Physics · Science General

Heat Transfer Mechanisms

828 Questions

Heat Transfer Mechanisms deal with the movement of thermal energy through conduction, convection, and radiation. It involves practical applications like ocean heat storage and atmospheric heating. These fundamental physics concepts are frequently asked in general science sections.

Thermal conductionHeat convectionThermal radiationSpecific heatStates of matter

Heat Transfer Mechanisms Questions

Multiple choice physics transfer of heat applications of convection convection thermal energy transfers

In which of the following process, convection does not takes place primarily?

  1. sea and land breeze

  2. Boiling of water

  3. warming of glass of bulb due to filament

  4. heating air around a furnace
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation
Heat transfer of glass bulb from filament is through radiation. 
A medium is required for convection process.
As a bulb is almost evacuated, heat from the filament is transmitted through radiation.
Multiple choice physics thermal energy transfers applications of convection convection transfer of heat

While measuring the thermal conductivity of liquids, the upper part is kept hot and lower one cool so that :

  1. convectional flow is stopped

  2. radiation is stopped

  3. conduction is easier

  4. it is easier to perform the experiment
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Heat flows from hotter part to colder part and the above arrangement is done to avoid convection currents.

Multiple choice physics transfer of heat applications of convection convection thermal energy transfers

It is hotter at the same distance over the top of a fire than it is on the side of it mainly because

  1. Heat is radiated upwards

  2. Air conducts heat upwards

  3. Convection takes more heat upwards

  4. Conduction, convection and radiation all contribute significantly in transferring heat upwards
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Convection significantly transferring heat upwards. The hot gases from the combustion rises so we get both the radiation heat from the flame to our hand and the convective heat transfer from the hot air to your hand.

Multiple choice physics transfer of heat applications of convection convection thermal energy transfers

Why it is more hotter for same distance over the top of a candle than it in the side of its flame?

  1. Conduction of heat in air is upward

  2. Heat is maximum radiated in upward direction

  3. Radiation and conduction both contribute in transferring heat upwards

  4. Convection takes more heat in upward direction
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The hot air is less dense (lighter) than the cooler air around it, so it naturally goes upward, making the area above the flame hotter than the area to the sides of it.

On the side of the fire you’re only getting the radiant heat from the burn. On the top you are getting the convective hot air and actual burn chemicals (smoke, etc.) rising with the less dense air.
Hence the D option is correct .

Multiple choice physics transfer of heat applications of convection convection thermal energy transfers

A sphere ,a cone and a circular plate, all of same material and same mass are initially heated to same high temperature, then

  1. Plate will cool fastest and cone the slowest

  2. Sphere will cool fastest and the cone slowest

  3. Plate will cool fastest and the cone slowest

  4. Cone will cool fastest and plate slowest
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Cooling rate depends on surface area to volume ratio. A plate has the highest surface area for a given volume, leading to fastest cooling, while a sphere has the lowest, leading to slowest cooling.

Multiple choice physics transfer of heat applications of convection convection thermal energy transfers

In natural convection, a heated portion of a liquid moves because

  1. Its molecular motion becomes aligned

  2. Of molecular collisions within it

  3. Its density is less than that of the
    surrounding fluid

  4. Of currents of the surrounding fluid
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

In natural convection, heating a fluid decreases its density, causing it to rise due to buoyancy forces, while cooler, denser fluid moves in to replace it.

Multiple choice physics transfer of heat applications of convection convection thermal energy transfers

It is hotter at some distance over the flames than in front of it because :

  1. air conducts heat upwards only

  2. heat is radiated upwards only

  3. convection of heat occurs upwards only

  4. heat is radiated downwards only
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Near flames, heat transfer occurs by both convection and radiation (and some conduction too).
But convection occurs only in upward direction. So, above the flame, it is hotter rather than in front of the flame.

Multiple choice physics transfer of heat applications of convection convection thermal energy transfers

Which of the following process(es) occur(s) during the convective transfer of heat within a large volume of air? 

  1. A continuous flow of warmer and cooler parts of air is established.

  2. The flow of air molecules results in flow of heat.

  3. The cooler portion of the air surrounding a warmer part exerts a buoyant force on it.

  4. As the warmer part of the air moves, it is replaced by cooler air that is subsequently warmed up.
Reveal answer Fill a bubble to check yourself
A,B,C,D Correct answer
Explanation

A continuous flow of warmer and cooler parts of air is established in which the air molecules are responsible for flow of heat. As the warmer air molecules move up, they are replaced bu cooler air molecules.

Multiple choice kirchoff's laws black body radiation heat transfer thermal properties physics

Good absorbers of heat are 

  1. Poor emitters of heat

  2. Good emitters of heat

  3. Only black bodies

  4. None of these
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation
Good absorbers of heat are good emitters of heat.
It is better explained by the Kirchoff's law of radiation.
For eg - Black Body is a good emitter as well as good absorber of heat.
Multiple choice kirchoff's laws black body radiation heat transfer thermal properties physics

In steady state

  1. Heat received is partly conducted and partly radiated

  2. Heat is not absorbed

  3. both A and B

  4. All the heat is conducted
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

In steady state, heat received by any small element = heat given by it
So, heat is partially radiated and no net heat is absorbed.
We give an argument for this.
suppose that the heat is being given out by the element.
In this case, the element's temperature will decrease.
But, by definition , at steady state, all the temperatures remain constant.
So, this will violate the steady state condition.

Multiple choice kirchoff's laws black body radiation heat transfer thermal properties physics

An ideal black body at room temperature is thrown into a furnace.It is observed that

  1. initially it is the darkest body and at later times the brightest.

  2. it is darkest body at all times.

  3. it cannot be distinguished at all times.

  4. initially it is darkest and at later times it cannot be distinguished.
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

An ideal black body is a good emitter and good absorber. Initially it will absorb all the energy and will appear dark. As soon as it attains the temperature equal to that of the furnace, it will start emitting better than the rest of furnace and hence appears the brightest.

Multiple choice kirchoff's laws black body radiation heat transfer thermal properties physics

Choose the correct statements from the following

  1. Good reflectors are good emitters of thermal radiation.
  2. Burns caused by water at 100$^{o}$C are more severe than those caused by steam at 100$^{o}$C

  3. If the earth did not have atmosphere, it would become intolerably cold.

  4. It is impossible to construct a heat engine of a 100% efficiency.
Reveal answer Fill a bubble to check yourself
C,D Correct answer
Explanation
Statement (1) is incorrect. According to Kirchhoff's law, a good emitter is good absorber.
Statement (2) is incorrect because 100°C steam has more energy compared to 100°C water. 
Statement (3) is correct. The thermal radiation from the sun warms the earth during the day since air is poor conductor of heat, the atmosphere acts as a blanket for the earth and keeps the earth warm during the night. Moon is very cold because it has no atmosphere. 
Statement (4) is also correct , $\eta=1-T _2/T _1$ where $T _2$ is the temperature of the sink. To have 
an efficiency, $\eta = 1 (or\ 100\%)$, $T _2=0K$. Since absolute zero cannot be achieved, even an ideal heat engine cannot have a $100\%$ efficiency.