Pressure in Fluid Systems

1. Pressure in Fluid Systems UNIT 3

2. Unit 3 Pressure Pages 43-60 • Fluid • Hydraulic System • Pneumatic System • Density • Specific gravity • Buoyant force • Hydrometer • Pressure • PSI • Atmospheric Pressure • Absolute pressure • Gage pressure • Manometer

3. Pressure in a Fluid System • Unit 3 Review • Page 53 • #1-15

4. Fluid • Gas or liquid that conforms to the shape of the container • “Anything that flows”

5. Hydraulic system • Fluid system that uses liquid as the fluid Pneumatic system • Fluid system that uses air or gas as the fluid

6. Why does a hot air balloon float?Why does motor oil rise to the top of water? Density • Amount of matter in a given amount of substance • = Mass/Volume

7. Density • SI measured in: • Kg/m3 or gm/cm3 • English measured in: • Lbm/ft3 or lb/ft3

8. Density • What is the density of gold if you have a 1.036cm3 piece that had a mass of 20grams? • D=m/v • D=20g/1.036cm3 • D=19.3g/cm3

9. Density • What is the density of gold if you have a 3.108cm3 piece that had a mass of 60grams? • D=m/v • D=60g/3.108cm3 • D=19.3g/cm3

11. Other Densities • Platinum • Diamond • Chromium • Tin (white) • Tin (gray) • 21.45 • 3.5-3.53 • 7.15 • 7.265 • 5.769

12. Density • What is the mass in grams of mercury with a volume of 1cm3? • D = m / v • 13.6 g/cm3 = x / 1cm3 • 13.6 g = x

13. Density • What is the mass in kilograms of balsa wood with a volume of 1m3? • 1m3 = __cm3 • 1m3 = 100cm x 100cm x 100cm • = 1,000,000 cm3 • D = m / v • .3g / cm3 = x / 1m3 • .3g / cm3 = x / 1,000,000cm3 • 300,000 g = x • 300 kg =x

14. Specific Gravity • Density of a substance divided by the density of water • Because specific gravity is density/density the units cancel out and is written as a whole number

15. Specific Gravity • Copper has a density of 8.9g/cm3 • What is its specific gravity? • Specific Gravity = density of substance • = density of water • S.G. = (8.9g/cm3)/ (1.0g/cm3) • S.G. = 8.9

17. Buoyant Force • The upward force on a substance from a fluid • Will lead sink or float in water? • Will it sink or float in mercury?

19. Hydrometer • Instrument that measures density or specific gravity of fluids • Can you drown in quick sand?

20. Pressure • Force per unit area exerted by a fluid

21. Force on Airplane Windows • An airplane window has a surface area of 136 square inches. • Air pressure inside the cabin is 12.3 lb/in2 Find: • The force pushing on the window

22. Pressure • What happens to the pressure as we move away from the earth? • http://www.sciencedaily.com/videos/2006/1201-home_runs_amp_holeinone.htm

23. Force on Airplane Windows • An airplane window has a surface area of 144 square inches. • Air pressure inside the cabin is 14.7 lb/in2 • Air pressure outside the window is 6.7 lb/in2 Find: • The force pushing in the window • The force pushing out the window • Net force on window

24. Inward force on window • F = P x A • F = (6.7 lb/in2)(144in2) • F= 964.8lb Outward force on window • F = P x A • F = (14.7lb/in2)(144in2) • F = 2116.8 lb

25. Net Force on window • Net Force = Force out – Force in • Net Force = 2116.8 lb – 964.8 lb • Net Force = 1152 lb • The window is being pushed outward with a net force of 1152 lb.

