28 Thermal Physics Concepts [Syllabus 6093]

Temperature1. X°C=(X+273)K2. Goodthermometer● Sensitivetosmall temperaturechanges● Abletomeasuretherequiredrangeof temperatures3. Thermometric properties vary uniformly and continuously withtemperature4. Typesof thermometersProperty Examples Volumeof fixedmassof liquid Mercury-in-glassthermometer Alcohol-in-glassthermometer Electrical Resistance ResistanceThermometer ElectromotiveForce Thermocouplethermometer5. Icepoint (lower fixedpoint), puremeltingwaterSteampoint (upper fixedpoint), pureboilingwaterWhen mercurylevel inthermometer stemremainssteady, makeamarkL0 andL100 respectivelyKineticModel of Matter(Shape, volume, densityandcompressibility)6. Statesof matterState Arrangement /Motion Property Solid Most closely packedinaregular pattern Highest Density Incompressible Strongest intermolecularforces = particlesvibrate about fixedpositions Fixedshapeandvolume Liquid Closely packed andrandomlyarranged Higher density Incompressible Strong intermolecularforces = particles moveabout freely within theliquid Fixed volume but takestheshapeof container

Gas Far apart from oneanother and randomlyarranged Lowest density Compressible Weakest intermolecularforces = particles moveabout randomly at highspeeds No fixed shape andvolume 7. All matter is made up of molecules which are always incontinuousrandommotion8. Brownian motion: random, haphazard and constant motion oftiny particles suspended in a gas or liquid due to unevenbombardment ondierent sidesbythemoleculesinaction9. Duetobrownianmotion, moleculeswill evenspreadout10.Gas pressure due to collision of gas molecules with the walls ofcontainer (forceisexertedwhenair moleculehitsthewall)11.PV/Tisaconstant● Pressure-Temperature(Volumeisconstant)By kinetic model of gases, a rise in the temperature of thegas causes an increase in the kinetic energy andhence thespeedof gasmolecules. Thegasmoleculeswill bombardthewalls more vigorously and more frequently. The averageforce acting on the wall. Thus, pressure increases since P=F/A.● Volume-Temperature(Pressureisconstant)Repeat P-T + However, in order for the pressure to remainconstant, thegaswill expandeventually.● Pressure-Volume(Temperatureisconstant)When the volume of container decreases, the number ofgas molecules per unit volumeincreases. Thegasmoleculeswill bombard the walls more frequently. The average forceacting on the wall increases. Thus, pressure increases sinceP=F/A. Transfer of Thermal Energy12.Net heat flow from hotter object to cooler object until thermalequilibriumisreached13.Molecular Vibration:When one end of an object is heated, the particles at this endgain kinetic energy and vibrate faster and further apart. Theycollide with their less energetic neighbouring particles. Some oftheir energy is transferred to their neighbouring particles, whichin turn gain kinetic energy. In this way, heat is passedalongthe

object byvibratingparticles.14.FreeElectronDiusion:In addition to that, free electron diusion takes place in metals.When heated, free electrons gain energy and move faster. Theywill travel in the spaces between particles freely before collidingwith other electrons and transferring some of their energy tothem.15.Heat conduction by free electron diusion is much faster thanthat byvibrationof molecules16.Conduction cannot take place in a vacuum where there are nomoleculestotransfer heat17.Convection:When the liquidat the bottomisheated, it expandsandbecomesless dense and rises. The colder and denser liquid sinks andreplaces it. Thisinturngetsheatedup. Aconvectioncurrent issetup.18.Convection cannot take place in a vacuum where there are noparticlestomovebyconvection19.Convection cannot take place in solids as particles are held infixedpositionsandcannot bemovedbyconvectioncurrents20.For ↑inrateof infraredradiation,● Dull and black surfaces as compared to shiny and whitesurfaces● Higher temperaturerelativetosurroundingtemperature● Greater surfacearea21.RadiationcanpassthroughvacuumThermal Propertiesof Matter22.E(J) =m(kg) xc(J/kgK) xΔ θ(changeintemperatureinK)E(J) =heat capacity(J/K) xΔ θE(J) =mxspecificlatent heat of fusion(J/kg)E(J) =mxspecificlatent heat of vaporisation(J/kg)23.At higher temperatures, more heat is lost tothe surroundingperunit time. By using a more powerful heater, more heat energy issupplied by the heater to the water per unit time, the water willboil withinashorter timeandprovideamoreaccurateresult.24.Riseintemperature:Heat energyabsorbedisconvertedto:● Internal kineticenergy=vibrate/movefaster● Internal potential energy=movefurther apartViceversafor heat energyreleased25.Constant temperature:Heat energyabsorbedconvertedto:

● Internal potential energy toovercome intermolecular forcetochangethesubstancestatefrom_to_Heat energy releasedtoformintermolecular force tochange thesubstancestatefrom_to_26.FactorsaectingmeltingandboilingpointMeltingPoint BoilingPoint ↑Pressure Lowered RaisedImpurities27.CoolingbyevaporationBy the kinetic model of matter, particles of liquidareinconstant,random motion. They move at dierent speeds. Evaporationoccurs when the faster movingparticles escape fromthe surfaceof the liquid, leavingbehindslower movingparticles. Theaveragespeed, and therefore the average kinetic energy decreases. Asthe average kinetic energy of particles is proportional to thetemperatureof theliquid, thetemperatureof liquiddecreases.28.Factorsaectingevaporation● The higher the temperature, the greater the number ofmolecules energetic enoughtoescapefromthesurface, thefaster therateof evaporation● The greater the exposed surface area, the greater thenumber of molecules that can escape fromthe surface,thefaster therateof evaporation● The faster the motion of air, the more water vapour formedby evaporation it carries away, the faster the rate ofevaporation● Thelower thehumidity, thefaster therateof evaporation● The lower the external pressure, the faster the rate ofevaporation● The lower the boiling point, the faster the rate ofevaporation