Tag: physics

Questions Related to physics

Conduction cannot take place in:

physics temperature and heat modes of heat transfer - conduction conduction heat and modes of heat transfer

  1. Copper

  2. Iron

  3. Aluminium

  4. Vacuum

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Correct Option: D
Explanation:

A solid medium is required for conduction to take place. Since, vaccum does not have a medium, conduction does not take place.

Transmission of heat by the movement of heated particles is called

physics temperature and heat modes of heat transfer - conduction conduction heat and modes of heat transfer

  1. Conduction

  2. Convection

  3. Radiation

  4. Regelation

  5. Expansion

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Correct Option: B
Explanation:

Convection is a mode of heat transfer where in the molecules near the heat source get heated up, they expand , becomes less dense , and rise upwards .whereas the relatively colder sink to the bottom and eventually get heated up. Thus transmission of heat by the movement of heated particles of heat particles is called convection.

The only liquid which is a good conductor of heat is-

physics temperature and heat modes of heat transfer - conduction conduction heat and modes of heat transfer

  1. water

  2. turpentine

  3. alcohol

  4. mercury

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Correct Option: D
Explanation:
The liquid which is good conductor of heat is mercury.
 Thermal conductivity is $8.30W/(mK)$ of mercury.

State whether given statement is True or False
Air conducts heat

physics temperature and heat modes of heat transfer - conduction conduction heat and modes of heat transfer

  1. True

  2. False

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Correct Option: B
Explanation:

False, air do not conducts heat.

But air is poor conductor of heat because its molecules are not in continuous contact with one another unlike a solid. Heat by conduction passes more quickly in solid because the molecules vibrations immediately affect the neighbours. Air molecules only transfer energy upon collision and the collision time is too small for significant energy transfer to occur. Heat transfer in air is mainly due to convection, which in general is less efficient than in conduction.

Give reason : Birds puff up their feathers in winter 

physics temperature and heat modes of heat transfer - conduction conduction heat and modes of heat transfer

  1. so that they can trap more air

  2. so that can develop charge

  3. cant say

  4. so that friction reduces

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Correct Option: A
Explanation:

Birds puff up their feathers in winter because by doing so they trap a large amount of air in their body which in turn acts as an insulator and does not allow the heat from their bodies to flow out

Using a heating blanket to get warm is an example of-

physics temperature and heat modes of heat transfer - conduction conduction heat and modes of heat transfer

  1. Conduction

  2. Convection

  3. Radiation

  4. none of these

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Correct Option: A
Explanation:
Using a heating blanket to get warm is example of conduction.
As conduction is the transfer of heat from one body to another by direct contact without mixing of masses. When we cover ourselves with blanket, there is direct contact. And blanket insulates the heat present in the body and does not out heat to escape and loose warmth.

Same quantity of ice is filled in each of the two metal containers P and Q having the same size, shape and wall thickness but made of different materials. The containers are kept in identical surroundings. The ice in P melts completely in time $t _1$ whereas that in Q takes a time $t _2$. The ratio of thermal conductivities of the materials of P and Q is

physics temperature and heat modes of heat transfer - conduction conduction heat and modes of heat transfer

  1. $t _2 : t _1$

  2. $t _1 : t _2$

  3. $t _1^2 : t _2^2$

  4. $t _2^2 : t _1^2$

Show answer
Correct Option: A
Explanation:

The power transported per unit area is given as

$\dfrac{\Delta Q}{A\Delta t}=-\kappa\dfrac{\Delta T}{\Delta x}$
Thus $\Delta Q=-\kappa \Delta tA\dfrac{\Delta T}{\Delta x}$
Same amount of heat was needed to melt ice in both P and Q. Hence 
$\kappa _{P}t _1=\kappa _Qt _2$
$\implies \dfrac{\kappa _P}{\kappa _Q}=\dfrac{t _2}{t _1}$

Two thermometers are used to record the temperature of a room. If the bulb of one is wrapped in wet hanky:

physics temperature and heat modes of heat transfer - conduction conduction heat and modes of heat transfer

  1. The temperature recorded by both will be same.

  2. The temperature recorded by the wet-bulb thermometer will be greater than that recorded by the other.

  3. The temperature recorded by dry-bulb thermometer will be greater than that recorded by the other

  4. None of the above

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Correct Option: C
Explanation:

The thermometer whose bulb is wrapped by wet hanky; undergoes evaporation process; and we know that evaporation results in cooling, so the wet bulb, thermometer will record less temperature than that of dry bulb thermometer.

Two plates of same thickness form a composite plate. The temperature on one side of the plate is $0^0 C$. If the ratio of thermal conductivities is 3 : 1 and the plate with higher thermal conductivity has one of its faces at $0^0 C$, then the temperature of the interface is 

physics temperature and heat modes of heat transfer - conduction conduction heat and modes of heat transfer

  1. $45^0 C$

  2. $40^0 C$

  3. $20^0 C$

  4. $15^0 C$

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Correct Option: D

A body P is connected to a large body Q through a conducting rod of length. I crossectional  area. A and thermal conductivity K. This assembly is placed in an an atmosplere of temperature ${ T } _{ A }$ and body Q is also maintained at temperature ${ T } _{ A }$. Let beat capacity of body P is C and it is unitally at temperature ${ T } _{ 1 }$. If in time t second temperature of body P falls to ${ T } _{ 2 }$. Then chose the correct option .

physics temperature and heat modes of heat transfer - conduction conduction heat and modes of heat transfer

  1. $log\left[ \dfrac { { T } _{ 2 }-{ T } _{ A } }{ { T } _{ 1 }-{ T } _{ A } } \right] =\left[ { K } _{ 1 }\dfrac { KA }{ LC } \right] t$

  2. $log\left[ \dfrac { { T } _{ 2 }-{ T } _{ A } }{ { T } _{ 12 }-{ T } _{ A } } \right] =\left[ { K } _{ 1 }\dfrac { KA }{ LC } \right] t$

  3. $log\left[ \dfrac { { T } _{ 1 }-{ T } _{ A } }{ { T } _{ 2 }-{ T } _{ A } } \right] =\left[ { K } _{ 1 }\dfrac { KA }{ LC } .t \right]$

  4. $log\left[ \dfrac { { T } _{ 1 }-{ T } _{ A } }{ { T } _{ 2 }-{ T } _{ A } } \right] =\left[ { K } _{ 1 }\dfrac { KA }{ LC } \right] t$

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Correct Option: C