The atomic hypothesis

zAll things are made of atoms zAtoms are little particles that move around in perpetual motion zAtoms attract each other when they are more than about m = 0.1 nm = 1 Å apart zAtoms repel on being squeezed together

Question Why does air spread out to fill the room? a) The molecules repel each other so the air expands to fill up the available space b) The molecules move randomly, and end up all over the place c) The energy of the system is lowered when the air fills the entire room

A glass of water zMicroscope: jiggling (Brownian motion) zBillion times magnification: atoms move together in groups (molecules) zMolecules attract each other

Question zAt what distance do you expect to find the water molecules? a) At the minimum in the potential energy b) In a range around the minimum in P.E. c) Anywhere as long as the P.E. is negative d) Anywhere as long as the P.E. is positive

Motion and temperature zJiggling molecules have K.E. zTemperature is a measure of K.E. zHeat measures energy transfer zAdd energy (“heat up”)  motion   attraction can’t bind molecules  steam

Question Consider a block of copper in a room at 20 °C a) The air molecules have more K.E. b) The copper atoms have more K.E. c) They have the same K.E.

Boiling zEvaporation mainly near surface zSometimes many energetic molecules come together and form a bubble zIf ambient pressure higher than pressure inside bubble (vapour pressure) it gets squashed zWhen pressure sufficiently high: bubble to surface, boiling

Question zAt altitude the air pressure is lower than at sea level. Cooking rice at altitude takes longer because a) There is less oxygen in the air b) The temperature is lower c) The boiling point of water is lower d) The rice molecules get bigger

Steam in a vessel zMolecules bounce off wall, give push zMany molecules (N  /l)  many pushes/s zWe observe average push (force) zPressure is force/area

Question zKeeping volume and temperature constant, the number of molecules is doubled. a) The pressure remains the same b) The pressure is roughly halved c) The pressure is roughly doubled d) We can’t say what happens to the pressure

Question zKeeping volume and number of molecules constant, the temperature is raised. a) The pressure remains the same b) The pressure increases c) The pressure decreases d) We can’t say what happens to the pressure

Question zKeeping the number of molecules constant, the container is made smaller. a) T and p will remain the same b) T will go up, p will remain the same c) T will remain the same, p will go up d) T and p will both go up

Gauge pressure (an aside) zGauge pressure (in car tyres) = 2 bar… zmeans the absolute pressure = = 3 bar (atmospheric pressure  1 bar) z“Pressure” can mean either zThermodynamics: (usually) pressure = absolute pressure

Solidification zCool water down: ymolecules fall back into potential well yattraction stronger ysolidification zSolid: ymolecules in definite places ythey still jiggle and vibrate: T  0 yIce: density smaller than water (exceptional)

Evaporation and dissolved air zEvaporation (e.g. pool of water): ymolecule near surface gets knock and escapes ynote: energetic molecule gone  T  zDissolving air in water: yair molecules near surface dive in yreduce pressure: bubbles xdanger for divers when resurfacing quickly xdanger for you when opening a Coke bottle

Questions yBefore you get an injection your arm is cleaned with isopropyl alcohol. Why does this make your arm feel cold? yAn ice cube placed on a kitchen table will stay frozen longer when wrapped in a wet paper towel. Why? yWater in a tray placed in a freezer will not freeze all at once. Why not?

A pot of water with a lid zEnergetic molecules near escape: ywater vapour near liquid surface yT  zMolecules in steam dive into water yattracted  T  ! zAfter a while: equal numbers leave and enter water. T constant

More... zDissolving/crystallising zChemical reactions zSmell: ymolecules wander into your nose

PS225 – Thermal Physics topics zThe atomic hypothesisThe atomic hypothesis zHeat and heat transferHeat and heat transfer zKinetic theoryKinetic theory zThe Boltzmann factorThe Boltzmann factor zThe First Law of ThermodynamicsThe First Law of Thermodynamics zSpecific HeatSpecific Heat zEntropyEntropy zHeat enginesHeat engines zPhase transitionsPhase transitions