Measurable and immeasurable properties

The quantities in thermodynamics that we can directly measure with simple instruments are pressure P, temperature T, mass m, and volume VI Although we can argue about whether these are direct (in a thermocouple we measure an emf and use a table to convert that value to a temperature), all of these values are measurable without recourse to thermodynamic calculations. We commonly combine m and V to get v= V/m or its reciprocal, density = p = l/v = m/V. Such measurements are normally called PvT measurements. 

There is no known direct measurement of m or s (or the properties derived from them, h, g, and a, or the other convenience properties we will define in Chapter 7). These must be calculated from PvT (and heat capacity, see below) measurements. (In so doing we often rely on electrical measurements in which we measure heat flow rates as the product of voltage and current.) Thus, all the values you will ever see of. s and u are based at least in part on calculations, based on the above four measurable variables. 

N m = J = V C = W s = several other combinations. In the English engineering system there is no comparable simplification. 

For heat there is no comparably simple, mechanical way of defining a unit quantity. The historic choice was to measure the amount of heat needed to raise the temperature of 1 unit mass of water by 1 degree and base the unit on that. The results of that are 

The calorie is an unpractically small engineering unit we normally use the kilocalorie (kcal) = 1000 cal. (The calorie in diet books is the kilocalorie. A normal adult doing moderate work needs to eat about 2500 kcal of food per day.) The Btu is also impracttcally small for industrial size equipment we often use 10 Btu as the working unit. (In 2012 the world wholesale price of natural gas was about 4/10 Btu, that of coal about 2/10 Btu.) 

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