Tag: conduction

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Multiple choice physics temperature and heat modes of heat transfer - conduction conduction heat and modes of heat transfer

A spherical black body of radius $(R)$ when heated to certain temperature and left in vaccum. cools at a rate $'x' $ Now a caity of radius $(R/2)$ is made concentrically from this sphere. The rate of cooling of the remaining sphere will be.....................

  1. $\cfrac { x }{ 8 } $

  2. $\cfrac { 7x }{ 8 } $

  3. $\cfrac { 8x }{ 8 } $

  4. $\cfrac { x }{ 7 } $
Reveal answer Fill a bubble to check yourself
A Correct answer
Multiple choice physics temperature and heat modes of heat transfer - conduction conduction heat and modes of heat transfer

A hollow copper sphere and a hollow copper cube, of same surface area and negligible thickness, are filled with warm water of same temperature and placed in an enclosure of constant temperature, a few degrees below that of the bodies. Then in the beginning:

  1. the rate of energy lost by the sphere is greater than that by the cube

  2. the rate of fall of temperature for sphere is greater than that for the cube.

  3. the rate of energy lost by the sphere is less than that by the cube

  4. the rate of fall of temperature for sphere is less than that for the cube.
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Rate of cooling dT/dt = (sigma * A * (T^4 - Ta^4)) / (m * c). For the same surface area A, the body with the smaller mass m will have a higher rate of temperature fall. A hollow sphere has less volume (and thus less mass) than a hollow cube of the same surface area.

Multiple choice physics temperature and heat modes of heat transfer - conduction conduction heat and modes of heat transfer

A copper block of mass $500gm$ and $Sp.$ Heat $0.1 cal/gm/^{o}{C}$ is heated from ${30}^{o}C$ to ${40}^{o}C$. Another identical copper block $B$ of same mass is heated from ${35}^{o}C$ to ${40}^{o}C$. The ratio of their thermal capacities is 

  1. $1:2$

  2. $2:1$

  3. $1:1$

  4. $1:4$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Thermal capacity is defined as mass * specific heat (m * c). Since both blocks have the same mass and are made of the same material (copper), their thermal capacities are identical, resulting in a 1:1 ratio.

Multiple choice physics temperature and heat modes of heat transfer - conduction conduction heat and modes of heat transfer

Heat required to convert one gram of ice at $0^{ _-^0}C$ into steam at $100^{ _-^0}C$ is (given $L _{steam}$ = 536 cal/gm)-

  1. 100 calorie

  2. 0.01 kilocalorie

  3. 716 calorie

  4. 1 kilocaorie
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Heat = (m * L_fusion) + (m * c_water * dT) + (m * L_vaporization). For 1g: (1 * 80) + (1 * 1 * 100) + (1 * 536) = 80 + 100 + 536 = 716 calories.

Multiple choice physics temperature and heat modes of heat transfer - conduction conduction heat and modes of heat transfer

Air is filled at $60^o$C in a vessel of open mouth. The vessel is heated at temperature T so that $\dfrac{1}{4}$ th part of air escapes. Assuming volume of container remaining constant, find value of T. 

  1. $80^oC$

  2. $444^oC$

  3. $333^oC$

  4. $171^oC$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Using PV = nRT, since P and V are constant, n1*T1 = n2*T2. If 1/4 escapes, n2 = 3/4 n1. So n1 * (60 + 273) = (3/4)n1 * (T + 273). 333 = 0.75 * (T + 273). 444 = T + 273, so T = 171 C.

Multiple choice physics temperature and heat modes of heat transfer - conduction conduction heat and modes of heat transfer

A metal rod of length $2m$ has cross sectional area $2A$ and as shown in figure$.$ The ends are maintained at temperature $100^0C$ and $70^0C$.$ The tem[temperature at middle point C is

  1. $90^0C$

  2. <span>$30^0C$</span>

  3. <span>$45^0C$</span>

  4. <span>$60^0C$</span>
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

$\begin{array}{l} Let\, \, q\, \, be\, \, temperatrure\, \, middle\, \, po{ { int } }\, \, C\, and\, \, in\, \, series\, \, rate\, \, of\, \, heat\, \, flow\, \, is\, \, same \ K\left( { 2A } \right) \left( { 100-\theta  } \right) =KA\left( { \theta -70 } \right)  \ 200-2\theta =\theta -70 \ 3\theta =270 \ \theta ={ 90^{ 0 } }C \end{array}$

Hence,
option $(A)$ is correct answer.