WHAT IS THERMAL EXPANSION? - (Part 2)

THERMAL EXPANSION

THERMAL EXPANSION: You can know read the second part of thermal expansion if you want to read thermal expansion from start then go to (part 1)

CONSEQUENCE OF THERMAL EXPANSION 

                Why gaps are left in railway tracks? The expansion of solid may damage the bridge, railway tracks and rods as they are constantly subjected to temperature changes. So provision is made during contraction for expansion and contraction with temperature. For example, railway tracks buckled on a hot summer day due to expansion if gaps are not left between sections.

   

     Bridges are made of steel girders also expand during the day and contract during night. They will bend if their ends are fixed. To allow thermal expansion, one end is fixed wile the other end of the girder resets on rollers in the gap left for expansion. Overhead transmission lines are also given a certain amount of sag so that they can contract in winter without snapping.

APPLICATION OF THE THERMAL EXPANSION

 Thermal expansion  is used in our daily life. In thermometers, thermal expansion is used in temperature measurements. To open the cap of the bottle that is tight enough, immerse it in hot water for a minute or so. Metal cap expands and becomes loose. It would be easy to turn it to open.

      

   To join steel plates tightly together, red hot rivets are forced through holes in the plates as shown bellow. The ends of the hot rivet is then hammered. On cooling, the rivets contact and bring the plates tightly gripped.

    

   Iron rims are fixed on wooden wheels of carts. Iron rims are heated. Thermal expansion allows them to slip over the wooden wheel. Water is poured on it to cool. The rim contracts and becomes tight over the wheel.

BI METAL STRIP 

 A bi-metal strip consist of two thin strips of different metals such as brass and iron join together. On heating the strip, brass expand more than iron. This unequal expansion causes bending of the strip as shown in fig bellow.

  

  Bi metal strips are used for various purposes. Bi metal thermometer are used to measure temperatures especially in furnaces and ovens. bi metal strips are also used in thermostat switch that is used to control the temperature of heater coil in an electric iron.

THERMAL EXPANSION OF LIQUIDS

  The molecules of liquid are free to move in all direction with in the liquid. On heating a liquid, the average amplitude of vibration of its molecules increases. The molecules push each other and need more space to occupy. This accounts for the expansion of the liquid when heated. The thermal expansion in liquids is greater than solids due to the weak forces between their molecules. Therefore, the coefficient of volume expansion of liquids is greater than solids.

       

       Liquid have no definite shape of their own. A liquid always attains shape of the container in which it is poured. Therefore, when a liquid is heated, both liquid and the container undergo a change in their volume. Thus, their are two types of thermal volume expansion for liquid.

• Apparent volume expansion
• Real volume expansion