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Lesson 2 Thermodynamics and Heat Transfer

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Lesson 2 Thermodynamics and Heat Transfer In the first lesson we did a historical review. Now we will work through the basic theory necessary for being able to measure temperature at all – thermodynamics and heat transfer. an also vibrate. This vibration or heat spreads because the vibrational LESSON 2 THERMODYNAMICS energy is transferred from atom to atom. This is called thermal AND HEAT TRANSFER conduction. See the figure and table below. A TEMPERATURE MODEL the same temperature, no heat is Medium Conductivity – THE KINETIC THEORY transferred. We understand this av sig själv från en an The smallest constituent parts of a intuitively but it is not stated in any Solid materials W / (m K) - Ag, Cu 420, 380 hot body have greater movement )energi i sina minsta of the other laws so it was added - - Al (the element) 220 than those of a cold body. The afterwards and put first. All temper- - Stainless steel 15 average value of the kinetic vibra- ature measurement and calibration - Glass 0.93 tional energy of atoms and mol- are based on this axiom. - Wood, pine 0.1 – 0.4 ecules in a body is a measurement - Glass wool 0.035 övergår of its temperature. The first law: Liquids (rörelse - Water 20 – 100 °C 0.60 – 0.68 Energy can neither be created nor Gases destroyed but only converted be- - Air 20 – 100 – 300 °C 0.025 –bara omvandlas mellan olika 0.032 – 0.045 aldrig tween different energy forms. Heat transfer by conduction. av sig själv från en – Vibrational energy is transferred from The second law: atom to atom. In general, metals are )energi i sina minsta This can be formulated in various - the best conductors. Gases are the ways but the most suitable formu- worst. Kunskap om de olika utbredningssätten lation for measurement techniques is that heat never moves by itself Metals are generally excellent övergår from a cold body to a hot one. thermal conductors.baseras på denna sats. In liquids and (rörelse gases the molecules maintain a HEAT TRANSFER greater distance from each other, so bara omvandlas mellan olika strålning There are three different the vibrations cannot be spread so aldrig mechanisms of heat transfer and easily. A vacuum does not contain – they can occur simultaneously any molecules so it cannoteller förstöras conduct och or separately: heat conduc- any heat. tion, c onvection and radiation. Know ledge about these various Convection . Kunskap om de olika utbredningssätten Convection is motion within a fluid (= a sätt men ger att värme distribution methods is important och kalibrering Livlighetsteorin for understanding what is happen- liquid or gas) baseras på denna sats. - Termodynamik och värmeöverföring. ing in a measurement situation. Natural convection involves så därför lade man till denna efteråt och placerade den först. den och placerade efteråt denna man till lade därför så - movements generated by den- This understanding makes it Detta förstår man intuitivt men det framgår inte av någon av de av någon av inte framgår det men intuitivt man förstår Detta konvektion sity differences due to temperature possible to prevent measurement strålning error by arranging the installation differences. A hot fluid is “lighter” , so that the sensor does not disrupt than a cold one and “rises”. Forced convection is achievedeller förstöras by the measurement process by e.g. och using mixers, pumps and fans. The creating new transport routes for I första I lektionen gjorde vi enhistorisk tillbakablick. Nu In accordance with the temperature model shown above, in which atoms (or the heat flow. temperature of the fluid can thereby molecules) are vibrating, we can understandbecome that an moreatom evenwith athroughout high level itsof ledning vibration will soon influence its neighbour to also vibrate. This vibration or heat volume. a sätt men ger att värme delar, medan kallare materia innehåller lägre kinetisk energinivå. Dessutom spreadsConduction because the vibrational energy is transferred from atom to atom. This is This figure indicates that hot materials och kalibrering Livlighetsteorin Heat transfer from a fluid to a have greater kinetic energy (energy calledIn accordance thermal conduction with the. Se ynamik och värmeöverföring. temperae the figure- and table below. - solid body is facilitated by increas- Termodynamik och värmeöverföring. så därför lade man till denna efteråt och placerade den först. den och placerade efteråt denna man till lade därför så from motion) in their smallest consti- ture model shown above, in which ing the flow of the fluid. Similarly, it - tuent parts whereas colder materials Metals are generally excellent thermal conductors. In liquids and gases the Detta förstår man intuitivt men det framgår inte av någon av de någon av inte framgår det men intuitivt man förstår Detta atoms (or molecules) are vibrating, modynamikens nollte huvudsats: is easier to transfer heat with a liquid have a lower kinetic energy level. moleculeswe can understandmaintain a greater that an distance atom from each other, so the vibrations cannot be konvektion Higher energy levels also usually than with a gas. This is because the , spreadwith a so high easily. level A vacuum of vibration does notwill contain anyliquid molecules contains so itatoms cannot or conduct molecules any förebygga mätfel genom att arrangera installationen så givaren inte stör mätningen genom att skapa nya transportvägar för värmeflödet. Lektion 2. 