The Fluid States

1. The Fluid States Section 13.1 Physics

2. Objectives • Describe how fluids create pressure and relate Pascal’s principle to some everyday occurrences. • Apply Archimedes’ and Bernoulli’s principles. • Explain how forces within liquids cause surface tension and capillary action, and relate the kinetic model to evaporation and condensation.

3. Properties of Fluids • Fluids: materials that flow and have no definite shape of their own. • For the most part, Newton’s Laws and the Laws of Conservation can be applied to fluids. • When we apply a force to a fluid, what do we call it?

4. Pressure • We call a force applied to a fluid: pressure. • Pressure is represented by the following equation: • P = F/A • Pressure equals force divided by area.

5. Pressure • The force is assumed to be perpendicular to the affected surface area. • Pressure is measured in Pascals, Pa. • The Pascal: 1N/m². • Which has a greater pressure? • The area under an elephant’s foot or the area under a woman’s high heel shoe? • The area under a high heel shoe.

6. Pressure from a Gas? • It is easy to see how solids and fluids exert pressure on a surface. • But how does a gas exert pressure on an area? • The tiny gas particles are constantly hitting surfaces and applying a pressure value.

7. Pressure from a Gas • Are we under pressure right now? • Yep. • In fact, on every square centimeter of Earth’s surface at sea level, the atmosphere exerts a force of approximately 10 N = 1 kg. • Blue Barometer Demo. http://www.cabelas.com/cabelas/en/content/Item/61/60/81/i616081sq02.jpg

8. Practice Problems • Pg 303 • 1-4

9. Fluids at Rest • Water also exerts pressure just as the atmosphere exerts pressure. • Think of a diver, the pressure felt when under water does not depend on whether the diver is upright, sideways, or up-side-down. • Pascal’s Principle accounts for this pressure.

10. Pascal’s Principle • Pascal’s Principle: any change in pressure applied at any point on a confined fluid is transmitted undiminished throughout the fluid. • How does toothpaste demonstrate this principle? http://www.gauweiler.net/photos/art/toothpaste.jpg

11. Pascal’s Principle • So why is this important? • There are more important reasons than that of toothpaste. • Hydraulic systems use this principle • Force Exerted by lift = F = (F1A2)/A1

12. Practice Problem • Pg 304 • 5

13. Swimming Under Pressure • When you dive under water, your body feels increasing pressure as you dive deeper. • The downward pressure of water is illustrated by the following equation. • P = ρhg • Rho; density of fluid, h; depth, g; gravity.

14. Swimming Under Pressure • While swimming, you may also notice the upward force from the water you are immersed in. • This force is the buoyant force. • Fbuoyant = ρVg • Rho; density, V; volume, g; gravity. • The net upward force is equal to the weight of the fluid displaced by the object.

15. Archimedes’ Principle • The buoyant force and weight of displaced fluid relationship was discovered by the Greek scientist Archimedes. • Archimedes’ Principle: states that an object immersed in a fluid has an upward force on it equal to the weight of the fluid displaced by the object; it does not depend on the weight of the object.

16. So Will It Sink Or Float? • When an object is placed in a fluid, it has the force of gravity pressing down and the buoyant force pressing up. • The difference between the two determines if the object will sink or float. • Fapparent = Fg – Fbuoyant • A negative value for Fapparent indicates the object will float.

17. Practice Problems • Pg 309 • 6-7 http://home.earthlink.net/~divegeek/archimedes_files/buoyancy.gif

18. Fluids in Motion • Ok, we have looked at static fluids, but fluids do not just stay still. • Bernoulli’s Principle: describes the relationship between the velocity and pressure exerted by a moving fluid. • Bernoulli’s Principle states that as the velocity of a fluid increases, the pressure exerted by that fluid decreases.

19. Fluids in Motion • Most airplanes get part of their lift by utilizing this principle. • The curvature of the top wing is greater than that of the bottom. • As the wing travels through the air, ;the air moving over the top surface travels faster than air moving past the bottom surface.

20. Fluids in Motion • The decreased air pressure created on the top surface results in a net upward pressure that produces an upward force on the wings, or lift, which helps hold the plane up. • Race cars use this same principle…How?

21. Forces Within Liquids • All of the fluids examined thus far have been ideal fluids. • In real liquids, particles exert electromagnetic forces of attraction on each other. • These forces affect the behavior of liquids.

22. Forces Within Liquids • These forces are cohesive forces. • Cohesive forces are the forces of attraction between particles of a fluid. • These forces account for water’s surface tension.

23. Forces Within Liquids • Another force that affects the way a fluid behaves is adhesion. • Adhesive forces are the attraction between particles of a fluid and the particles of another surface. • This is why capillary action occurs.