Pressure; Pascal’s Principle

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# Pressure; Pascal’s Principle - PowerPoint PPT Presentation

Pressure; Pascal’s Principle. PRESSURE OF A FLUID. pressure = height of mercury column . air pressure. Barometer. Gauge pressure – pressure above the atmospheric pressure. absolute (total) pressure = gauge pressure + atmospheric press. Pressure is produced by the

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## Pressure; Pascal’s Principle

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### Pressure; Pascal’s Principle

PRESSURE OF A FLUID

pressure = height of mercury

column

air pressure

Barometer

Gauge pressure – pressure

above the atmospheric pressure.

absolute (total) pressure =

gauge pressure + atmospheric press.

Pressure is produced by the

weight of the fluid above the

surface.

force

pressure = ------

area

weight of fluid

= ----------------

area

mg

= -----

area

density · volume · g

= -------------------------

area

density · (area · height) · g

= --------------------------------

area

pressure = density · height · g

P = ρhg gauge pressure

P = ρhg + P0 absolute pressure

SI units:

pressure: N/m2 = Pascals (Pa)

density: kg/m3

height (depth): m

Standard atmospheric pressure

760 mm Hg ≈ 30 in Hg

Calculate 1 atm in Pascals

P = ρgh

ρHg = 13.6 x 103 kg/m3

g = 9.8 N/kg h = 0.760 m

P = 1.013 x 105 Pa

Why does a barometer use mercury

and not water?

If p = 1 atmosphere = 1.013 x 105 Pa

ρ = 1.00 g/cm3 = 1.00 x 103 kg/m3

Find height.

10.3 m

Pressure does not depend on volume,

only on height (depth).

“Water seeks its own level”

Pascal’s Principle – The pressure

in an enclosed fluid is constant

throughout the fluid.

p1 = p2

F1 F2

--- = ---

A1 A2

1 N

?

A = 1 cm2

A = 50 cm2

p1 = p2

1 N F2

------- = ------

1 cm2 50 cm2

50 N

If piston on left moves 10 cm,

what distance does piston on right

move?

work1 = work2

F1 d1 = F2 d2

(1) (10) = (50) d2

0.2 cm

Area of brake cylinder > area of brake line

force of brake cylinder > force of brake pedal