Fiveable
Fiveable
Fiveable
Fiveable

2.2 Thermodynamic Properties of Fluids

3 min readLast Updated on July 19, 2024

Fluid properties like bulk modulus and thermal expansion are crucial in understanding how fluids behave under different conditions. These properties affect how fluids respond to pressure and temperature changes, impacting their use in various applications.

The ideal gas law and fluid statics principles help us predict fluid behavior in real-world situations. From calculating pressure changes in gases to determining hydrostatic pressure in liquids, these concepts are essential for designing and analyzing fluid systems.

Fluid Properties

Bulk modulus of elasticity

Top images from around the web for Bulk modulus of elasticity
Top images from around the web for Bulk modulus of elasticity
  • Measures a fluid's resistance to compression, defined as the ratio of the change in pressure to the fractional change in volume
  • Mathematically expressed as Ev=VdPdVE_v = -V \frac{dP}{dV}, where VV is volume and PP is pressure
  • Fluids with higher bulk modulus are less compressible (water has a higher bulk modulus than air)
  • Important property in hydraulic systems and high-pressure applications (hydraulic presses, pumps)

Thermal expansion and compressibility

  • Thermal expansion: tendency of a fluid to change its volume in response to temperature changes
    • Most fluids expand when heated and contract when cooled (water, oil)
    • Volume thermal expansion coefficient (β\beta) quantifies the fractional change in volume per unit change in temperature, expressed as β=1VdVdT\beta = \frac{1}{V} \frac{dV}{dT}, where TT is temperature
  • Compressibility: tendency of a fluid to change its volume in response to pressure changes
    • Isothermal compressibility (κ\kappa) quantifies the fractional change in volume per unit change in pressure at constant temperature, expressed as κ=1VdVdP\kappa = -\frac{1}{V} \frac{dV}{dP}
    • Adiabatic compressibility (α\alpha) quantifies the fractional change in volume per unit change in pressure during an adiabatic process (no heat transfer), expressed as α=1V(dVdP)s\alpha = -\frac{1}{V} \left(\frac{dV}{dP}\right)_s, where ss denotes constant entropy

Ideal Gas Law and Fluid Statics

Applications of ideal gas law

  • Relates pressure (PP), volume (VV), temperature (TT), and amount of gas (nn) using the equation PV=nRTPV = nRT, where RR is the universal gas constant
  • Assumes gas molecules have negligible volume and do not interact with each other, a good approximation for many gases at moderate temperatures and pressures (air, nitrogen)
  • Calculates changes in pressure, volume, or temperature when other variables are known
    • Doubling the volume of a gas at constant temperature decreases the pressure by half
    • Heating a gas at constant volume increases its pressure proportionally to the temperature change

Pressure-depth relationship in fluids

  • In a static fluid, pressure increases linearly with depth due to the weight of the fluid above
    • Hydrostatic pressure at a given depth is calculated using P=P0+ρghP = P_0 + \rho g h, where P0P_0 is the pressure at the surface, ρ\rho is the fluid density, gg is the acceleration due to gravity, and hh is the depth below the surface
  • Pressure difference between two points in a static fluid depends only on the vertical distance between them and the fluid density (Pascal's law)
  • Hydrostatic pressure acts equally in all directions at a given depth, a property used in hydraulic systems to transmit force and motion (hydraulic lifts, brakes)
© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.


© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.

© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.
Glossary
Glossary