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.
- Find the volume:
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
- 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