Properties of fluids

Property

• Any characteristic of a system

• ome familiar properties are pressure P, temperature T, volume V, and mass m. Properties are either intensive or extensive.

• Sebarang ciri bagi sesuatu sistem

• Beberapa sifat yang biasa ialah tekanan (P), suhu (T), isipadu (V), dan jisim (m)

• Sifat-sifat ini sama ada bersifat intensif atau ekstensif

Intensive properties

• Those that are independent of the mass of the system, such as temperature, pressure, and density.

• Yang tidak bergantung kepada jisim sistem, seperti suhu, tekanan, dan ketumpatan.

Extensive properties

• those whose values depend on the size—or extent—of the system. Total mass, total volume V, and total momentum are some examples of extensive properties.

• Extensive properties per unit mass are called specific properties. Some examples of specific properties are specific volume (v = V/m).

• Sifat yang bergantung pada saiz atau jumlah sistem

• Contoh: jumlah jisim, jumlah isipadu (V), dan jumlah momentum

• Jika sifat ekstensif dibahagi dengan jisim → dipanggil sifat tentu (specific properties)

• Contoh: isipadu tentu, v=V/mv = V/mv=V/m

Continuum

• Matter is made up of atoms that are widely spaced in the gas phase

• It is convenient to ignore the atomic nature and treat it as continuous, homogeneous matter (a continuum)

• The continuum idealization allows properties to be treated as point functions

• Properties vary continuously in space with no sudden jumps

• This idealization is valid when the system size is large compared to the space between molecules

• This applies to almost all practical problems

• Jirim terdiri daripada atom yang berjauhan antara satu sama lain dalam fasa gas

• Namun, lebih mudah untuk mengabaikan sifat atom dan menganggapnya sebagai bahan berterusan dan homogen (kontinuum)

• Pendekatan kontinuum membolehkan sifat dianggap sebagai fungsi titik

• Sifat berubah secara berterusan dalam ruang tanpa perubahan mendadak

• Pendekatan ini sah apabila saiz sistem jauh lebih besar berbanding jarak antara molekul

• Ini berlaku dalam hampir semua masalah praktikal

• (Caveman explanation):

  • Gas = tiny atoms, far apart

  • But brain say: “too messy, ignore atoms”

  • Treat it like smooth stuff, no holes → continuum

  • Means we can look at one point and say its property (pressure, temp)

  • No sudden jump, everything change smoothly

  • Works because system BIG, atoms SMALL

Density

mass per unit volume

Specific volume

volume per unit mass

Specific gravity

can be defined as the ratio of the density of a substance to the density of some standard substance at a specified temperature

(usually water at 4°C, for which 𝜌H2O = 1000 kg/m3).

Density of Ideal Gases

• The densities of liquids are essentially constant, and thus they can often be approximated as being incompressible substances during most processes without sacrificing much in accuracy

• How about density of gases???

• Equation of state: Any equation that relates the pressure, temperature, and density (or specific volume) of a substance.

• Ideal-gas equation of state: The simplest and best-known equation of state for substances in the gas phase

Ideal gas law

• An ideal gas is a hypothetical substance that obeys the relation Pv=RTPv = RTPv=RT

• The ideal-gas relation closely approximates real gases at low densities

• At low pressure and high temperature, gas density decreases and behaves like an ideal gas

• Many common gases (air, nitrogen, oxygen, hydrogen, helium, argon, neon, krypton, carbon dioxide) can be treated as ideal gases with negligible error

• Dense gases like water vapor in steam power plants and refrigerant vapor should NOT be treated as ideal gases

• This is because they usually exist near saturation conditions

• (Caveman explanation):

  • Ideal gas = “perfect gas” (not real, just idea)

  • It follows simple rule: push, volume, temp all nicely related

  • Real gas sometimes messy, but…

  • Low pressure + high temperature → gas spread out → act like ideal gas

  • Many gases (air, oxygen, nitrogen) → behave like ideal gas → easy use

  • BUT… thick/dense gas (steam, refrigerant) → too crowded

  • When crowded → no longer “perfect” → cannot use ideal gas rule

Viscosity

• A property that represents the internal resistance of a fluid to motion or the “fluidity”.

• Sifat yang mewakili rintangan dalaman sesuatu bendalir terhadap pergerakan atau “kelikatan”.

Drag force

• The force a flowing fluid exerts on a body in the flow direction. The magnitude of this force depends, in part, on viscosity.

• Daya yang dikenakan oleh bendalir yang mengalir ke atas suatu objek dalam arah aliran. Magnitud daya ini bergantung, sebahagiannya, pada kelikatan.

A fluid moving relative to a body exerts a drag force on the body, partly because of friction caused by viscosity.