pressure and pressure differences

• A fluid

  can be either a liquid or a gas.
• The pressure in fluids

  causes a force normal (at right angles) to any surface
• The pressure at the surface of a fluid can be calculated using the

  equation:

  pressure = force / area of that surface

  p = F/A

  pressure, p, in pascals, Pa

  force, F, in newtons, N

  area, A, in metres squared, m2
• The pressure due to a column of liquid can be calculated using the

  equation:

  pressure = height of the column × density of the liquid

  × gravitational field strength

  [ p = h ρ g ]

  pressure, p, in pascals, Pa

  height of the 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 in a liquid

  The pressure in a liquid is different at different depths.

  • Pressure increases as the depth increases. The pressure in a liquid is due to the weight of the column of water above.
  • Since the particles in a liquid are tightly packed, this pressure acts in all directions
• upthrust


  A partially (or totally) submerged object experiences a greater

  pressure on the bottom surface than on the top surface. This

  creates a resultant force upwards.
• What factors affect floating and sinking?


  • The sinking or floating of an object depends on the mass and volume of the object.
  • An object will float if its density is less than density of water.
  • If the density of an object exceeds that of water, it will sink.
• The atmosphere

  a thin layer (relative to the size of the Earth) of air round the Earth. The atmosphere gets less dense with increasing altitude
• atmospheric pressure

  • Air molecules colliding with a surface create atmospheric pressure.
  • The number of air molecules (and so the weight of air) above a

  surface decreases as the height of the surface above ground level

  increases.

  • as height increases atmospheric pressure decreases