The Physics Classroom Tutorial

Understanding Pressure: Concepts and Calculations

• Definition of Pressure: Pressure is defined as the total force exerted over a specific area, expressed by the formula: Pressure (P) = Force (F) / Area (A).
• Molecular Basis: Pressure in gases is caused by the constant, random motion of molecules colliding with the walls of their container.
  • Increasing the number of molecules increases the frequency of collisions, thereby increasing pressure.
  • Increasing the temperature increases the average kinetic energy (speed) of the molecules, which also increases the force of collisions and the resulting pressure.
• Key Relationships:
  • Force: For a constant area, increasing the force increases pressure.
  • Area: For a constant force, increasing the surface area decreases pressure (e.g., snowshoes prevent sinking by distributing weight over a larger area).
• Units of Measurement:
  • Pascal (Pa): The standard SI unit, where 1 Pa = 1 N/m².
  • Atmosphere (atm): A standard reference point representing average air pressure at sea level.
  • Other units: Includes kiloPascals (kPa), millimeters of mercury (mm Hg), Torr, Bar, and pounds per square inch (psi).
• Conversion Standard:
  • 1.00 atm = 101,325 Pa = 760 mm Hg = 760 torr = 14.7 psi.
• Historical Context: The Pascal unit is named after Blaise Pascal, a French physicist and mathematician known for his contributions to fluid mechanics.