Tag: thermal energy transfers

Questions Related to thermal energy transfers

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

physics transfer of heat applications of convection convection thermal energy transfers

  1. sea and land breeze

  2. Boiling of water

  3. warming of glass of bulb due to filament

  4. heating air around a furnace

Show answer
Correct Option: C
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.

A double-plane window consist of two glass sheets each of area $1m^2$ and thickness $0.01m$ separated by a $0.05m$ thick stagnant air space In the steady state, the room glass interface and the glass outdoor interface are at constant temperature of $27^oC$ and $0^oC$ respectively. The rate of heat flow through the window plane is (Given , $k _{glass}=0.8\,\,W\,\,m^{-1}K^{-1},K _{air}=0.08\,\,W\,\,m^{-1}K^{-1})$

physics transfer of heat applications of convection convection thermal energy transfers

  1. $41.5\,\,W$

  2. $31.5\,\,W$

  3. $21.5\,\,W$

  4. $11.5\,\,W$

Show answer
Correct Option: A
Explanation:

Total thermal resistance is 
$R=\dfrac{t _1}{K _1A _1}+\dfrac{t _2}{K _2A _2}+\dfrac{t _1}{K _1A _1}$


$R=2\times \dfrac{0.01}{0.8\times 1}+\dfrac{0.05}{0.08\times 1}= 0.65W^{-1}K$

$\therefore $Heat current $,H=\dfrac{\triangle T}{R}=\dfrac{27-0}{0.65}=41.5W$

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

physics thermal energy transfers applications of convection convection transfer of heat

  1. convectional flow is stopped

  2. radiation is stopped

  3. conduction is easier

  4. it is easier to perform the experiment

Show answer
Correct Option: A
Explanation:

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

Fireman crawl when entering a burning building because:

physics transfer of heat applications of convection convection thermal energy transfers

  1. It is easier to crawl

  2. Smoke rises high in the air

  3. It helps to move faster

  4. To resist more heat

Show answer
Correct Option: B
Explanation:

Smoke is lighter than air so it rises above the air, hence it is easier to breath crawling than while standing up.

Convection of heat takes place in:

physics thermal energy transfers applications of convection convection transfer of heat

  1. Metals only

  2. Liquids only

  3. Gases only

  4. Liquids and gases

Show answer
Correct Option: D
Explanation:

The molecules of liquids and gases are not rigidly bound to each other and can move freely. This makes the transfer of heat by convection possible.

Birds glide effortlessly in the air with the help of

physics transfer of heat applications of convection convection thermal energy transfers

  1. Conduction of heat in the air

  2. Radiation of light through the atmosphere

  3. Convection currents of air

  4. More sugar

Show answer
Correct Option: C
Explanation:

When birds flap their wings the density of the air decreases nearby since the air particles move apart. This less dense air rises and helps the bird to keep flying. The denser air replaces this space. This movement is known as convection current of air. 

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

physics transfer of heat applications of convection convection thermal energy transfers

  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

Show answer
Correct Option: C
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.

State whether the given statement is True or False.
Most of the heat transfer that is taking place on the earth is by Radiation.

physics transfer of heat applications of convection convection thermal energy transfers

  1. True

  2. False

Show answer
Correct Option: B
Explanation:

Most of heat transfer taking place on earth is due to conduction and convection .

If there were no atmosphere around the earth, the temperature of the earth will

physics transfer of heat applications of convection convection thermal energy transfers

  1. Increase

  2. Decrease

  3. Increase during day and decrease during night

  4. Remain unaffected

Show answer
Correct Option: B
Explanation:
If there were no atmosphere around the earth, the temperature will decrease.
Earth's atmosphere traps heat from sun and keeps it near to the surface of earth, popularly known as green house effect. It keeps warming the planet and sustaining life on earth. If there were no atmosphere, average temperature drops below zero.
So, we can say that it will decrease.

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

physics transfer of heat applications of convection convection thermal energy transfers

  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

Show answer
Correct Option: D
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 .