Topic 3: Thermal Physics3.1 Thermal concepts This chapter is an introduction to thermal physics. It introduces the concepts of temperature, heat, internal energy and thermal equilibrium. • You should: • understand the concept of thermal equilibrium; • relate the Kelvin and Celsius scales of temperature; • know that internal energy is the total kinetic energy of the molecules of a system plus the potential energy associated with the molecular forces.
Temperature Intuitively: concept of “hotness” or “coldness” of a substancewithrespecttosomethingelse. Tomeasurethetemperature of a bodyweneedtofind a property of thebodythatchanges as the “hotness” changes. In 1742, Andreas Celsius createdthetemperaturescalethatisknownbyhisname. 0°C – Freezing point of water 100°C – Boiling point of water
ThermalEquilibrium Twoor more bodies are in thermalequilibriumwhentheyhavethesametemperature. Body A and body B are each in thermalequilibriumwithbody C. Thereforethey are in thermalequilibriumwitheachother and hencehavethesametemperature (ZerothLaw of Thermodynamics).
Absolute temperature scale Thetemperaturescaleused in Physicsistheabsolutetemperaturescaleor Kelvin scale. Itsunitisthekelvin (K) Temperature has a lowerlimit 0 K orabsolutezero 0 K = -273.15 °C or 0°C = 273.15 K For practical purposes: T = TC + 273 TC = T -273 T = TC + 273.15 TC = T -273.15
Heat As Energy Heatisenergythatistransferredfromonebody and intoanother as a result of a difference of temperature. Thus, when a hotobjectisbrought in contactwith a colderbody, heatwillbetransferredtothecolderbody and increaseitstemperature. Wesaythatthecolderbody has been “heated”. Heat and work, unliketemperature, pressure, and volume, are notintrinsicproperties of a system.
InternalEnergy Allsubstancesconsist of molecules in constantmotion. Theythereforehavekineticenergy. In addition, there are forcesbetweenmolecules (electrical in nature). Increasingtheaverageseparation of twomolecules requieres worktobe done. Thisworkgoesintopotentialenergyassociatedwith intermolecular forces. Internalenergyisthe total kineticenergy of themolecules of a sustance, plus anypotentialenergyassociatedwithforcesbetweenthemolecules. Theheatthatistransferredfrom a hotto a coldbodyincreasestheinternalenergy of thecoldbody (and decreasestheinternalenergy of thehotbodybythesameamount
Temperature, again Theabsolutetemperatureis a measure of theaveragekineticenergy of themolecules of a substance. Theaveragekineticenergy of themoleculesisdirectlyproportionaltotheabsolutetemperature in kelvin. Wethereforehave a relationshipbetween a microscopic concept and a macroscopic concept.
A hotbodyisbroughtintocontactwith a colderbodyuntiltheirtemperatures are thesame. Assumethat no otherbodies are around. Istheheatlostbyonebodyequaltotheheatgainedbytheother? Isthetemperaturedrop of onebodyequaltothetemperatureincreasebytheother? A body at a givenuniformtemperature of 300 K and internalenergy 8 x 106 J issplitintotwoequalhalves. Has anyheatbeenexchanged? Whatisthetemperature of eachhalf? Whatistheinternalenergy of eachhalf?
The giant hornet Vespa mandarinia japonica preys on Japanese bees. However, if one of the hornets attempts to invade a bee hive, several hundred of the bees quickly form a compact ball around the hornet to stop it. After 20 minutes the hornet is dead, although the bees do not sting, bite, crush, or suffocate it. Why, then, does the hornet die?
Heat transfer (conduction) Thermalconductionistheprocessbywhich a temperaturedifference causes the transfer o thermalenergyfromthehotterregion of thebodytothecolderregionbyparticlecollisionwithouttherebeingany net movement of thesubstanceitself. Conduction can occur in solids, liquids and gases. Gases: Duetothecollisionbetweenfast and slowmovingparticleswherekineticenergyistranferedfromthefasttotheslowparticle. Liquids: Duetoincreasedvibrationalenergy. Becausethemajority of theparticles are coupledtootherparticlestheyalsobegintovibrate more energetically. Solids: Twoways. Similarlytoliquidsorbymobileelectrons.
Heat transfer (convection) Thermalconvectionistheprocess in which a temperaturedifference causes themassmovement of fluid particlesfromareas of highthermalenergytoareas of lowthermalenergy (thecolderregion). Liquids and gases can transfer heatreadilybyconvection.
Heattranfer (radiation) Thermalradiationisenergyproducedby a sourcebecause of itstemperaturethattravels as electromagneticwaves. Itdoesnotneedthepresence of matterforitstranfer. Thermalradiationismainlyelectromagneticwaves in the infra-red region of theelectromagneticspectrum at temperaturesbelow 1000°C.
The mole, molar mass and Avogadro’snumber Themass of anatomisexceedinglysmall. Forexample, theisotope carbon-12 is 1.99 x10-23g. In 1961 the International Union of Pure and AppliedChemistrydefinedthemasses of atomsrelativeto carbon-12 thatwasassigned a value of 12.0. Therefore, therelativeatomicmassisdefined as themass of anatomwhencomparedwith 1/12 themass of carbon-12 atom. Mass of carbon-12 = 12.000 u Mass of oxygen = 16.000 u = 16/12 Mass of carbon-12
Mole The SI fundamental unitfortheamount of a substanceisthe mole (mol). The mole istheamount of substancethatcontains as manyparticles (atoms, molecules) as there are in 12 g of carbon-12 Avogadrofoundthatequalvolumes of gases at thesametemperature and pressurecontainedthesamenumber of particles. One mole of any gas containstheAvogadronumber of particles NA=6.02 x 1023, and itoccupies 22.4 dm3 at 0°C and 101.3kPa pressure (STP). Molar mass of carbon-12 = 12 g/mol Molar mass of oxygen = 16 g/ mol
Molar mass Where M isthe molar mass (g/mol) , m isthemass (g), and n istheamount of substance in moles. M = m/n Where N isthe total number of particles n = N/NA Important note: Whenusingthe mole, theatomsormoleculesshouldbeclearlystipulated. Forexample, one mole of copperatomscontains 6.02 x 1023cooperatoms. One mole of nitrogenmolecules (N2) contains 6.02 x 1023 of nitrogenmolecules and 12.04 x 1023nitrogenatoms.
Example Calculatethenumber of moles of oxygenmoleculescontained in 64 g of oxygen gas, O2. Calculatethenumber of oxygenmolecules in part 1 of thisexample. Determine thevolume of oxygen gas thatwouldbepresent at STP. Calculatethemass in 0.75 mol of carbondioxide gas.