Tag: heat and heat transfer

Questions Related to heat and heat transfer

$\triangle { H }^{ \circ  }$ for reaction: 
${ F } _{ 2 }+2HCI\rightarrow 2HF+{ CI } _{ 2 }$
is equal to -352.8kJ. If $\triangle { H } _{ f }^{ \circ  }$ for HF is -268.3 kJ ${ mol }^{ -1 }$ then ${ \triangle H } _{ f }^{ \circ  }$ of HCI would be:- 

physics heat energy transfers heat and heat transfer heat energy transfer transfer of heat

  1. -22 kJ ${ mol }^{ -1 }$

  2. 88.0 kJ ${ mol }^{ -1 }$

  3. 01.0 kJ${ mol }^{ -1 }$

  4. None

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Correct Option: C

A hetaed smooth metallic body is allowed to cool in air. Which of the following statements about its heat loss is incorrect?

physics heat energy transfers heat and heat transfer heat energy transfer transfer of heat

  1. Convection currents in the air aid the process of heat loss.

  2. Conduction is the main mode of heta loss as the body is a good conductor of heat

  3. The rate of heat loss by thermal radiation is increased due to its metallic surface.

  4. The rate of heat loss decreases as the temperature of teh body nears room temperature

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Correct Option: C

Spheres $P$ and $Q$ are uniformly constructed from the same material which is a good conductor of heat and the radius of $Q$is thrice the radius of $P$. The rate of fall of temperature of $P$ is $x$ times that of $Q$ when both are at the same surface temperature. The value of $x$ is:

physics heat energy transfers heat and heat transfer heat energy transfer transfer of heat

  1. $1/4$

  2. $1/3$

  3. $3$

  4. $4$

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Correct Option: C

If we convert a sphere of radius $'r'$ into a cylinder of radius $r$ at same temperature the rate of heat loss will  becomes if both are placed at same surrounding-

physics heat energy transfers heat and heat transfer heat energy transfer transfer of heat

  1. $7/6$

  2. $6/7$

  3. $4/3$

  4. remain same

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Correct Option: A

A sphere, a cube and a thin circular plate, all of same material and same mass, are initially heated to same high temperature. Choose the correct statement.

physics heat energy transfers heat and heat transfer heat energy transfer transfer of heat

  1. The plate will cool fastest and cube the slowest

  2. The sphere will cool faster and cube and the slowest

  3. The plate will cool faster and sphere the slowest

  4. The cube will cool fastest and plate the slowest.

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Correct Option: C

A metal cylinder of mass 0.5 kg is heated electrically by a 12 W heater in a room and cylinder temperature rises uniformly to $ 25^oC in 5 min $ excess temperature surroundings,

physics heat energy transfers heat and heat transfer heat energy transfer transfer of heat

  1. The rate of loss of heat of the cylinder to surrounding at $ 20^oC $ is 2 W

  2. The rate of loss heat of the cylinder to surrounding at $ 45^oC $ is 12 W

  3. Specific heat capacity of metal is $ \frac {240}{Ib(3/2)} $

  4. None of these

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Correct Option: B

Heat current is maximum in which of the following (rods are of identical dimension)

physics heat energy transfers heat and heat transfer heat energy transfer transfer of heat

  1. Copper

  2. Copper steel

  3. Steel Copper

  4. Steel

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Correct Option: A

Assertion (A): When hot water is poured in athick glass tumbler then the tumbler breaks.
Reason (R): Glass is a bad conductor of heat

physics heat energy transfers heat and heat transfer heat energy transfer transfer of heat

  1. A and R are correct and R is correct explanation for A

  2. A and R are correct and R is not correct explanation for A

  3. A is true and R is false

  4. A is wrong and R is true.

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Correct Option: A
Explanation:

Since glass is bad conductor of heat the heat will try to escape but the glass would not allow it and in the process ultimately glass breaks down.

Metals are good conductors of heat because

physics heat energy transfers heat and heat transfer heat energy transfer transfer of heat

  1. They contain large number of free electrons

  2. Their atoms are relatively apart

  3. Their atoms collide frequently

  4. They have reflecting surfaces

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Correct Option: A
Explanation:

Free electrons are not associated to any atom in particular. They are usually the valence electrons of the metal atoms and are loosely bound. These electrons form a "charge fluid" that travels from one place to other in the conductor and thus convey electric current.

In the Ingen Hausz's experiment, the wax melts up to lengths $10cm$ and $25cm$ on two identical rods of different materials. The ratio of thermal conductivities of the two materials is

physics heat energy transfers heat and heat transfer heat energy transfer transfer of heat

  1. $1:6.25$

  2. $6.25:1$

  3. $1: $$\sqrt{2.5}$

  4. $1:2.5$

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Correct Option: D
Explanation:

In the Ingen Hauszs experiment, as rate of heat transfer is constant
$\dfrac { { K } _{ 1 } }{ { K } _{ 2 } } =\dfrac { { l } _{ 1 } }{ { { l } _{ 2 } } } $
So, putting the value in above formula's we find
$\dfrac{{K} _{1}}{{K} _{2}}=\dfrac{10}{25}=\dfrac{1}{2.5}$