)/(2) ((1)/(2))^(2) = (1)/(8)` |
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| 111. |
The heat is flowing through two cylinderical 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 rates of flow of heat through them will beA. `1 : 1`B. `2 : 1`C. `1 : 4`D. `1 : 8` |
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Answer» `k_(1)=k_(2)=k` `2d_(1)=d_(2)` `(l_(1))/(l_(2))=2impliesl_(1)=2l_(2)` `DeltaT_(1)=DeltaT_(2)` `(I_(1))/(I_(2))=((k_(1)ADeltaT_(1)))/(l_(1))((k_(2)ADeltaT_(2)))/(l_(2))` `(I_(1))/(I_(2))=((d_(1))/(d_(2)))^(2)xx((l_(1))/(l_(2)))xx((1)/(2))^(2)xx((1)/(2))=(1)/(8)` |
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| 112. |
A solid sphere and a hollow sphere of the same material and of equal radii are heated to the same temperature.A. in the beginning both will emit equal amount of radiation per unit timeB. in the beginning both will absorb equal amount of radiation per unit timeC. both sphers will have same rate of fall of temperature `(dT//dt)`D. both spheres will have equal temperature at any constant |
| Answer» Correct Answer - A::B | |
| 113. |
A cylinderical rod of length 50cm and cross sectional area `1cm^(2)` is fitted between a large ice chamber at `0^(@)C` and an evacuated chamber maintained at `27^(@)C` as shown in figure. Only small protions of the rod are insid ethe chamber and the rest is thermally insulated from the surrounding. The cross section going inti the evacuted chamber is blackened so that it completely absorbe any radiation falling on it. The temperatuere of the blackened end is `17^(@)C` when steady state is reachhed. Stefan constant `sigma=6xx10^(-s)Wm^(-2)K^(-4)` . Find the thermal conductivity of the material of the rod. |
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Answer» Correct Answer - A::C (Q/t)=eA rho(T_(2)^(4)-T_(1)^(4))` `implies (q)/(At)=1xx6xx10^(-8)[(300)^4-(290)^4)]` `=6xx10^(-8)xx(81xx10^8xx-70.7xx10^8)` `=6xx10.3` `(Q/t)=(K(theta_1-theta_2)/l`. |
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| 114. |
A cylindrical rod of length `50cm` and cross sectional area `1cm^(2)` is fitted between a large ice chamber at `0^(@)C` and an evacuated chamber maintained at `27^(@)C` as shown in figure. Only small portions of the rod are inside the chamber and the rest is thermally insulated from the surrounding. The cross section going into the evacuated chamber is blackened so that it completely absorbs any radiation falling on it. The temperature of the blackened end is `17^(@)C` when steady state is reached. Stefan constant `sigma=6xx10^(-s)Wm^(-2)K^(-4)` . Find the thermal conductivity of the material of the rod. |
| Answer» Correct Answer - 3 | |
| 115. |
A rod CD of thermal resistance `5.0KW_(-1) is joined at the middle of an identical rod AB as shown in figure. The ends A, B and D are maintained at `100^(@)C` , `0(@)C and 25(@)C respectively. Find the heat current in CD. |
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Answer» The thermal resistance of AC is equal to that of CB and is equal to `2.5KW_(-1)` . Suppose, the temperature at C is (theta). The heat current through AG, CB and CD are ``(DeltaQ_(1))/(Deltat)=(100^(@)C-theta)/(2.5KW^(-1)` . `(DeltaQ_(2))/(Deltat)=(theta-0^(@)C)/(2.5KW^(-1)` . and `(DeltaQ_(3))/(Deltat)=(theta-25^(@)C)/(5.0KW^(-1)` . We also have `(DeltaQ_(1))/(Deltat)=(DeltaQ_(2))/(Deltat)+(DeltaQ_(3))/(Deltat)` . or, ``(100^(@)C-theta)/(2.5)=(theta-0^(@)C)/(2.5)+(theta-25^(@)C)/(5)` . or, `225^(@)C=5theta` . or, `theta=45^(@)C` . Thus, `(DeltaQ_(3))/(Deltat)=(45^(@)C-45^(@)C)/(5.0KW^(-1))+(20K)/(5.0KW^(-1))` . `=4.0W` . |
