The Physics
Hypertextbook
Opus in profectus

# Pressure

## Summary

• Pressure is the ratio of normal force to area.
• Although both force and area are vectors, pressure is a scalar quantity and has no direction.
• Pressure is a way to describe force in a region of a continuous system.
 P = F A
• Units
• The SI unit of pressure is the pascal [Pa = N/m2 = kg/ms2].
• One hundred thousand pascals are sometimes called a bar [100,000 Pa = 1 bar].
• The unit atmosphere is 101,325 Pa by definition.
• Absolute vs. Gauge
• The pressure of a region, as defined above, is its absolute pressure (P).
• The pressure difference between a system and its environment (P0) is known as gauge pressure (Pg).
 P = P0 + Pg
• Pressure in a fluid (P) at rest…
• is equal to the weight of a column of fluid divided by the area on which it rests, so that it…
• increases uniformly with depth (h)
• is directly proportional to the density of the fluid (ρ)
• depends on the surface pressure (P0)
 P = P0 + ρgh
• acts equally in all directions and therefore…
• exerts a net force perpendicular to any surface that it contacts
• Pascal's principle: Pressure changes applied to the surface of an enclosed fluid are transmitted evenly throughout the fluid.
• Hydraulics increase force, but  P1 = F1 = F2 = P2 A1 A2
• decrease distance (since energy is conserved)  W1 = F1s1 = F2s2 = W2