Tag: heat energy transfers

Quizzes (5) Questions (137)

Questions Related to heat energy transfers

Multiple choice physics heat energy transfers heat and heat transfer heat energy transfer transfer of heat

Two walls of thickness $d _1$ and $d _2$, thermal conductivities $K _1$ and $K _2$ are in contact. In the steady state if the temperatures at the outer surfaces are $T _1$ and $T _2$, the temperature at the common wall will be

  1. $\dfrac{K _1T _1+K _2T _2}{d _1+d _2}$

  2. $\dfrac{K _1T _1d _2+K _2T _2d _1}{K _1d _2+K _2d _1}$

  3. $\dfrac{(K _1d _1+K _2d _2)T _1T _2}{T _1+T _2}$

  4. $\dfrac{K _1d _1T _1+K _2d _2T _2}{K _1d _1+K _2d _2}$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

In steady state, the heat flow rate through both layers must be equal. Solving the equation (K1/d1)(T1 - T) = (K2/d2)(T - T2) for T gives the interface temperature.

Multiple choice physics heat energy transfers heat and heat transfer heat energy transfer transfer of heat

$\triangle { H }^{ \circ  }$ 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:- 

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

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

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

  4. None
Reveal answer Fill a bubble to check yourself
C Correct answer
Multiple choice physics heat energy transfers heat and heat transfer heat energy transfer transfer of heat

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

  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
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The statement that the rate of heat loss by radiation is increased due to a metallic surface is incorrect because metallic surfaces (especially smooth ones) have low emissivity, which reduces radiative heat loss. Convection and conduction are standard modes of heat transfer, and heat loss rate naturally decreases as the body approaches ambient temperature.

Multiple choice physics heat energy transfers heat and heat transfer heat energy transfer transfer of heat

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:

  1. $1/4$

  2. $1/3$

  3. $3$

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

The rate of cooling (dT/dt) is proportional to the surface area divided by the volume (A/V). For a sphere, A/V = (4*pi*r^2) / ((4/3)*pi*r^3) = 3/r. Since the radius of Q is 3 times that of P, the rate of cooling for P is 3 times the rate for Q (x = 3/1 = 3).

Multiple choice physics heat energy transfers heat and heat transfer heat energy transfer transfer of heat

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.

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

Cooling rate is proportional to the surface area to volume ratio (A/V). For a fixed mass and material, the object with the largest surface area will have the highest rate of heat loss. A thin circular plate has the highest surface area, while a sphere has the lowest surface area for a given volume.

Multiple choice physics heat energy transfers heat and heat transfer heat energy transfer transfer of heat

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,

  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
Reveal answer Fill a bubble to check yourself
B Correct answer
Multiple choice physics heat energy transfers heat and heat transfer heat energy transfer transfer of heat

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

  1. Copper

  2. Copper steel

  3. Steel Copper

  4. Steel
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Heat current (dQ/dt) is proportional to thermal conductivity (k). Copper has a much higher thermal conductivity than steel, so a copper rod will conduct heat faster than a steel rod of identical dimensions.

Multiple choice physics heat energy transfers heat and heat transfer heat energy transfer transfer of heat

Assertion (A): When hot water is poured in athick glass tumbler then the tumbler breaks.
Reason (R): Glass is a bad conductor 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.
Reveal answer Fill a bubble to check yourself
A Correct answer
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.

Multiple choice physics heat energy transfers heat and heat transfer heat energy transfer transfer of heat

Metals are good conductors of heat because

  1. They contain large number of free electrons

  2. Their atoms are relatively apart

  3. Their atoms collide frequently

  4. They have reflecting surfaces
Reveal answer Fill a bubble to check yourself
A Correct answer
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.