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| 116. |
Figure shows an aluminium rod joined to a copper rod. Each of the rods has a length of 20cm and area of ccross section `0.20cm^(2)` . The junction is maintained at a constant temperature `40^(@)C` and the two ends are maintained at 80^(@)C` . Calculate the amount of heat taken out from the cold junction in one minute after the steady state is reached. The conductivities are `K_(Al)=200Wm^(-1)`^(@)C^(-1)` . and `K_(Cu)=400Wm^(-1)`^(@)C^(-1)` . |
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Answer» Correct Answer - A::D `K_(Al)=200W//m^(@)C,` `K_(U)=400W//m^(@)C,` `A=0.2 cm^(2)=2xx10^(-5)m^(2)` `l=20cm=2xx10^(-1)m` Heat drawn per sec. `Q_(Al)+Q_(Cu)=(K_(Al)xxAxx(80-40))/l` +(K_(Cu)xxAxx(80-40))/l` ltbrge(2xx10^(-5)xx40)/(2xx10^(-1))[200+400]` `=2.4 J.` Heat drown per minute =2.4xx60=144J.` |
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| 117. |
One end of a rod length 20cm is inserted in a furnace at 800K. The sides of the rod are coved with an insulating material and the other end emits radiation like a blackbody. The temperature of this end is 750K in the steady state. The temperature of the surrounding air is 300K. Assuming radiation to be the only important mode of energy tranfer between the surrounding and the open end of the rod, find the thermal conductivity of the rod. Stefan constant `sigmas=6.0xx10^(-1)Wm^(-2)K^(-4)` . |
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Answer» Correct Answer - A::D `Q/t = eArho(T_2^4 - T_1^4)` `implies Q/At = 1 xx 6 xx 10^(-8) [(300)^(4) - (290)^(4)]` `= 6 xx 10.3` `Q/t = (K(theta_(1) - theta_(2))/(l)` `implies `Q/(tA) = (K(theta_(1) - theta_(2))/(l) = (K xx 17)/(0.5)` Thus, `6 xx 10.3 = (K xx 17)/(0.5)` `implies K = (6 xx 10.3 xx 0.5)/(17) = 1.8` |
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| 118. |
A blackbody of surface area `1cm^(2)` is placed inside an enclosure. The enclosure has a constant temperature `27(@)C` and the blackbody is maintained at `327(@)C` by heating it electrically. What electric power is needed to maintain the temperature? `sigma=6.0xx10^(-s)Wm^(-2)K(-2)` . |
| Answer» The area of the blackbody is `A=10^(-4)m^(2)` , its temperature is `T_(1)=327(@)C=600K` . and the temperature of the enclosure is `T_(2)=27(@)C=300K` . The blackbody emits radiation at the rate of `AsigmaT_(1)^(4)` . The radiation falls on it (and gets absorbed) at the rate of `AsigmaT_(2)^(4)` .The net rate of loss of energy is `Asigma(T_(1)^(4)-T_(2)^(4))` . The heater must supply this much of power. thus, the power needed is `Asigma(T_(1)^(4)-T_(2)^(4))` . `=(10^(-4)m^(2)(6.0xx10^(-s)Wm^(-2)K^(-4)^(4)-(300K)^(4)]`. `=0.73W` . | |
| 119. |
A blackbody of sarface area `10cm^(2) is heated to `127^(@)C` and is suspended in a room at temperature `27^(@)C` calculate the initial rate of loss of heat from the body to the room. |
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Answer» For a blackbody at temperature T, the rate of emission is `u=sigmaAT^(4)` . When it is kept in a room at temperature `T_(0)`, the rate of absorption is `u_(0)=sigmaA(T^(4)-(t^(4)0)` .Here `A=10xx10^(4)m^(2)` , `T=400K` and `T_(0)=300K` . Thus, `u-u_(0)` `=(5.67xx10^(-8)Wm^(-2)K^(-4)(10xx10^(-4)m^(2))(400^(4)-(300^(4))K^(4)` `=0.99W`. |
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| 120. |
