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LIQUIDS Molecules in a Liquid ● Chemical bonds can break when heated – ● Leaving individual molecules free to “roam” Liquids: no “large-scale” structure – – Molecules → roughly confined by a “crowd” of other molecules Molecules bounce around in a random pattern Pressure ● Liquid → molecules bounce off each other – – ● Therefore exert forces on each other! And exert forces on the container of the liquid To measure pressure: – – 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 ● Deep under the surface of a liquid: – – 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 ● Solid object in contact with a liquid – – ● Submerged object feels compressive stress – – – ● 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 – – Must ascend and descend slowly Allows internal and external pressures to equalize Buoyancy ● Pressure increases with depth – – – ● If weight > buoyant force – ● So does force on an object! Upward force > Downward force Object feels upward “buoyant” force! Object sinks! If buoyant force > weight – – Object floats! Occurs when object is less dense than liquid F buoyant W Archimedes' Principle ● Submerging object pushes liquid out of its way – ● Buoyant force = weight of displaced liquid Ancient people used this to measure density – – Object less dense than liquid → float to top Object more dense than liquid → sink to bottom Man-Made Floats and Boats ● Ancient times → Need for travel on water – ● To float people: – – ● 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 – Adds buoyant force without adding weight – The average density determines floating or sinking Boat Example Simple boat → steel box with air inside Average Density = ● Steel is heaver than water – ● mass of steelmass of air volume But the average density of the steel/air combo is light Put a person on top – – More air needs to be submerged to balance extra weight Boat still floats, but now a little lower Pascal's Principle ● For a body of liquid: – – 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 ● This is an excellent way to “transmit” force – Example: Car brakes (foot force → force on brake pads) Hydraulics ● Hydraulics use Pascal's Principle as a “force multiplier” – 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 ● Molecules in a liquid are free to bounce around – – ● Surface Tension – – – ● 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 – – Molecules don't just “spread out” in the air Near-spherical shape held together by surface tension Surface Tension Example ● Insects called water striders can use surface tension as a “floor” – – – 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”) ● ● Molecules in a liquid often “adhere” to surfaces – They “stick” to their container – Water level at edge rises up... – 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 ● Difference between liquids and solids: – ● Difference between liquids and gases: – ● 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 – Due to uneven pressure in the liquid