Units

 Units

All engineered systems require measurements to chararcterise an object's performance as well as to determine its size, weight, speed, etc. It's important to comprehend how to use these units.

the single most significant goal of this book because it pertains to every aspect of engineering and everything an engineer performs. Understanding units involves much more than just being able to convert between feet and meters or the other way around; integrating and converting units from many sources is a difficult subject. Combining and converting units from several sources is a difficult problem. For example, meters to meters or vice versa. How may thermal conductivity, measured in Watts per meter per degree Celsius, be converted from, for instance, building insulation defined in BTU inches per hour per square foot per degree Fahrenheit? Or is it convertible? Are the measurements made by the two units equivalent or not? (For instance, the natural gas flow was insufficient in a new engine laboratory facility that was being constructed for me. In order to produce a minimum of 50 cubic feet per minute (cfm) of natural gas at a pressure of 5 pounds per square inch, I informed the contractor that I needed a system (psi). What is the conversion between cfm and psi, he asked in response. Naturally, the answer is "no," as cfm and psi are measurements of flow rate and pressure, respectively.) These myths present a daily challenge for engineers.

The USCS (US Customary System) and the English systems of units and measurements are burdensome for American engineers! and (2) the SI ( Saystem International d' Unités) metric system. Both systems have a set of base units, which are created based on a standard measure such the quantity of wavelengths produced by a specific light source. These fundamental units comprise:

👉Length (meters, centimeter, feet, inches); 1 m = 100 cm = 3.281 ft = 39.37 in

👉Mass ( l bm, slugs, kilograms); (1 kg = 2.205 l bm = 0.06853 slug) ( l bm = "pounds mass")

👉Time (seconds; the standard contraction is "s" not "sec") (same units in USCS and SI!)

👉Electric flow (truly electric charge in units of coulombs [abbreviation: 'coul' is the base

     unit and the determined unit is current = charge/time) (1 coulomb = charge on 6.241506 x 1018

     electrons) (1 ampere [abbreviation: amp]= 1 coul/s)

In spite of frequent reports to the contrary, moles are only an accounting convenience used to avoid lugging around factors of 10^23 everywhere. The number of particles in a mole of particles is wholly arbitrary; by convention, Avogadro's number is given as NA = 6.0221415 x 10^23, with particles/mole (or persons of any sort, not only particles) serving as the unit of measurement.

Temperature is frequently used as a base unit, although it is not; rather, it is a derived unit, which means that it was produced by combining foundation units. In essence, temperature is a unit of energy multiplied by Boltzman's constant. An ideal gas molecule's average kinetic energy in a three-dimensional box is 1.5kT, where k is Boltzman's constant, which is 1.380622 x 10^-23 J/K (actually (Joules/molecule)/K). The kinetic energy of an ideal gas molecule (and only an ideal gas; no other substance) is 1.5 kT = 2.0709 x 10^-23 J at 1 Kelvin, which is the temperature at which this occurs.

The ideal gas constant (ℜ) with which are you are very familiar is simply Boltzman’s constant multiplied by Avogadro’s number, i.e. 

There’s also another type of gas constant R = ℜ/M, where M = molecular mass of the gas; R depends on the type of gas whereas ℜ is the “universal” gas constant