A hollow metallic sphere of radius `20cm` surrounds a concentric metallic sphere of radius `5cm`. The space between the two sphere is filled with a nonmetallic material. The inner and outer sphere are maintained at `50^(@)C` and `10^(@)C` respectively and it is found that `100J` of heat passes from the inner sphere to the outer sphere per second. Find the thermal conductivity of the material between the sphere. |
| Answer» Correct Answer - `15 W//m-^(@)C` | |
| 121. |
For a black body at temperature `727^@C`, its radiating power is 60 watt and temperature of surrounding is `227^@C`. If temperature of black body is changed to `1227^@C` then its radiating power will be-A. 304 WB. 320 WC. 240 WD. 120 W |
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Answer» Correct Answer - B `60=K(1000^(4)-500^(4))....(i)` `E=K(1500^(4)-500^(4)).....(ii)` from (i) and (ii) `E/60 (1500^(4)-500^(4))/(1000^(4)-500^(4)) rArr E=320` |
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| 122. |
A blackbody does notA. emit radiationB. absorb raddiationC. reflect radiationD. refract radiationo. |
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Answer» Correct Answer - C::D reflect radiation,refract radiation |
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| 123. |
A blackbody of surface area `1cm^(2)` is placed inside an enclosure. The enclosure has a constant temperature `27^(@)C` and the blackbody is maintained at `327^(@)C` by heating it electrically. What electric power is needed to maintain the temperature? `sigma=6.0xx10^(-s)Wm^(-2)K(-2)` . |
| Answer» Correct Answer - `0.73 W` | |
| 124. |
Which of the following is nearest to blackbody-A. An enclosure with a small holeB. carbon blackC. AboniteD. none of these |
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Answer» Correct Answer - A An enclosure with a small hole |
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| 125. |
The temperature gradient in a rod of `0.5 m` length is `80^(@)C//m`. It the temperature of hotter end of the rod is `30^(@)C`, then the temperature of the cooler end isA. `40^(@)C`B. `- 10^(@)C`C. `10^(@)C`D. `0^(@)C` |
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Answer» Correct Answer - B (2) Temperature gradient `(Delta theta)/(Delta x) = (theta_(1) - theta_(2))/(l)` `80 = (30 - theta_(2))/(0.5)` `theta_(2) = 10^(@)C` |
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| 126. |
A cylindrical rod having temperature `T_(1)` and `T_(2)` at its ends. The rate of flow of heat is `Q_(1) cal//sec`. If all the linear dimensions are doubled keeping temperature constant, then rate of flow of heat `Q_(2)` will beA. `4 Q_(1)`B. `2 Q_(1)`C. `(Q_(1))/(4)`D. `(Q_(1))/(2)` |
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Answer» Correct Answer - B (2) `(Q_(1))/(Q_(2)) = (R_(2))/(R_(1)) = (2 l //K . Pi (2 r)^(2))/(l..K . Pi r^(2)) = 1//2` `Q_(2) = 2Q_(1)` |
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| 127. |
A cylindrical rod having temperature `T_(1)` and `T_(2)` at its ends. The rate of flow of heat is `Q_(1) cal//sec`. If all the linear dimensions are doubled keeping temperature constant, then rate of flow of heat `Q_(2)` will beA. `4 Q_(1)`B. `2Q_(1)`C. `(Q_(1))/4`D. `(Q_(1))/2` |
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Answer» Correct Answer - B `Q_(1)=(DeltaT)/((l/(k pir^(2)))), Q_(2)=(DeltaT)/((2l)/(kpi(2r)^(2)))rArr (Q_(2))/(Q_(1))=2` |
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