More on Fluids and pressure

Fluids vs Solids Solid: Has a definite shape and volume Fluids: Includes liquids and gases Take on the shape of their container (no definite volume) Liquids: no definite shape but have a definite volume Gases: no definite shape or volume

Fluids vs Solids – Particle Model -Neatly arranged -Strong forces of attraction between particles (physical bonds) -No real movement of particles -Close together but not as structured as a solid -Weak forces of attraction between particles -Particles can slide over each other -Far apart -No forces of attraction between particles -Move a lot!

Compressible vs Incompressible Fluids Their volume can be changed as a response to pressure They can be squished Incompressible fluids Their volume does not change as a response to pressure They cannot be squished

Compressible vs Incompressible Add Pressure Compressible! Add Pressure Incompressible!

Pressure Pressure is the result of a force applied in a perpendicular fashion to a surface Pressure is therefore affected by: The amount of force applied Increased force = The surface area Increased surface area = Increased pressure Decreased pressure

𝑃= 𝐹 𝐴 Pressure P is pressure, usually measured in Pascals (Pa) 𝑃= 𝐹 𝐴 P is pressure, usually measured in Pascals (Pa) F is force measured in Newtons (N) A is the surface area measured in m2

Pressure In m2 𝑃= 𝐹 𝐴 A= 𝑐 2 10N A= (0.4𝑚) 2 A= 0.16𝑚 2 1) What is the pressure exerted by the following box? In m2 𝑃= 𝐹 𝐴 A= 𝑐 2 10N A= (0.4𝑚) 2 A= 0.16𝑚 2 𝑃= 10𝑁 0.16 𝑚 2 40cm 𝑃=62.5𝑃𝑎 40cm

Pressure In m2 𝑃= 𝐹 𝐴 A= 𝑐 2 A= (0.2𝑚) 2 10N A= 0.04𝑚 2 2) What is the pressure exerted by the following box? In m2 𝑃= 𝐹 𝐴 A= 𝑐 2 A= (0.2𝑚) 2 10N A= 0.04𝑚 2 𝑃= 10𝑁 0.04 𝑚 2 20cm 𝑃=250𝑃𝑎 20cm

Pressure Area ↓ Pressure ↑ 10N 10N 𝑃=62.5𝑃𝑎 𝑃=250𝑃𝑎 If we keep the force the same but change the surface area…. 10N 10N Area ↓ Pressure ↑ 20cm 40cm 20cm 40cm 𝑃=62.5𝑃𝑎 𝑃=250𝑃𝑎

Pressure and Fluids In incompressible fluids (aka liquids) pressure is a result of the force exerted by the mass of the particles The more liquid above an object, the more mass above the object, therefore the more force exerted and the greater the pressure Pressure is also dependent on density ↑ density = ↑ pressure

Pressure and Fluids In compressible fluids (aka gases) pressure is a result of the force exerted by the particles when they collide with an object or each other. The sum of all of these forces = pressure Pressure is dependent on the number of collisions. More collisions = higher pressure Collisions are dependent on: Number of particles Temperature Volume

Pressure and Fluids Collisions are dependent on: Number of particles ↑ particles = ↑ pressure Temperature ↑ temperature = ↑ pressure Volume ↑ volume = ↓ pressure

If the air is exerting pressure on us all the time, why aren’t we being crushed?

Air pressure If the air is exerting pressure on us all the time, why aren’t we being crushed? The air particles are hitting us from all sides equally and so their effects cancel each other out!

Principles of Fluid Pressure First Principle: Fluids move from areas of HIGH pressure to LOW pressure Think about when we breathe! This principle is also applied in the circulatory system

Principles of Fluid Pressure Second Principle: Pressure applied to the surface of a fluid inside a closed container gets uniformly distributed to every part of the fluid Ex: pressing on a tube of toothpaste! Or a syringe

Principles of Fluid Pressure Third Principle: The transfer of pressure in a fluid can increase the force involved The carjack!

Principles of Fluid Pressure As you press on the small piston you transfer pressure (principle 2) Let’s say 20 Pa This same pressure gets transferred to the other side but is now spread over a larger area

Principles of Fluid Pressure Small piston: 𝑃= 𝐹 𝐴 𝑃= 20𝑁 0.1 𝑚 2 P = 200 Pa Large piston: F=𝑃𝐴 F= (200 Pa)(1 𝑚 2 ) F = 200 N Force of 20N became 200N!

Homework Textbook: p. 85-87 #4, 6, 7, 8, 12, 14, 16 & 21