Molecules in a Liquid

Transcription

1 LIQUIDS

2 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

3 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 Pressure = Force Area Bigger detector more area more force Pressure is the same no matter the size of the detector

4 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

5 Pressure on Submerged Object Solid object in contact with a liquid Feels forces due to molecular collisions Force is always perpendicular to object surface Submerged object feels compressive stress 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

6 Buoyancy Pressure increases with depth F buoyant So does force on an object! Upward force > Downward force Object feels upward buoyant force! W If weight > buoyant force Object sinks! If buoyant force > weight Object floats! Occurs when object is less dense than liquid

7 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

8 Man-Made Floats and Boats Ancient times Need for travel on water For food, transportation, exploration To float people: 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

9 Boat Example Simple boat steel box with air inside Average Density = mass of steel mass of air volume Steel is heaver than water 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

10 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)

11 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)

12 Surface Tension Molecules in a liquid are free to bounce around Except through the surface of the liquid Glass of water Why don't molecules jump out? Surface Tension 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

13 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

14 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

15 SUMMARY Difference between liquids and solids: Individual molecules in liquid are free to roam around Difference between liquids and gases: Liquids are held to a particular volume by surface tension Liquids exert an upward buoyant force Due to uneven pressure in the liquid