Gravity causes fluid molecules to press on molecules below {pressure, fluid} {fluid pressure}|. Deeper molecules have more pressure, because more molecules are above them. Pressure P, force F per area A, at point below fluid surface is density d times depth h times gravity acceleration g: P = F / A = (m * g) * (h / V) = (m / V) * g * h = d * g * h. Pressure is directly proportional to gravity acceleration, because acceleration times mass is force. Pressure is directly proportional to density, because density relates to molecule mass. Pressure is directly proportional to depth, because depth relates to molecule number. Pressure does not depend on total surface area, because pressure is force per unit area.
fluid level
Liquids rise to equal heights at all openings to atmosphere, because pressures and potential energies are equal at liquid surfaces.
cause
Random molecule motions cause fluid pressure. Random motion exerts force and pressure equally in all directions, even upward or at angle. Container wall slope has no effect. Pressure is the same at all points at same depth. The net effect of random motions is that pressure is perpendicular to fluid surface, because random motions are symmetric around perpendicular.
temperature
Temperature increase increases fluid pressure, because molecules move faster.
density
More and/or heavier molecules have higher density and exert more pressure.
gas
Gas has random translational kinetic energy per unit volume, making force per unit area on container walls. Random translational kinetic energy depends on mass and molecule average velocity. If volume decreases, pressure goes up. If temperature decreases, pressure goes down. If pressure decreases, volume goes up. If pressure increases, temperature goes up.
Because pressure transmits equally throughout fluids, pressure sum, and energy sum, is constant {Bernoulli's theorem} {Bernoulli theorem} {Bernoulli's principle}. At pipe points, energy conserves. Pressure stays constant throughout fluid.
If fluid goes through narrower pipe area, fluid speeds up, because mass cannot build up. Higher speed makes more kinetic energy and directed pressure. Sideways pressure decreases, to conserve energy. If fluid slows, kinetic energy goes down, and potential energy and sideways pressure increase.
examples
Fountains, tanks with holes, aspirators, sprayers, Venturi meters, wings, pipes, rubber tubes, and curve balls demonstrate Bernoulli's principle.
In confined fluids, force in one direction can transmit pressure to all directions {hydraulics}|. Increased pressure changes into increased random molecule motions. For example, pushing a piston in a long thin cylinder can do work on fluid and increase molecule random kinetic energy, which can do work on a piston in a short wide cylinder.
area
At depth, force per area pressure is the same throughout confined fluid. Small force acting over long distance on small area can make big force acting over short distance on large area, because energy in and energy out are equal. Large area can apply large total force.
examples
Dams, breathing using lung double cavity, manometer, Bourdon gauge, barometer, hydraulic brakes, hydraulic lifts, syringe, and fluid tank show hydraulic effects.
Simple-fluid hydrodynamics {hydrodynamics}| has no viscosity or heat exchange and follows Euler equation: mass m per volume V times acceleration g equals negative of pressure P gradient perpendicular to velocity vector: (m / V) * g = - dP / ds.
Equal areas have same pressure in confined fluid {Pascal's principle} {Pascal principle}.
Fluid discharge velocity from small hole at depth below open surface is square root of two times gravity acceleration g times depth h {Torricelli's theorem} {Torricelli theorem}: (2*g*h)^0.5.
Removing gas molecules {vacuum, gas}| reduces pressure. Vacuum pumps remove gas molecules.
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Date Modified: 2022.0225