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# Pressure - PowerPoint PPT Presentation

Pressure. Pressure changes provide the push that drive ocean currents Key is the hydrostatic pressure Hydrostatic pressure is simply the weight of water acting on a unit area at depth Total pressure at depth will be sum of the hydrostatic & atmospheric, or p t = p h + p a.

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## PowerPoint Slideshow about 'Pressure' - garin

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Presentation Transcript

• Pressure changes provide the push that drive ocean currents

• Key is the hydrostatic pressure

• Hydrostatic pressure is simply the weight of water acting on a unit area at depth

• Total pressure at depth will be sum of the hydrostatic & atmospheric, or pt = ph + pa

• Hydrostatic pressure is simply the weight of water acting on a unit area at depth

• Mass seawater in column = r A D = [kg]

• A = cross-sectional area of column [m2]

• D = depth of water column [m]

• Weight column = (r A D) * g

• Mass * acceleration gravity (g = 9.8 m s-2)

• Hydrostatic pressure is the weight per unit area

• ph = r g A D / A

ph= r g D

Holds for r = constant

Often ph= - r g z (z+ up)

D

ph = r g D

• Let, D = 100 m & r = 1025 kg m-3

• Hydrostatic Pressure, ph= r g D

= (1025 kg m-3) (9.8 m s-2) (100 m)

= 1,004,500 kg m-1 s-2 [=N/m2]

• Pressure is a stress (like tw) but normal to the surface not along it

• ph = 1,004,500 N m-2

• 1 N m-2 = 1 Pascal pressure

• 105 Pa = 1 bar = 10 db

• ph = 1,004,500 Pa (10 db/105 Pa)

= 100.45 db

• First, 100 m depth gave a ph = 100.45 db

• Rule of thumb:

1 db pressure ~ 1 m depth

• Total pressure = hydrostatic + atmospheric

pt = ph + pa

• pa varies from 950 to 1050 mb (9.5-10.5 db)

• pa = ph(@~10 m)

• Mass atmosphere = mass top 10 m ocean

• Density is a f(depth)

• Taking a layer approach

dp = r(z) g dz

dz = layer thickness [m]

• Summing over D

ph= S r(z) g dz (where S over depth, D)

D

r1 = 1025 kg m-3

r2 = increases from 1025 to 1026 kg m-3

What is ph(100m)??

• Sum over the top 2 layers

ph(100 m) = ph(layer 1) + ph(layer 2)

• Layer 1:

ph(1) = (1025 kg m-3) (9.8 m s-2) (50 m)

= 502,250 N m-2 (or Pa)

105 Pa = 10 db

ph(1) = 50.22 db

• Layer 2:

Trick: Use average density!!

ph(2) = (1025.5 kg m-3) (9.8 m s-2) (50 m)

= 502,500 Pa = 50.25 db

• Sum over top 2 layers

ph(100 m) = ph(1) + ph(2)

= 50.22 + 50.25 = 100.47 db

• Hydrostatic relationship: ph = r g D

• Links water properties (r) to pressure

• Given r(z), we can calculate ph

• Proved that 1 db ~ 1 m depth

• Showed the atmospheric pressure is small part of the total seen at depth