Pressure in Fluids

Pressure in Fluids

1. Pressure in a fluid

   • A fluid can be either a liquid or a gas
   • Pressure in fluids causes a force at right angles (normal) to any surface
   • Pressure at surface of liquid
     • pressure = force normal to surface ÷ area of that surface
     • pressure, P, in pascals, Pa
     • force, F, in newtons, N
     • area, A, in metres squared, m²
     • P = F ÷ A
   • Pressure due to a column of liquid
     • pressure = height of column x density of liquid x gravitational field strength
     • P = hρg
     • pressure, p, in pascals, Pa
     • height of column, h, in metres, m
     • density, ρ, in kilograms per metre cubed, kg/m³
     • gravitational field strength, g, in newtons per kilogram, N/kg
   • Pressure is a measure of force per unit area
   • As depth of liquid increases, mass of column of fluid directly above point increases
   • As mass increases, weight force acting on that point increases
     • Pressure increases
   • If liquid has greater density, pressure will also be greater at same point compared to a liquid of lower density
     • Mass is directly proportional to density
       • Larger density will mean larger mass, so a greater weight of liquid above that point
   • Upthrust
     • A partially (or totally) submerged object experiences a greater pressure on the bottom surface than on the top surface
       • Creates resultant force upwards
         • Upthrust
     • An object floats if upthrust is equal to weight of object
       • There is no resultant force acting downwards
       • Object is less dense than fluid it is in
     • An object sinks if upthrust is less than weight of object
       • There is resultant force acting downwards
       • Object is more dense than fluid it is in

2. Pascals Vases

   • Level of water in each of tubes is constant
   • Pressure at bottom of each tube is same
     • Same height of water above it

3. Atmospheric Pressure

   • Atmosphere is a thin layer (relative to size of Earth) of air around Earth
   • Atmosphere gets less dense with increasing altitude
   • Air molecules colliding with a surface create atmospheric pressure
   • Number of air molecules (and so weight of air) above a surface decreases as high of surface above ground level increases
     • As height increases there is always less air above a surface than there is at a lower height
       • Atmospheric pressure decreases with an increase in height