Heat Transfer

4. Convection

  • Convection is transfer of heat by actual motion of matter
  • If material is forced to move by a blower or pump the process is called forced convection.
  • If the material flows due to difference in density for example that caused by thermal expansion then the process is called natural of free convection.
  • Meahanism of heat transfer in human body is forced convection. Here heart serves as the pump and blood as the circulating fluid.

5. Radiation

  • Radiation process does not need any material medium for heat transfer.
  • Term Radiation refers to the continous emission of energy from surface of all bodies and this energy is called radient energy.
  • Radiant energy is in the form of Electro Magnetic waves.
  • Radiant energy emitted by a sunface depends on the temperature and nature of the surface.
  • All bodies whether they are solid, liquid or gas emit radiant energy.
  • EM radiations emitted by a body by virtue of increased temperature of a body are called thermal radiation.
  • Thermal radiation falling on a body can partly be absorbed and partly be reflected by the body and this absorption and reflection of radiation depends on the color of body.
  • Thermal radiation travels through vacuum on straight line and with the velocity of light.
  • Thermal radiations can be reflected and refracted.

6. Black Body Radiation

  • A body that absorbs all the radiation falling on it is called a black body.
  • Radiation emitted by black body is called Black Body radiation.
  • A black body is also called an ideal radiator.
  • For practical purpose black body can be considered as an enclosure painted black from inside and a small hole is made in the wall.

    Black Body Radiation                          
  • Once radiation enters the enclosure it has very little chance to come out of the hole and it gets absorbad after multiple refrections inside th enclosure.
  • Concept of a perfact black body is an ideal one.

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