Key Terms
SI unit
Pascal (Pa) = 1 N/m^2
Standard atmospheric pressure at sea level
P_atm = 1.013 x 10^5 Pa = 1 atm = 101.3 kPa
To find force a fluid exerts on a surface
F = P_avg * A
Average pressure on a dam with water depth h
P_avg = rho * g * (h/2) (average depth is halfway down)
Absolute pressure
Total actual pressure P_abs = P_gauge + P_atm
Gauge pressure
Pressure relative to atmospheric pressure (what most instruments read)
Example
Tire gauge reads 34 psi. Absolute pressure = 34 + 14.7 = 48.7 psi.
Manometer (open-tube)
U-shaped tube, one side open to atmosphere. Fluid height difference measures gauge pressure.
Barometer
Measures atmospheric pressure. Closed tube, vacuum at top.
Aneroid gauge
Uses flexible bellows connected to a mechanical indicator; no fluid required.
Buoyant force
The net upward force on any object in any fluid.
Cohesive forces
Attractive forces between molecules of the SAME type (water-water) Adhesive forces: attractive forces between molecules
Surface tension (gamma)
Force per unit length exerted by a liquid surface. gamma = F / L
Gauge pressure inside a spherical bubble
P_gauge = 4 * gamma / r
Contact angle (theta)
Angle between the liquid surface tangent and the solid surface.