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GASES, PLASMAS, AND STARS Gases ● When a liquid substance is heated... – – ● Gases have pressure and temperature – ● Molecules gain KE → break through the liquid surface Molecules “drift” until they collide with another molecule Just like liquids! Example: “Atmospheric” Pressure – Very large due to weight of air molecules above – 1 atmosphere = about 105 N / m2 ! – Average human body has 200,000 N of force pushing in But air is also in the lungs → force pushing outward – Pressure Changes in Gases ● Molecule collisions → gas pressure – – ● Compress a gas → it “pushes” outward – – – ● If gas expands → fewer collisions → decreased pressure If gas is heated → more collisions → increased pressure Expand a gas → it “pulls” inward These can be combined to form a pump Pulls air from one place and pushes it to another Unequal pressures can push objects – – Example: drinking straw Lower pressure in lungs → atmospheric pressure pushes liquid through straw into mouth Pressure Example: Siphon ● Device → uses pressure differences to move liquid – Example: transfer liquid from one cup to another With air in hose When hose is filled (using a pump) Atmospheric pressure on entire surface of liquid – nothing happens Left end → below water surface – Pressure > P atmosphere Right end → open to air – Pressure = P atmosphere P submerged end > P open end Uneven pressure pushes liquid! Once siphon is started → no pump Measuring Pressure: Barometer ● One way to measure gas pressure: – ● Simple Barometer – – – ● Measure how much weight it can lift Fill tube with liquid... Then flip it into a dish No air bubbles allowed! Why doesn't all the liquid fall into the dish? – – – Empty space inside the tube is a vacuum → no pressure Atmospheric pressure pushes on liquid in dish Measure height of liquid in tube → can find Patm Air Pressure and Altitude ● Air pressure exists due to weight of air above – ● Effect is noticeable on mountains – – ● Altitude sickness / shortness of breath Water boils at lower temperature than usual Can measure air pressure to estimate altitude – ● Higher elevation → less air above → lower air pressure Device is called an “altimeter” Why are airplane cabins made airtight? – Low pressure outside → air from inside wants to escape Buoyant Force in Air ● Pressure gets weaker with height → buoyant force – ● Similar to liquids, pressure difference causes an upward force To “float” on air: – An object must be less dense than air! ( very light ) Common ways to do this: 1) Heat air inside a balloon – It expands and becomes less dense 2) Use a light gas like helium or hydrogen – Why is hydrogen a bad idea? Energy in Fluids (Liquids and Gases) ● 3 common forms of energy in fluids: – – – ● Energy in a fluid can change forms – – ● Kinetic Energy → motion of individual molecules Potential Energy → height of molecules “Pressure Energy” → unequal pressures can exert forces Exhaling: Higher pressure in lungs → KE of air Hot air balloon: Higher pressure on ground → PE Energy can also be converted into other forms – Wind turbine: KE of air → electrical energy Bernoulli's Principle ● Applies conservation of energy to fluids – ● Important result: – – ● Important for understanding flowing gases and liquids When the speed of a fluid increases... The pressure of that fluid must decrease! F Low P Air Application: We can make a fluid exert a force – – By creating unequal fluid speeds on 2 sides of an object Object is pushed toward high speed region Hi P Forces Exerted by Fluids Using the fluid's KE Put an object in the fluid's path → “pushing” force Using the fluid pressure Can exert force by setting up unequal speeds Force is perpendicular to fluid motion Aerodynamics ● Study of moving fluids – Determine how to shape an object to get a desired force Plasma ● When a gas is heated to very high temperature: – – ● Atomic collisions become more violent Electrons are blasted away from their nuclei The result is a “soup” of free e– and nuclei – – Called a plasma Particles have electric charge → very strange behavior Applications of Plasma ● Source of light – ● Source of energy – – – ● When positively charged nuclei collide with negatively charged electrons, light is emitted If a plasma can be made hot enough: Nuclear fusion begins to occur → Energy is released Very difficult to control (about 10 million degrees) Spacecraft propulsion – – Plasma rockets are relatively weak... But can operate for a long time with little fuel Stars ● Stars are giant bodies of hot plasma – ● Energy comes pouring out of stars – ● In the form of light, radiation, and fast-moving particles Huge outward pressure due to escaping energy – ● Nuclear fusion occurs at an incredible rate inside stars Just enough to balance gravity's inward pressure When stars run out of nuclear “fuel” – – Gravity dominates → star implodes in a supernova Becomes a white dwarf, neutron star, or black hole