WHAT IS CONDUCTION? | Conductor And Non Conductor | Examples of Conductors

CONDUCTION

                         CONDUCTION:    The handle of metal spoon held in hot water soon gets warm. But in case of wooden spoon, the handle does not get warm. Both the materials behave differently regarding the transfer of heat. Both metals and non-metals conduct heat. Metal are generally better conductor than non-metal.

                        In solids, atoms and molecules are packed close together as shown in fig bellow. They continue to vibrate about their mean position. What happens when one of its ends is heated? The atoms or molecules Present at that end begin to vibrate more rapidly. They also collide with their neighboring atoms or molecules. In doing so, they pass some of their energy to neighboring atoms or molecules during collisions with them with the increase in their vibrations. These atoms or molecules in turn pass on a part of the energy to their neighboring particles. In this way some heat reaches the other parts of the solids. This is a slow process and very small transfer of heat takes place from hot to cold parts in solids.

                          How does these heat flow from hot to cold parts in metal so rapidly then non-metal? These free electrons move with high velocities within the metal objects. They carry energy at very fast rate from hot to cold part of the object as they move. Thus, heat reaches the cold parts of the metal object from its hot part much more quickly than non-metals.

                           The mode of transfer of heat by vibrating atoms and free electrons in solids from hot to cold part of body  is called conduction of heat.

                             All metals are good conductor of heat. The substances through which heat does not conduct easily are called bad conductor of insulators. Wood, cork, cotton, wool, glass, rubber, etc. are bad conductor or insulators.

THERMAL CONDUCTIVITY 

            Conduction of heat occurs at different rates in different materials. In metals, heat flows rapidly as compares such as wood or rubber as compare to insulator consider a solid block as shown in fig bellow. One of two opposite faces each of cross-sectional area.

            The amount of heat that flows in unit time is called the rate of flow of heat.

         Thus,

                                          Rate of flow of heat = Q / t

            

           It is observe that the rate at which heat flow through a solid object depends upon various factors.

CROSS - SECTIONAL AREA OF THE SOLID

           Large cross - sectional area A of a solid contains large number of molecules and free electrons on each layer parallel to its cross-sectional area and hence greater will be the rate of flow of heat through the solid. Thus 

                Rate of flow of heat     Q / t ∝ A

LENGTH OF THE SOLID

           Larger is the length between the hot and cold end of the solid , more time it will take to conduct heat to the colder end and smaller will be the rate of flow of heat. Thus

                 Rate of flow of heat Q / t ∝ 1 / L

TEMPERATURE DIFFERENCE BETWEEN ENDS

          Greater is the temperature difference T1 - T2 between hot and cold faces of the solid, greater will be the rate of flow of heat. Thus

                      Rate of flow of heat Q / t ∝ (T1 - T2)

     Combining the above factors, We get

                                                   Q / t ∝ A(T1 - T2) / L

    

     Rate of flow of heat Q / t = K A (T1 - T2) / L

             Here K is proportionality consent called the thermal conductivity of the solid. Its value depends on the nature of the substance and is different for different materials.

                                                    K = Q / t × L / A(T1 - T2)

              

             Thus, thermal conductivity of a substance can be defined as:

           The rate of flow of heat across the opposite face of meter cube of the substance to maintained at a temperature difference of one Kelvin is called the thermal conductivity of that substance.

USE OF CONDUCTOR AND NON CONDUCTOR

           In houses, good thermal insulation means lower competition of fuel. For this , following measures may be taken to save energy.

• Hot water tanks are insulated by plastic or foam lagging.
• Wall cavities are filled with plastic foam or wool.
• ceiling of rooms is covered by insulating materials.
• Double glazed window panes are used. These window panes have air between glass sheets that provides good insulation. 

Good conductor are used when quick transfer of heat is required through a body. Thus cookers, cooking plate, boiler, radiators and condensers of refrigerators etc. are made of metals such as aluminum and copper. Similarly, metal boxes are used for making ice, ice cream, etc.