26. Net Force on window • If the plane rises to a higher altitude and the pressure outside the plane changes to 5.4 lb/in2 • How much stronger will the windows need to be in order to hold the pressure

27. Outward force on window • F = P x A • F = (14.7lb/in2)(144in2) • F = 2116.8 lb Inward force on window • F = P x A • F = (5.4 lb/in2)(144in2) • F= 777.6lb

28. Net Force on window • Net Force = Force out – Force in • Net Force = 2116.8 lb – 777.6 lb • Net Force = 1339.2 lb • The window was originally pushing outward with a net force of 1152 lb. • Therefore it needs to hold 187.2 more pounds of pressure (1339.2 – 1152)

29. Pressure • Pressure acts equally in all direction at any point in a fluid and therefore it is a scalar

30. Absolute vs. Gage Pressure • When we fill a tire to 30lb/in2 is that the absolute or the gage pressure? • Atmospheric pressure = 14.7 lb/in2

31. Absolute Pressure • Total pressure compared to a perfect vacuum Gage Pressure • Pressure measured above atmospheric pressure G.P = Total pressure – atmospheric pressure

32. Gage pressure is generally measured “with a gage” Total Pressure

33. Pressure • Tire gage reads 38lb/in2 • What is the atmospheric pressure? • What is the gage pressure? • What is the total pressure?

34. Pressure • Tire gage reads 38lb/in2 • What is the atmospheric pressure? • What is the gage pressure? • What is the total pressure? 14.7 lb/in2 38 lb/in2 38 lb/in2 + 14.7 lb/in2 = 52.7 lb/in2

35. How does pressure change with depth? • Where is the pressure greater the shallow end or the deep end? • Why?

36. Pressure increases with depth • There is more water sitting on top of the deep end • There is twice as much weight • Twice as much force • Twice as much pressure

37. Relationship between pressure and depth

38. Water Pressure Calculation • Given: • The height of the water in a storage tank is 100 ft above the valve. The weight density of water is 62.4 lb/ft3 • Find: • The pressure at the valve in lb/ft2

39. Water Pressure Calculation • P = pw x h • P = (62.4 lb/ft3)(100ft) • P = 6240 lb/ft2 • Given: 1 ft2 = 144 in2 • Now find: • Pressure in PSI

40. Water Pressure Calculation • P = pw x h • P = (62.4 lb/ft3)(100ft) • P = 6240 lb/ft2 • Given: 1 ft2 = 144 in2 • p = (6240 lb/ft2)(1ft2/144in2) P = 43.3 lb/in2 (psi)

41. Balanced pressure across the valve

42. Unbalanced pressure across the valve

43. Pressure on bottom does not depend on the size of the tank

44. Pressure acts like forces • Pressure is a prime mover Measuring Pressures • Manometer – instrument used to measure fluid pressure

45. Hydraulic lift • Liquids are incompressible • Air compressor increases the pressure to the fluid • Large pushing force is exerted on the lifting piston

46. Hydraulic jack? • Large cylinder to a small cylinder • Same pressure = more force in the smaller cylinder • Small to large = allowable force but small increments?

47. An enclosed fluid under pressure exerts that pressure throughout its volume and against any surface containing it. That's called 'Pascal's Principle', and allows a hydraulic lift to generate large amounts of FORCE from the application of a small FORCE. Assume a small piston (one square inch area) applies a weight of 1 lbs. to a confined hydraulic fluid. That provides a pressure of 1 lbs. per square inch throughout the fluid. If another larger piston with an area of 10 square inches is in contact with the fluid, that piston will feel a force of 1 lbs/square inch x 10 square inches = 10 lbs.

48. So we can apply 1 lbs. to the small piston and get 10 lbs. of force to lift a heavy object with the large piston. Is this 'getting something for nothing'? Unfortunately, no. Just as a lever provides more force near the fulcrum in exchange for more distance further away, the hydraulic lift merely converts work (force x distance) at the smaller piston for the SAME work at the larger one. In the example, when the smaller piston moves a distance of 10 inches it displaces 10 cubic inch of fluid. That 10 cubic inch displaced at the 10 square inch piston moves it only 1 inch, so a small force and larger distance has been exchanged for a large force through a smaller distance.