Transmission of Heat

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Transmission of Heat

Transfer of Heat

Heat can transferred by the following 3 ways :

1. Conduction Heat Transfer :

In this process heat transferred without bodily movement of the particles of medium.

2. Convection Heat Transfer :

In this process heat is transferred by the bodily movement of particles of the medium due to difference in densities of different parts of the medium.

3. Radiation Heat Transfer :

Quickest way of transmission of heat is known as radiation.

In this mode of heat transmission heat is transferred from one place to another without effecting the intervening medium.

Mercury though a liquid, is heated by conduction and not by convection.

All liquids and gases are heated by convection. Heat comes to us from the sun by radiation.

Perfectly Black Body

Perfectly black body is that which neither reflects nor transmits the radiations falling on it, inspite of it the black body absorbs all radiations falling on it and hence it appears black.

It is clear that, when heated, the black body will radiate all the energy absorbed by it.

Memorable Points
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  • All metals are good conductors of heat. Silver is the best conductor.
  • Good conductors of heat are good conductors of electricity. Mica is an exception which although being a good conductor of heat and a bad conductor of electricity.
  • Cooking utensils are provided with wooden or ebonite handles, since wood or ebonite is a bad conductor of heat.
  • In winter, wooden chair appears hotter than the metal one because metal is a good conductor of heat while wood is a bad conductor of heat.
  • Convection takes place in liquids and gases but not in solids.
  • In a room, ventilators are provided at ceiling, to escape the hot air by convection.
  • Principle of chimney used in the kitchen or a factory is based on the convection.
  • Land and sea breezes are due to the convection.
  • Temperature of the upper of the flame is more then the temperature on the sides, because the currents of air carry the heat upwards.
  • Dull black surfaces are good emitters and good absorbers while they are bad reflectors.
  • Highly polished surfaces are bad absorbers and bad emitters but they are good reflectors.
  • If a thermous flask containing tea is shaken vigorously, temperature of the tea rises because of increase in internal energy.
  • If the door of refrigerator is kept open, it will not cool the room, it will increases the temperature of the room because heat rejected by the refrigerator to the room will be more than the heat taken by the refrigerator from the room.
  • If heat rejection portion of the refrigerator is outside the closed room, the opening of refrigerator door will cool the room gradually.

Latent Heat Physics

It is defined as the amount of heat absorbed or given out by a body during the change of state. It is of two types :

(1) Latent Heat of Fusion

The latent heat of fusion of a substance is quantity of heat required to change unit mass of solid substance from solid state to liquid state.

The latent heat of fusion of ice is 80 cals.

(2) Latent Heat of Vaporization

  • It is the quantity of heat required to change unit mass of liquid substance from liquid to vapour state while temperature remaining constant.
  • The latent heat of vaporization of water is 536 cals.
  • Latent heat of vaporization is maximum at 0°C and decreases on increasing temperature.
  • Latent heat of vaporization of water decreases on increasing pressure or we may say that the latent heat of vaporization decreases with increase in boiling point.
  • Ice of 0°C feels colder than water at 0°C.
  • Hot water (100°C) burns are less serve than steam burns (100°C), because steam has high latent heat.
  • Melting point decreases on adding impurity.
  • The melting point of solids which contract on melting decreases with increase in pressure e.g., ice and cast iron. These solids float on their corresponding liquid.
  • The melting point of solids which expand on melting, increase with increase in pressure e.g., wax, glass, gold etc.,
  • Boiling point increases on adding impurity.
  • Boiling point increases on increasing pressure.
  • Cooling a liquid below freezing point without turning it to a solid is called super – cooling. Water can be super – cooled to temperature as low as -12°C.
  • Heating a liquid above its boiling point without converting it vapour state is called super – heating. Water can be heated up to 137°C without boiling.

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