THERMAL EXPANSION

GATEWAY OF KNOWLEDGE

1.In case of freezing of a liquid,cubical expansitivity can not be defined.In this process,change in temperature is zero,i.e.,delta theta=0. 2.There is a principle of thermal expansion that during expansion the separation between any two points in the body increases.For example,a steel plate has a central hole and if it is heated then size of hole increases. 3.The positive values of slpha,beta and gamma for the substances indicate that they expand with increasing temperature. 4.The cubical expansivity of a material that shows anomalous behaviour within certain range of temperature (e.g. water) is negative. 5.The pendulum rod is made of invar (alloy of nickel and steel). This alloy has negligible coefficient of linear expansion.So,it is not affected by temperature.That is why,a pendulum clock made of invar gives the correct time in summer and winter as well. 6.The metal of higher coefficient of expansion (brass) bends more than the other of lesser coefficient of expansion (invar steel). 7.A device used to maintain a constant temperature is called thermostat. 8.Vacuum is a perfect bad conductornof heat. 9.In general,the good conductors of heat are also the good conductor of electricity.Mica is an exception to this rule.It is good conductor of heat but insulator of electricity. 10.If a brass plate with a hole in it expands ,does the hole get larger or smaller? Answer:If a brass plate with a hole in it expands,the hole gets larger. 11.A solid ball of metal has spherical cavity inside it.The volume of cavity is increased if the ball is heated.This is because during the expansion the separation between two points of a body increases. 12.The pendulum rod is made of invar (alloy of nickel and steel).This alloy has negligible coefficient of linear expansion.The metal of higher coefficient of expansion (brass) bends more than other of lesser coefficient of expansion (invar steel). 13.Material of zero thermal expansion! We know that materials expand or contract with changes in temperature.There are only a few special materials that barely or do not alter their shape and size in response to temperature, and this normally only occurs over a relatively narrow temperature range.This property is called zero thermal expansion.For example, antiperovskite manganese nitride has zero thermal expansion.Invar (64FeNi) is a nickel - iron alloy whose thermal expansivity is very, very low. 14.Materials of negative thermal expansion! having negative thermal expansion contract upon heating,rather than expand as most other materials do.The most well - known material with negative thermal expansion is water at 0 to 4 °C.Similarly,rubber elasticity shows negative thermal expansivity at normal temperature,carbon fibers shows between 20°C and 500°C, fairly pure silicon (Si) shows between about 18 K and 120 K.