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Lecture 5

# PHYS 1041 Lecture Notes - Lecture 5: Barometer, Net Force, Pressure Measurement

2 pages73 viewsWinter 2014

Department
Physics
Course Code
PHYS 1041
Professor
Dr.David Fleming
Lecture
5

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Physics 1041
March 3rd
Pressure
fluid: matter that flows under influence of external force
comprised of both liquids and gases
fluids are incompressible; density is constant
gases are compressible; density is variable
density:
)(
)(
3
mvolume
kgmass
=
ρ
density of water:
3
/.1000
2
mkg
OH
=
ρ
density of mercury:
3
/.13600 mkg
Hg
=
ρ
pressure: measure normal force per unit area exerted by a fluid
pressure:
)(
)(
2
marea
Nforce
P
=
the unit of N/m2 is a Pascal (Pa)
Example: Pressure changes with depth in a fluid
pressure at water surface is the same as air pressure
pressure increases as depth increases
for a fluid to remain at rest, net force everywhere must be equal to zero
more and more fluid is above you pushing you down the deeper you are; pressure
must increase to counteract that effect
this is called hydrostatic equilibrium
hydrostatic equilibrium:
ghPP
O
ρ
+=
P=pressure
PO=pressure of liquid at surface
ρ=density of liquid
g=acceleration due to gravity
h=height (depth) of fluid
barometer: instrument used to measure pressure
air pressure acts on an open pool/liquid
there is no pressure inside a barometer; when pressure comes from outside, it
causes the liquid inside the barometer to rise
the difference in pressure between 2 ends of a tube results in a height difference
between 2 liquid surfaces
gauge pressure: the difference between pressure of your point of interest and an
atmospheric pressure
atmospheric pressure: on average, 101.3kPa
Pascal's Law: an increase in surface pressure results in a pressure increase
throughout the fluid