Download LIQUIDS

LIQUIDS
Molecules in a Liquid
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Chemical bonds can break when heated
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Leaving individual molecules free to “roam”
Liquids: no “large-scale” structure
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Molecules → roughly confined by a “crowd” of other molecules
Molecules bounce around in a random pattern
Pressure
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Liquid → molecules bounce off each other
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Therefore exert forces on each other!
And exert forces on the container of the liquid
To measure pressure:
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Insert a detector with a known surface area into liquid
Measure force exerted on detector
Force
Pressure =
Area
Bigger detector → more area → more force
Pressure is the same → no matter the size
of the detector
Pressure vs. Depth
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Deep under the surface of a liquid:
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Lots of liquid above, weighing down
Liquid pressure is greater than it is near the surface
Bottom stream has
same horizontal range
but less time to fall
Velocity of bottom
stream must be greater
So pressure at bottom
hole must be greater
Pressure on Submerged Object
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Solid object in contact with a liquid
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Submerged object feels compressive stress
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Feels forces due to molecular collisions
Force is always perpendicular to object surface
At large depths below the liquid surface:
Compressive forces become enormous
Very expensive and dangerous to explore the ocean floor
Deep sea divers
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Must ascend and descend slowly
Allows internal and external pressures to equalize
Buoyancy
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Pressure increases with depth
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If weight > buoyant force
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So does force on an object!
Upward force > Downward force
Object feels upward “buoyant” force!
Object sinks!
If buoyant force > weight
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Object floats!
Occurs when object is less dense than liquid
F
buoyant
W
Archimedes' Principle
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Submerging object pushes liquid out of its way
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Buoyant force = weight of displaced liquid
Ancient people used this to measure density
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Object less dense than liquid → float to top
Object more dense than liquid → sink to bottom
Man-Made Floats and Boats
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Ancient times → Need for travel on water
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To float people:
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For food, transportation, exploration
Buoyant force must balance weight of boat and people
Standing on floating material not good enough
Clever trick: enclose a volume of air
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Adds buoyant force without adding weight
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The average density determines floating or sinking
Boat Example
Simple boat → steel box with air inside
Average Density =
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Steel is heaver than water
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mass of steelmass of air
volume
But the average density of the steel/air combo is light
Put a person on top
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More air needs to be submerged to balance extra weight
Boat still floats, but now a little lower
Pascal's Principle
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For a body of liquid:
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Every point at the same height has the same pressure
Pushing on one edge increases the pressure everywhere
Pushing on the red piston will
increase the pressure everywhere
in the fluid by the same amount
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This is an excellent way to “transmit” force
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Example: Car brakes (foot force → force on brake pads)
Hydraulics
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Hydraulics use Pascal's Principle as a “force multiplier”
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Like a pulley or lever → greater force over shorter distance
Hydraulic systems are often
used to:
Lift heavy objects (like cars)
Apply large forces (e.g. brakes)
Surface Tension
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Molecules in a liquid are free to bounce around
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Surface Tension
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Except through the surface of the liquid
Glass of water → Why don't molecules “jump out”?
Edge of a liquid acts like a “loose balloon”
Holds liquid molecules to a confined volume
Due to weak molecular attraction at edges
Example: Raindrop
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Molecules don't just “spread out” in the air
Near-spherical shape held together by surface tension
Surface Tension Example
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Insects called water striders can use surface
tension as a “floor”
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Their legs are covered in fine hairs so they stay dry
Otherwise, surface tension couldn't support them
Note: insects are not floating, but standing on surface
Capillary Action (“Capillarity”)
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Molecules in a liquid often “adhere” to surfaces
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They “stick” to their container
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Water level at edge rises up...
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Until the extra weight of water balances the “stickiness”
The thinner the tube → the higher the liquid lifts
This is how tall trees are able to lift water to the
highest branches → through capillarity
SUMMARY
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Difference between liquids and solids:
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Difference between liquids and gases:
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Individual molecules in liquid are free to roam around
Liquids are held to a particular volume by surface tension
Liquids exert an upward buoyant force
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Due to uneven pressure in the liquid