Transmission of heat energy

1. Heat or thermal energy is one of the vital energy source on earth. This heat energy needs to be transferable if life processes on earth are to continue. Heat is a form of energy that flows from a region of   higher temperature to another of lower temperature.
2. The three main types of heat transfers are
   (i) conduction,     (ii) convection and    (iii) radiation
   
   (i) Conduction:
        - The process of heat transfer, where there is no physical visibility of matter moving from their
          position, but pass heat in the form of kinetic energy is called as conduction.
        - Those substances which are capable of conducting heat are called as conductors and those
          substances through which heat cannot pass through are called as insulators.
        - Usually metals are good conductors of heat, while wood, thermacoal, plastics, ceramic are
          examples of insulators. Vacumm is considered to be the best insulator.
   (ii) Convection:     - Fluids, (liquids and gases) are usually bad conductors of heat. In fluids heat travels by the
          process called convection. Here the molecules themselves move, carrying the heat energy with
          them in the form of kinetic energy.
        - The upward movement of the hot fluid and the downward movement of the cold fluid causes the
          movement in the fluid called the convection current.
        - Land and sea breeze are caused by convection current.
   (iii) Radiation:     - The transfer of heat energy from the sun through vacuum in the form of electromagnetic waves
           is called radiation.
        - The sun gives off heat energy in the form of rays or radiant energy. When these rays fall on an
           object, they are absorbed and the object gets heated up.
        - All hot bodies give off radiant heat even if they are not glowing. White, silvery or highly polished
           surfaces are good reflectors of radiant heat. Black or dark bodies are good absorbers of heat.
           Good absorbers of heat are also good radiators.
3. Heat absorbed or given out due to radiant energy depends on
   (i) Colour of the object     (ii) Texture of the object     (iii) Surface area     (iv) Temperature
   Black and rough surfaces absorb and radiate more heat energy than light and smooth surfaces. Objects with more surface area and at high temperature absorb and radiate more heat energy than the objects with low temperature and less surface area.
4. Applications of conduction, convection and radiation
   Conduction: Pans used for cooking are made of metals, while the handles are made of insulators.
   Convection: Boiling of water in an electric kettle, land and sea breeze.
   Radiation: Black umbrellas, rough black surfaces behind refrigerators.
5. Thermos flaskVacuum flasks also called commonly as Thermos flask maintains the hot or cold temperature of the contents inside. It reduces the heat transfer from inside which could occur by the three modes viz conduction, convection and radiation. The flask is made in such a way that heat is not lost or gained by conduction, convection and radiation. Thus, the temperature inside a thermos flask is maintained.