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Energy that flows from something warm tosomething coolerA hotter substance gives KE to a cooler oneHeat is the transfer of energy between two objectsdue to temperature differencesHeat is measured in Joules (J) because it is a formof energyHeat
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.
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 asubstanceFast motion = high temperatureSlower motion = lower temperature Relatively describes how the particles collide withthe surface of the thermometer Collisions transfer energy
Temperature Scales FahrenheitCommon in the USFreezing point of water is 32ºFBoiling point of water is 212ºF CelsiusCommon in the rest of the world (SI)Freezing point of water is 0ºCBoiling point of water is 100ºC KelvinUsed for scienceFreezing point of water is 273KBoiling point of water is 373K
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).
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.
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.
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.
First Law of Thermodynamics Energy loss is equal to energy gain.
Energy and Heat flowThere 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.
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
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
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
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.
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.
Thermal Conductivity The thermal conductivity of a material describeshow well the material conducts heat.
Heat Conduction EquationPH = k A (T2 -T1)LArea of cross section (m2)Length (m)Thermal conductivity(watts/moC)Heat flow(watts)Temperaturedifference (oC)
ConvectionConvection 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.
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.
Convection Both free and forced convection help to heathouses and cool car engines.
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.
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.
Heat Convection EquationPH = h A (T2 -T1)Area contacting fluids (m2)Heat transfer coefficient(watts/m2oC)Heat flow(watts)Temperaturedifference (oC)
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.
• 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.
Radiant Heat We do not see the thermalradiation because it occursat infrared wavelengthsinvisible to the human eye. Objects glow different colorsat different temperatures.
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.
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.
Stefan-BoltzmannformulaP = s AT4Surface area (m2)Stefan-Boltzmann constant5.67 x 10-8 watts/m2K4)Power(watts)Absolute temperature(K)