Summary of Forces, Pressure, and Diffusion

## Introduction Pressure and diffusion are two important ideas that explain how particles in fluids (gases and liquids) behave. In this material you will learn what causes pressure in gases and liquids, how atmospheric pressure changes with height, how hydraulic systems use liquid pressure, and why diffusion makes substances spread out. Clear definitions, examples, and comparisons are included to help you understand and remember the concepts. ## Key Definitions > **Pressure:** Force exerted per unit area when particles collide with a surface. > **Air (atmospheric) pressure:** Pressure caused by the weight and collisions of air particles in the atmosphere. > **Diffusion:** Movement of particles from a region of higher concentration to a region of lower concentration until they are evenly distributed. ## Pressure in Gases ### What causes pressure in a gas? - Gas particles are far apart and move randomly in all directions. - When particles collide with the walls of a container, they exert a force on the walls. The force per unit area is the **gas pressure**. ### Factors that change gas pressure 1. Number of particles: More particles in the same volume increase the collision frequency and so increase pressure. Example: Blowing more air into a balloon increases its pressure and inflates it. 2. Temperature: Increasing temperature gives particles more kinetic energy, so they move faster and collide more often and harder, increasing pressure. 3. Volume (qualitative): Reducing the volume (compressing the gas) makes particles hit the walls more often, raising pressure. Did you know that atmospheric pressure at sea level is about $1.01\times10^{5}\ \text{Pa}$? This is roughly the weight of a column of air above each square meter of Earth's surface. ### Why an inflated balloon is hard to compress - The gas particles inside the balloon are already colliding with the balloon wall and pushing outward. To compress it you must push particles closer together, which requires overcoming the pressure created by their collisions. ## Atmospheric (Air) Pressure - The atmosphere is a mixture of gases such as nitrogen, oxygen, and carbon dioxide. - Air particles move and collide with surfaces, producing air pressure. - Atmospheric pressure decreases with altitude because the number of air particles above a given area is smaller at higher altitudes. Example: A barometer measures atmospheric pressure. At higher mountains the barometer shows a lower reading than at sea level. ## Pressure in Liquids ### How liquids exert pressure - Liquid particles move randomly and collide with each other and the container walls; these collisions produce pressure in the liquid. - Unlike gases, liquids are nearly incompressible, so pressure is transmitted effectively through the liquid. ### Pascal's principle and hydraulic devices - **Pascal's principle:** A change in pressure applied to an enclosed fluid is transmitted undiminished to every part of the fluid. Practical application: Hydraulic lifts and brakes. - When a small force is applied to a small-area piston, the pressure transmits through the liquid to a larger-area piston, producing a larger output force that can lift heavy objects (for example, a car). This makes it easier for a person to lift a heavy load. Example steps in a hydraulic lift: 1. Push a small piston with force $F_{1}$ and area $A_{1}$ producing pressure $P = \frac{F_{1}}{A_{1}}$. 2. The same pressure acts on the larger piston with area $A_{2}$ producing force $F_{2} = P\,A_{2}$. 3. If $A_{2} > A_{1}$ then $F_{2} > F_{1}$, making lifting easier. ## Diffusion in Gases and Liquids ### What is diffusion? - Diffusion is the process by which particles spread from regions of higher concentration to regions of lower concentration until they are evenly distributed. > **Diffusion:** Particles moving from a higher concentration region to a lower concentration region due to random motion. ### How diffusion happens - Particles move randomly and c