Βασικές Εννοιες Ενέργειας. Εισαγωγή. Το λεξιλόγιο της Ενέργειας Επισκόπηση μονάδων Βασικές έννοιες Θερμοδυναμικής Σύστημα Κατάσταση Διεργασία Κύκλος Εννοιες Θερμοκρασίας και Κλίμακες Θερμοκρασίας, Πίεσης, Μετρήσεις. Θερμοδυναμική.
of matter is under study.
The portion of the internal energy of a system associated with the
The seven fundamental dimensions and their
units in SI (International System).
Pressure is defined as the force exerted by a fluid per unit area.
Units in SI are Pa=N/m2. The pressure unit Pascal is too small for pressure encountered in practice.
Therefore, kPa and MPa are commonly used.
Units in British are : psf = lbf/ft2, psi = lbf/in2
You have to convert from psi to psf ( 144 in2 = 1 ft2)
Absolute pressure, is measured relative to absolute vacuum (i.e., absolute zero pressure.)
Gauge pressure, is measured relative to atmospheric pressure
Variation of Pressure with Depth
The pressure variation in a constant density fluid is given as
P + Z = constant
Or P1+ Z1 = P2 + Z2
Z is the vertical coordinate ( positive upward).
is the specific weight of fluids, (N/m3)
For small to moderate distances, the variation of pressure with height is negligible for gases because of their low density.
Pressure at a Point
Pressure Variation in horizontal planes
Pressure is constant in horizontal planes provided the fluid does not change. ( this leads to Pascal’s principle.)
Noting that P1 = P2, the area ratio A2/A1 is called the ideal mechanical advantage. Using a hydraulic car jack with A2/A1 = 10, a person can lift a 1000-kg car by applying a force just 100 kg (= 908 N).
A device based on P + Z = constant is called a manometer (Right), and it is commonly used to measure small and moderate pressure differences.
P2 = Patm + h
The length or the cross-sectional area of the tube has no effect on the height of the fluid column of a barometer.
When solving problems, we will assume the given information to be accurate to at least 3 significant digits. Therefore, if the length of a pipe is given to be 40 m, we will assume it to be 40.0 m in order to justify using 3 significant digits in the final results.
A result with more significant digits than that of given data falsely implies more accuracy.