2. Lektion kunna taget överhuvud att för teori grundläggande igenom vi går mäta temperatur En temperaturmodell En varm kropp har större rörlighet i sina minsta element än en kall kropp. kropp en i molekyler och atomer hos vibrationsenergin kinetiska den av Medelvärdet är ett mått på dennas temperatur. kall kropp till en varm. Värmeöverföring eller samtidigt uppträda kan värmeöverföring,vilka för mekanismer olika tre Det finns var för sig. är väsentlig för att förstå vad som händer i en mätsituation. Med förståelsen kan m Ledning Figuren antyder att varm materia har stor kinetisk bestånds kräver normalt högre energinivåer mera utrymme. Termodynamik Första huvudsatsen: energiformer. Andra huvudsatsen: Kan formuleras på lite olik För att förstå temperaturmätning måste man ha grundläggande kunskaper i termod termisk i även är de då kropp, tredje termisk jämvikten i med är kroppar Om två har kroppar om värme ingen överförs så ord andra Med varandra. med jämvikt temperatur. samma huvudsatserna andra All tempereraturmätning Energi kan varken skapas require more space. heat. Ter soon influence its neighbour to that transfer its vibrational energy I första I lektionen gjorde vi enhistorisk tillbakablick. Nu THERMODYNAMICS Insulation To understand temperature meas- T1 T2 ledning urement you must have a basic delar, medan kallare materia innehåller lägre kinetisk energinivå. Dessutom knowledge of thermodynamics and )))))))))))) ) ) ) ) ) ynamik och värmeöverföring. ) ) ) ) ) ) ) ) ) ) heat transfer. The zeroth law of modynamikens nollte huvudsats: thermodynamics: kall kropp till en varm. Om två kroppar är i termisk jämvikt med en tredje kropp, då är de även i termisk i även är de då kropp, tredje termisk jämvikten i med är kroppar Om två har kroppar om värme ingen överförs så ord andra Med varandra. med jämvikt temperatur. samma huvudsatserna andra All tempereraturmätning bestånds kräver normalt högre energinivåer mera utrymme. Termodynamik För att förstå temperaturmätning måste man ha grundläggande kunskaper i termod Ter förebygga mätfel genom att arrangera installationen så givaren inte stör mätningen genom att skapa nya transportvägar för värmeflödet. Ledning Lektion 2. 2. Lektion kunna taget överhuvud att för teori grundläggande igenom vi går mäta temperatur En temperaturmodell En varm kropp har större rörlighet i sina minsta element än en kall kropp. kropp en i molekyler och atomer hos vibrationsenergin kinetiska den av Medelvärdet är ett mått på dennas temperatur. Värmeöverföring eller samtidigt uppträda kan värmeöverföring,vilka för mekanismer olika tre Det finns var för sig. är väsentlig för att förstå vad som händer i en mätsituation. Med förståelsen kan m Figuren antyder att varm materia har stor kinetisk If two bodies are in thermal equilib- Hot Första huvudsatsen: Energi kan varken skapas energiformer. Cold Andra huvudsatsen: Kan formuleras på lite olik rium with a third body, then the first T1>T2 sink two bodies must also be in ther- Source mal equilibrium with each other. Heat transfer by conduction. Vibrational energy is transferred from atom to atom.T2°C In other words, if the bodies have HeatIn general, transfer metals by conduction. are the bestVibrational conductors. energy Gasesis transferred are the from worst. atom to atom. In general, metals are the best conductors. Gases are the worst. Medium Conductivity Solid materials W / m K - Ag, Cu 420, 380 - Al (the element) 220 - Stainless steel 15 - Glass 0.93 - Wood, pine 0.1 – 0.4 - Glass wool 0.035 Liquids - Water 20 – 100 °C 0.60 – 0.68 Gases - Air 20 – 100 – 300 °C 0.025 – 0.032 – 0.045 This table shows the thermal conductivity of several different mediums. The values are approximate. Convection Convection is motion within a fluid (= liquid or gas) Natural convection involves movements generated by density differences due to temperature differences. A hot fluid is “lighter” than a cold one and “rises”. Påtvingad konvektion erhålls genom omrörare, pumpar och fläktar. Fluidens temperatur kan då bli jämnare över volymen. Värmeöverföring från fluid till fast kropp underlättas av ökad fluidomströmning. Likaså är det lättare att överföra värme med en vätska än med en gas. Det beror på att vätskan har atomer eller molekyler som överför sin vibrationsenergi tätare. I vakuum förekommer ingen konvektion i brist på molekyler. Figuren visarblocks exempel some of på the radiant heat, as most of us know from experience. konvektion iHeat eluppvärmd is transferred via convection ackumulatortank.between the person’s body and the air (e.g. via the ocean breeze). Heat Värmeväxlarenis conducted eller elpatronen between the body and värmer upp thevattnet sand. Thevars direction is from the molekyler påhigher grund temperature av ökad to the lower one. Thermal radiation is an vibrationsenergioften- ignoredblir mer source of error in åtskilda.
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