Heat & Thermodynamics

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Heat & Thermodynamics

  1. 1. By iTutor.comT- 1-855-694-8886Email- info@iTutor.com
  2. 2. Energy that flows from something warm tosomething coolerA hotter substance gives KE to a cooler oneHeat is the transfer of energy between two objectsdue to temperature differencesHeat is measured in Joules (J) because it is a formof energyHeat
  3. 3. Heat The name of the transfer process is heat. What getstransferred is energy. Heat is NOT a substance although it is veryconvenient to think of it that way.In fact, it used to be thought that heat was asubstance.There is a circular nature to the definitions used:(a) energy does work or produces heat, but(b) heat is a transfer of energy.
  4. 4. Temperature Temperature is the property which determines thedirection heat will flow when two objects arebrought into contact Based on the motion of the particles in asubstanceFast motion = high temperatureSlower motion = lower temperature Relatively describes how the particles collide withthe surface of the thermometer Collisions transfer energy
  5. 5. Temperature Scales FahrenheitCommon in the USFreezing point of water is 32ºFBoiling point of water is 212ºF CelsiusCommon in the rest of the world (SI)Freezing point of water is 0ºCBoiling point of water is 100ºC KelvinUsed for scienceFreezing point of water is 273KBoiling point of water is 373K
  6. 6. Temperature and the Phasesof Matter In the Fahrenheit scale, water freezes at 32degrees and boils at 212 degrees The Celsius scale divides the difference betweenthe freezing and boiling points of water into 100degrees (instead of 180).
  7. 7. Temperature and the Phasesof Matter All thermometers are basedon some physical property(such as color or volume)that changes withtemperature. A thermometer is a devicethat changes its electricalresistance as thetemperature changes. A thermocouple is anotherelectrical sensor thatmeasures temperature.
  8. 8. Heat and Thermal Energy Temperature is NOT the same as thermal energy. Thermal energy is energy stored in materialsbecause of differences in temperature. The thermal energy of an object is the totalamount of random kinetic energy for all the atomsin the object. Remember, temperature measures the randomkinetic energy of each atom.
  9. 9. Heat and Thermal Energy Imagine heating a cup ofcoffee to a temperature of100°C. Next think about heatingup 1,000 cups of coffee to100°C. The final temperature isthe same in both cases butthe amount of energyneeded is very different.
  10. 10. First Law of Thermodynamics Energy loss is equal to energy gain.
  11. 11. Second Law ofThermodynamics
  12. 12. Energy and Heat flowThere Energy, in the process we call heat or heat flow,is constantly flowing into and out of all objects,including living objects. Heat flow moves energy from a higher temperature toa lower temperature.The bigger the difference in temperature between twoobjects, the faster heat flows between them.When temperatures are the same there is no change inenergy due to heat flow.Radiation and Conduction are the two methods of heattransfer. Convection is a special type of conduction.Heat has the units of energy; heat flow has the units ofpower.
  13. 13.  Heat flow is energy moving. It has the same units aspower - energy per unit time . It means that during the given amount of time, duringwhich heat is flowing, a certain amount of energy istransferred or moved from one place to another placeEnergy and Heat flow
  14. 14.  Three easy things to know about the way heat flows:1) There has to be a temperature difference. Energyonly flows as heat if there is a temperature difference.2) Energy as heat flows from a higher temperature to alower temperature.3) The greater or larger the difference in temperature,the faster the energy flows.Heat flow
  15. 15.  The science of how heat flows is called heat transfer. There are three ways heat transfer works:conduction, convection, and radiation. Heat flow depends on the temperature difference.Heat flow
  16. 16. Heat flow
  17. 17. Heat Conduction Conduction occurs when two object atdifferent temperatures are in contactwith each other. Heat flows from the warmer to thecooler object until they are both at thesame temperature. Conduction is the movement of heatthrough a substance by the collision ofmolecules. At the place where the twoobject touch, the faster-moving moleculesof the warmer object collide with theslower moving molecules of the coolerobject.
  18. 18. Heat Conduction Conduction is the transfer of heat through materialsby the direct contact of matter. Dense metals like copper and aluminum are verygood thermal conductors. A thermal insulator is a material that conducts heatpoorly.
  19. 19. Thermal Conductivity The thermal conductivity of a material describeshow well the material conducts heat.
  20. 20. Heat Conduction EquationPH = k A (T2 -T1)LArea of cross section (m2)Length (m)Thermal conductivity(watts/moC)Heat flow(watts)Temperaturedifference (oC)
  21. 21. ConvectionConvection is the transfer ofheat by the motion of liquidsand gases.– Convection in a gas occursbecause gas expands whenheated.– Convection occurs becausecurrents flow when hot gas risesand cool gas sink.– Convection in liquids alsooccurs because of differences indensity.
  22. 22. Convection When the flow of gas orliquid comes fromdifferences in density andtemperature, it is calledfree convection. When the flow of gas orliquid is circulated bypumps or fans it is calledforced convection.
  23. 23. Convection Both free and forced convection help to heathouses and cool car engines.
  24. 24. Convection Convection depends on speed and surface area. Motion increases heat transfer by convection in allfluids. If the surface contacting the fluid is increased, the rateof heat transfer also increases.
  25. 25. Convection and SeaBreezes On a smaller scale nearcoastlines, convection isresponsible for sea breezes. During the daytime, land ismuch hotter than the ocean. A sea breeze is created whenhot air over the land rises dueto convection and is replacedby cooler air from the ocean. At night the temperaturereverses so a land breezeoccurs.
  26. 26. Heat Convection EquationPH = h A (T2 -T1)Area contacting fluids (m2)Heat transfer coefficient(watts/m2oC)Heat flow(watts)Temperaturedifference (oC)
  27. 27. Radiation Radiation is heat transfer byelectromagnetic waves. Thermal radiation iselectromagnetic waves(including light) produced byobjects because of theirtemperature. The higher the temperatureof an object, the morethermal radiation it gives off.
  28. 28. • RadiationRadiation Heat from the sun is thebest example of radiation.The sun is rays reach theearth without having anymaterial medium inbetween the earth and thesun.The electromagnetic rayscarry energy from the sun issurface to the earth andheat up the earth and itsatmosphere.
  29. 29. Radiant Heat We do not see the thermalradiation because it occursat infrared wavelengthsinvisible to the human eye. Objects glow different colorsat different temperatures.
  30. 30.  The graph of power versus wavelength for a perfectblackbody is called the blackbody spectrum.Radiant Heat A perfect blackbody is a surface that reflects nothingand emits pure thermal radiation.
  31. 31. Radiant Heat The total power emitted as thermal radiation by ablackbody depends on temperature (T) and surfacearea (A). Real surfaces usually emit less than the blackbodypower, typically between 10 and 90 percent. The Kelvin temperature scale is used in the Stefan-Boltzmann formula because thermal radiationdepends on the temperature above absolute zero.
  32. 32. Stefan-BoltzmannformulaP = s AT4Surface area (m2)Stefan-Boltzmann constant5.67 x 10-8 watts/m2K4)Power(watts)Absolute temperature(K)
  33. 33. The Endwww.iTutor.com

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