LAWS OF THERMODYNAMICS 169 



more efficient than others. If the job is done by the hypothetical frictionless 

 machine, with minimum loss of energy, it is then done the most efficiently. 

 By analogy, work can be extracted from a process in many ways, some more 

 efficient than others. The hypothetical conditions of no waste are given the 

 special name, reversible conditions; AF is therefore the maximum work avail- 

 able under reversible conditions. One practical system from which nearly 

 maximum work can be extracted is the electrochemical one, a battery for 

 example; or, more pertinent here, the concentration cells which exist and 

 deliver energy at living membranes. 



Very common are the processes which occur under nonreversible condi- 

 tions. The expression then becomes 



A// = AF' + q' + Q_ 



for the reaction of 1 mole, or 



A3"C = A3' + q' +Q 



for the living system as a whole. Here AF' (or AJF') and q' refer to the ex- 

 ternally available work and "frictional" loss, respectively. The latter of 

 course shows up as heat energy, which must be dissipated to the environ- 

 ment by any of the well-recognized methods of perspiration, excretion, 

 respiration, etc., which will be discussed later. 

 A useful efficiency can be defined as: 



8= AF'/AF, or £= AJF/Afr 



This ratio is the fraction of the reversible Tree energy change which is re- 

 alized as useful work in the process. The value can easily be demonstrated 

 with a flashlight dry-cell; it ranges from per cent if the dry-cell is short- 

 circuited by a screwdriver across the terminals; through any value up to 

 about 70 per cent when operating in a flashlight; to close to 100 per cent 

 when used only as a source of voltage with almost no current being drawn. 

 Corresponding values for man cannot be given numerically, but must range 

 from nearly zero for a football team which expends an unimaginable amount 

 of energy to move a 2-lb football a few feet, to very high values for the nerve 

 transmission and mental activity which occur during computation. Other 

 examples will be given later. 



The thermodynamic ratio AF/A/7, defined as T, is fixed by the value of 

 the unavailable energy, Q_. It is a more fundamental quantity than 8, in the 

 sense that it does not depend upon the frictional losses in the engine, or upon 

 the inefficiencies of the living machine. All processes of energy conversion 

 are producers or consumers of heat energy, and the conversion can take 

 place only as long as heat can be transferred from one part of the system to 

 another. When finally no further transfer is possible, the process ceases. It 



