Tag: convection

Questions Related to convection

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

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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 .

The process which are subjected to action of heat in absence of air is called

physics transfer of heat applications of convection convection thermal energy transfers

  1. Extraction

  2. Polishing

  3. Calcination

  4. None of these

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Correct Option: C
Explanation:

Calcination is defined as converting an ore into its oxide by heating strongly , below its melting point either in a limited supply of air or in the absence of air.

Heat is flowing through two cylindrical rods made of same materials whose ends are maintained at similar temperatures. If diameters of the rods are in ratio 1 : 2 and lengths in the ratio 2 : 1, then the ratio of thermal current through in steady state is:

physics transfer of heat applications of convection convection thermal energy transfers

  1. 1 : 8

  2. 1 : 4

  3. 1 : 6

  4. 4 : 1

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Correct Option: A
Explanation:

The thermal current is nothing but the rate of flow heat,


$I=\dfrac{Q}{t}=K\Delta T\dfrac{A}{l}$. . . . . .(1)

Where,

$\Delta T= $ change in temperature

$A=\pi r^2$, cross-sectional area of the cylindrical rod.

$K=$ Thermal conductivity.

$l=$ length of the rod

From equation (1)

Thermal current is $I=4\pi K\Delta T \dfrac{d^2}{l}$

$I\alpha \dfrac{d^2}{l}$ where, $d=$ diameter of the cylindrical rod

Given,

$l _1:l _2=2:1$

$d _1:d _2=1:2$

The ratio of the thermal current  is,

$\dfrac{I _1}{I _2}=(\dfrac{d _1}{d _2})^2\times \dfrac{l _2}{l _1}$

$\dfrac{I _1}{I _2}=(\dfrac{1}{2})^2\times \dfrac{1}{2}$

$\dfrac{I _1}{I _2}=\dfrac{1}{8}$

$I _1:I _2=1:8$

Option A is correct.

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

physics transfer of heat applications of convection convection thermal energy transfers

  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

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Correct Option: C

Temperature of a room is ${ -15 }^{ 0 }C$ and outside temperature is ${ 10 }^{ 0 }C.$ If room temperature is made ${ 40 }^{ 0 }C$, then find outside temperature, if rate of heat flow is same in both cases. 

physics transfer of heat applications of convection convection thermal energy transfers

  1. ${ 10 }^{ 0 }C$

  2. ${ 5 }^{ 0 }C$

  3. ${ 15 }^{ 0 }C$

  4. ${ 20 }^{ 0 }C$

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Correct Option: C

Ram want to drink tea, when it is at $50^0C$. He ordered tea which arrives at a temperature of $80^0C$.tea takes one minute to cool from $80^0Cto60^0C$. If room temperature is $30^0C$, how long he has to wait to drink the tea?

physics transfer of heat applications of convection convection thermal energy transfers

  1. Less than one minute

  2. Two more minutes

  3. Half a minute

  4. Nearly three minute

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Correct Option: C

A metallic sphere cools from $50^{\circ}C$ to $40^{\circ}C$ in $300\ s$. If the room temperature is $20^{\circ}C$, then its temperature in the next $5$ min will be

physics transfer of heat applications of convection convection thermal energy transfers

  1. $38^{\circ}C$

  2. $33.3^{\circ}C$

  3. $30^{\circ}C$

  4. $36^{\circ}C$

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Correct Option: C

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

physics transfer of heat applications of convection convection thermal energy transfers

  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

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Correct Option: C

A sphere of density d, statisfied heat s and radius r is hung by thermally insulating thread in an enclosure which is kept at a lower temperature than the sphere.The temperature of the sphere to drop at a rate which depends upon the temperature difference between the and the enclosure,If the temperature difference is $ \triangle T $ and surrounding temperature is $ T _o $ then rate of fall in temperature will be-

physics transfer of heat applications of convection convection thermal energy transfers

  1. $ \frac {4 \sigma T _o^2 \triangle T}{rdc} $

  2. $ \frac {12 \sigma T _o^2 \triangle T}{rdc} $

  3. $ \frac {12 \sigma T _o^4 \triangle T}{rdc} $

  4. $ \frac {12\sigma \triangle T}{rdc T _o^3 } $

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Correct Option: A

The correct statement of the following:
A) Water can be boiled inside the artificial satellite by convection
B) Heavy liquid can be boiled in artificial satellite by convection
Mark correct option/s

physics transfer of heat applications of convection convection thermal energy transfers

  1. Both statements are correct

  2. Both statements are wrong

  3. A is correct but B is wrong

  4. B is correct but A is wrong

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Correct Option: B
Explanation:

Conduction and Convection are non-existent in space, so both the statements are wrong.