My last post entertained briefly the First Law of Thermodynamics. Such a concept permeates most all systems including what is often referred to as a closed system. Within a closed system, matter can not cross a set boundary - enter or leave. Most textbooks refer only to energy as being able to cross the set boundary within a closed system. We demonstrated from earlier posts that work and energy were in fact synonomous, and as such, we can include work as being able to cross the set boundary of the system; this validates both th qualitative and quantitative definitions of the law: 1. conservation of energy; 2. delta U= Q-W - change in internal energy is equal to energy/heat added to the system minus the work done on the system. Let's discuss why most solids expand when energy/heat is added.
As a child, I can remember my grandmother running hot water over the lids of jars to help open them. This practice is based on the concept of linear expansion. When energy/heat is added to most solids the molecules of the material speed up their interactions, push outward and the solid expands. Quantitatively, and from a linear perspective, this is given by delta L = «a»L delta T - change in length is equal to the coefficient of linear expansion times the original length times the change in temperature. This «a» or coefficient of linear expansion will almost always be given in a stated problem. For a good list of coefficients of this type you can go to www.engineeringtoolbox.com and search for coefficient of linear expansion for materials. Here is a railroad example.
You are reviewing some specifications on a newly constructed railroad. As you are examining the 12 meter long rails, you notice there is 2.5 millimeter gap between the rails. The coefficient of linear expansion for the steel used in the rail is 1.2*10^-5 C^-1; the units for coefficient of linear expansion are 1/C or C^-1. You check the almanac and find that the normal temperature can increase as much as 50 degrees Celsius on a hot day. Did the chief engineer wisely design the rail system or should he retake high school physics?
Next time, we will.check the answer to this and tackle some other thermodynamic issues with rail transport.
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