Overview
This lecture covers the fundamentals of fluid pressure, including definitions, units, the difference between absolute and gauge pressure, the effect of depth, and practical calculation guidelines for fluids in static equilibrium.
Pressure Fundamentals
- Pressure is the normal force exerted by a fluid per unit area (force/area).
- Pressure is a scalar quantity and acts perpendicular to surfaces.
- SI unit of pressure is the pascal (Pa), which is one newton per square meter (N/m²).
- Other units include kilopascal (kPa), megapascal (MPa), pounds per square inch (psi), and kips per square inch (ksi; 1 kip = 1,000 pounds).
Atmospheric and Reference Pressures
- Atmospheric pressure at sea level is 101.325 kPa or 1 atmosphere (atm), equivalent to 14.696 psi.
- Absolute pressure is measured relative to a perfect vacuum (zero pressure).
- Gauge pressure measures pressure relative to local atmospheric pressure.
- Gauge pressure = Absolute pressure – Atmospheric pressure.
- Vacuum pressure refers to negative gauge pressure (pressure below atmospheric).
Pressure Variation with Depth
- In incompressible fluids, pressure increases linearly with depth.
- The change in pressure with depth is given by ΔP = ρgh, where:
- ρ = fluid density,
- g = acceleration due to gravity (9.81 m/s²),
- h = vertical depth or height.
- Pressure increases by adding ρgh as depth increases downward.
Pressure at Different Points in a Fluid
- Pressure at the same height in a connected fluid is equal, regardless of horizontal position.
- Moving downward in fluid: add ρgh to find the new pressure.
- Moving upward in fluid: subtract ρgh to find the new pressure.
Multiple Fluids and Interfaces
- At the interface between two different fluids, the pressures are equal.
- The pressure change across a gas layer is negligible compared to liquids, so it's often ignored in calculations.
Key Terms & Definitions
- Pressure — Force per unit area exerted by a fluid.
- Pascal (Pa) — SI unit of pressure, equivalent to N/m².
- Gauge Pressure — Pressure relative to atmospheric pressure.
- Absolute Pressure — Pressure relative to a perfect vacuum.
- Vacuum Pressure — Pressure below atmospheric (negative gauge pressure).
- ρgh — Formula for pressure change with depth (density × gravity × height).
Action Items / Next Steps
- Review and memorize the formula ΔP = ρgh for pressure change with depth.
- Study the conversion chart for pressure units provided on the formula sheet.
- Practice pressure calculation problems involving gauge, absolute, and depth-related scenarios.