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Density and Pressure

Density
  • Density = Mass/Volume
  • To measure the density of a liquid:
    • Place a measuring cylinder on a balance
    • Fill the measuring cylinder with the liquid
    • The change in mass is the mass of the liquid, and the volume is shown on the scale.
    • Then use the formula.
  • To get the density of a solid:
    • Find the volume:
      • To find the volume of a regular object, use its mathematical formula.
      • If the object is irregular, put the object into a measuring cylinder with water, and the rise of the water is the volume of the object.
    • Find the mass: Weigh the object on a scale
    • Then use the formula.
Flotation
  • The density of water is 1 g/cm3, if an object has a greater density than that, it will sink in water, and if the object’s density is less than that, then it will float in water.
  • For example, an orange with its peel has a density of 0.84 g/cm3, we can predict that it will float because it is less than 1 g/cm3. We can also say that an orange without its peel, which has a density of 1.16 g/cm3 will sink because it is greater than 1 g/cm3

    Photo Credit: CSIRO’s Tweet

PRESSURE

Pressure in Solids

  • Pressure (Pa) = Force (N)/Area (m2)
  • P = F/A
  • Unit: Pascals (Pa) = N/m2

Pressure in Liquids

  • Pressure (Pa) = Density (kg/m3) × Gravity (m/s2) × Height (m)
  • P = hρg
  • At a depth of 10 m in water, the pressure is always 100 000 Pa (1 bar) and is constant for each further 10 m.

Atmospheric Pressure

  • 1 atm = 101.325 × 103 Pa = 101kPa

Photo Credit: Manometer (ACSMASIndia.com), Barometer (Brainly.in)

Manometer

  • A manometer measures pressure difference.
  • The height difference shows the excess pressure in addition to the atmospheric pressure

Barometer

  • A barometer is a tube with vacuum at the top and mercury filling the rest.
  • The pressure of the air pushes down on the reservoir, forcing mercury up the tube.
  • Measure the height of the mercury.
  • 1 atm is approximately 760 mm of mercury.
Examples
  1. A shape with dimensions 5 cm × 4 cm × 2 cm. It has a mass of 100 g. What is the density of the material?
    • Density = Mass/Volume
    • Mass = 100 g
    • Volume = 5 × 4 × 2 = 40 cm³
    • ∴ Density = 100/40
    • ∴ Density = 2.5 g/cm³
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