Density and pressure

• Gas pressure is when air particals colide with each other inside a certain space
• If gas particals are but in a smaller space they will colide more and if they are put in a larger space they will colide less.
• When put in a smaller space as well as coliding more the gas pasrticals pressure increases
• Density is mass divided by volume
• Density


  The amount of mass in a unit volume
• Density tells us how tightly matter is packed together
• Calculating density


  Density = Mass/Volume ρ = m/V
• Mass


  The amount of matter in an object, measured in kilograms (kg) or grams (g)
• Volume


  Measured in cubic metres (m3) or cubic centimetres (cm3)
• Density units


  Kilograms per cubic metre (kg/m3) or kilograms per cubic centimetre (kg/cm3)
• Measuring density of a regular solid

  1. Find mass by placing on balance

  2. Find volume by measuring length, width and height and multiplying
• Measuring density of an irregular solid

  1. Find mass by placing on balance

  2. Find volume by placing in measuring cylinder or eureka jar and measuring displaced water
• Measuring density of a liquid


  1. Find mass by weighing empty measuring cylinder, then with liquid

  2. Find volume by reading from measuring cylinder
• Pressure


  A measure of how much force is applied over a certain area
• Calculating pressure


  Pressure = Force/Area P = F/A
• Calculating area


  Area = Force/Pressure A = F/P
• Calculating force


  Force = Pressure x Area F = P x A
• Pressure units


  Pascals (Pa.), where 1 Pa. = 1 newton per square metre (N/m2)
• The pressure in a liquid at a particular point acts equal in all directions
• Pressure in a liquid


  • Increases with depth
  • Increases with density
  • Increases with gravity
• When an object is in a liquid, there are 2 forces acting on it: its weight (↓) and the upthrust of the liquid (↑). If the object floats on the liquid, the upthrust is equal to the weight of the object
• Compressibility


  Liquids are very difficult to compress, gases are more easily compressible
• The compressibility of liquids and gases is used in hydraulic breaks and lifts
• How hydraulic breaks and lifts work

  1. A small force is exerted on a small area piston which produces a big pressure in a liquid or a gas

  2. That big pressure is transmitted to a bigger area piston, producing a large force
• Gas molecules


  • In constant, random motion
  • When a molecule collides with a surface it exerts a force on the surface as it changes its direction
• Gas pressure


  Equal to the total force exerted by the molecules divided by the area
• Increasing temperature of a fixed volume of gas

  Increases the pressure of the gas
• Increasing volume of a gas


  Decreases the pressure of the gas
• The air around us exerts a pressure because the random motion of the air molecules causes them to collide with us and other surfaces