Physics Unit 6: Pressure

Unit 6: Pressure

• Contents: States of Matter, Basic concepts of Pressure, Example, Pressure and Fluids, Absolute pressure, and Questions.

States of Matter

• Matter is typically classified into three states: solid, liquid, and gas.
• Solids have a definite volume and shape.
• Liquids have a definite volume but no definite shape.
• Gases have neither a definite volume nor a definite shape.

Pressure and States of Matter

• Solids exert pressure only downwards due to their weight.
• Liquids and gases exert pressure in all directions equally on surfaces.

Basic Concepts of Pressure

• Pressure is the magnitude of force applied perpendicularly to a unit area.
• Pressure = Force/Area (P = F/A)
• Pressure is a scalar quantity.
• The SI unit of pressure is Pascal (Pa).
• 1 Pascal = 1 N/m².
• Other non-SI units for pressure include atm, bar, mmHg (Torr).

Pressure and Area

• Increasing the area over which a force is applied decreases pressure.
• Decreasing the area over which a force is applied increases pressure.

Pressure and Area: Example

• A water bed is 2 meters long, 2 meters wide, and 0.3 meters deep.
• The weight of the water in the bed is approximately 11,800 Newtons.
• When the water bed is on the floor, the pressure exerted is approximately 2,940 Pascals.

Pressure and Fluid

• A fluid is a collection of randomly arranged molecules held together by weak cohesive forces.
• Both liquids and gases are fluids.
• In a non-moving fluid, the pressure exerted on an immersed object is perpendicular to the object's surfaces.
• Pressure is the same at all points at the same depth in a stationary fluid.

Fluid Pressure

• Hydrostatic pressure is the pressure of a static fluid at a point.
• Hydrostatic pressure is due to the force of gravity acting on the fluid.
• Hydrostatic pressure increases proportionally with depth.
• The pressure at the same elevation throughout a container is uniform regardless of shape.

Fluid Pressure Equation

• Static fluid pressure is calculated as: P = pgh, where:
  • P is pressure
  • p is density of fluid
  • g is acceleration due to gravity (≈ 9.8 m/s²)
  • h is the depth of the fluid

Absolute Pressure

• Absolute pressure is the pressure relative to a vacuum (zero pressure).
• Absolute pressure for a fluid open to the atmosphere is the sum of atmospheric pressure and the pressure due to the fluid. (P = P_o + pgh)

Fluid Pressure: Example

• The force exerted on the eardrum when swimming at a depth of 5 meters in a swimming pool is approximately 2.6 Newtons

Questions

• Q1: Pressure exerted by water on a diver at a depth of 10 meters in a lake (assuming water density = 1000 kg/m³; g = 9.8 m/s²)
• Q2: Pressure difference between two points in a fluid at depths of 5m and 15m (assuming the fluid density is 1200 kg/m³)
• Q3: Depth of water in a tank if the pressure at the bottom is 29,400 Pascals (assuming water density = 1000 kg/m³; g = 9.8 m/s²)