CBSE Class 11-science Answered

 

A BI-metallic strip works on the principle that different metals have different coefficients of expansion. A BI-metallic strip consists of two strips of different materials but of equal length. These two strips are riveted together at different points along their lengths such that they cannot slip sideways. Typically a welding process is used for bonding, but rivets, bolts, adhesive and other fasteners can also be used.

On heating the strip, the metal having the higher coefficient of linear expansion expands more than the other metal and therefore the strip bends. The bending is such that the strip is concave on the side of the metal of low coefficient of expansion and convex on the side of the metal of large coefficient of expansion. On cooling the strip, the reverse happens. Now, the metal having a higher coefficient of expansion contracts more and is therefore on the concave side while the metal of low coefficient of expansion contracts less and is on the convex side.

Example : A BI-metallic strip is brass on one side and steel on the other. When heated over a Bunsen burner the strip curves toward the steel side. When cooled in liquid nitrogen, it curves the other way.