First Law of Thermodynamics and Steam Locomotives

Recently, we have discussed work and energy concepts.  Let us now build upon those concepts within a basic steam locomotive.

We know from previous posts that there is more than one way to define work.  I am going to now offer another definition.  Work equals force times distance - W=Fd, is our most common definition. We also know this is equal to a change in energy.  Yet, if we consider pressure is equal to force divided by area - P=F/A, a common concept learned at least by middle school.  An easy manipulation tells us that F=PA.  Substituting this into our equation yields work equals pressure times area times distance - W=PAd.  But, area times distance is a definition of volume.  So work is equal to pressure times a change in volume - W=PdeltaV.  This concept of changing volume at constant pressure to move a piston is precisely the mechanism behind basic steam locomotive operation and owes its origination to the 1st Law of Thermodynamics;  the change in the internal energy of a system is equal to the heat added to a system minus the work done on the system - delta U=Q- W.

Our locomotive burns coal or oil to heat water. The water eventually changes to high pressure steam which increases its volume and expands pushing a piston within a cylnder.  This explains why locomotives must take on water periodically as its water supply is constantly be turned to steam.  The piston is connected to a rod apparatus which connects to coupling rods which move the wheels.  Incidently, the steam exaust is simply released by a valve under great pressure which accounts for the "choo choo choo choo" we hear and, the piston returns to its original position ready to be pushed again by pressure and volume expansion.

We now should begin to recognize some basic relationships among work, energy, power, and force with a locomotive.