HeatandInternalEnergy

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1、1Heat and Internal Energy Internal Energy U is the total energy associated with the microscopic components of the system Includes kinetic and potential energy associated with the random translational,rotational and vibrational motion of the atoms or molecules Also includes the intermolecular potenti

2、al energy Does not include macroscopic kinetic energy or external potential energy Heat refers to the transfer of energy between a system and its environment due to a temperature difference between them Amount of energy transferred by heat designated by symbol Q A system does not have heat,just like

3、 it does not have work(heat and work speak to transfer of energy)2Units of Heat The historical unit of heat was the calorie A calorie is the amount of energy necessary to raise the temperature of 1 g of water from 14.5C to 15.5C A Calorie(food calorie,with a capital C)is 1000 cal Since heat(like wor

4、k)is a measure of energy transfer,its SI unit is the joule 1 cal=4.186 J(“Mechanical Equivalent of Heat”)New definition of the calorie The unit of heat in the U.S.customary system is the British thermal unit(BTU)Defined as the amount of energy necessary to raise the temperature of 1 lb of water from

5、 63F to 64F3More About Heat Heat is a microscopic form of energy transfer involving large numbers of particles Energy exchange occurs due to individual interactions of the particles No macroscopic displacements or forces involved Heat flow is from a system at higher temperature to one at lower tempe

6、rature Flow of heat tends to equalize average microscopic kinetic energy of molecules When 2 systems are in thermal equilibrium,they are at the same temperature and there is no net heat flow Energy transferred by heat does not always mean there is a temperature change(see phase changes)4Heat Transfe

7、r SimulationSimulation presented in class.(ActivPhysics Online Exercise#8.6,copyright Addison Wesley publishing)5Specific Heat Every substance requires a unique amount of energy per unit mass to change the temperature of that substance by 1C The specific heat c of a substance is a measure of this am

8、ount,defined as:Or DT is always the final temperature minus the initial temperature When the temperature increases,DT and Q are considered to be positive and energy flows into the system When the temperature decreases,DT and Q are considered to be negative and energy flows out of the system c varies

9、 slightly with temperature TmQcD(units of J/kgoC)TmcQD6Consequences of Different Specific Heats Air circulation at the beach Water has a high specific heat compared to land On a hot day,the air above the land warms faster The warmer air flows upward and cooler air moves toward the beach,creating air

10、 circulation pattern Moderate winter temperatures in regions near large bodies of water Water transfers energy to air,which carries energy toward land(predominant on west coast rather than east coast)Similar effect creates thermals(rising layers of air)which help flight of eagles and hang gliders Se

11、ctions of land are at higher temp.than other areas7Calorimetry Calorimetry means“measuring heat”In practice,it is a technique used to measure specific heat Technique involves:Raising temperature of object(s)to some value Place object(s)in vessel containing cold water of known mass and temperature Me

12、asure temperature of object(s)+water after equilibrium is reached A calorimeter is a vessel providing good insulation that allows a thermal equilibrium to be achieved between substances without any energy loss to the environment(styrofoam cup or thermos with lid)Conservation of energy requires that:

13、0kQ(Q 0(0)when energy is gained(lost)8Example Problem#11.17Solution(details given in class):80 gAn aluminum cup contains 225 g of water and a 40-g copper stirrer,all at 27C.A 400-g sample of silver at an initial temperature of 87C is placed in the water.The stirrer is used to stir the mixture until

14、it reaches its final equilibrium temperature of 32C.Calculate the mass of the aluminum cup.9CQ1:Interactive Example Problem:Calorimetry(Physlet Physics Exploration#19.3,copyright PrenticeHall publishing)Part(a):What is the energy released via heat by the block?193 J 193 J193 kJ193 kJ4186 kJ 10CQ2:In

15、teractive Example Problem:Calorimetry(Physlet Physics Exploration#19.3,copyright PrenticeHall publishing)Part(c):What is the equilibrium temperature of the system?300.0 K 304.6 K319.0 K327.1 K1000 K 11Phase Transitions A phase transition occurs when the physical characteristics of the substance chan

16、ge from one form to another Common phase transitions are Solid liquid(melting)Liquid gas(boiling)Phase transitions involve a change in the internal energy,but no change in temperature Kinetic energy of molecules(which is related to temperature)is not changing,but their potential energy changes as wo

17、rk is done to change their positions Energy required to change the phase of a given mass m of a pure substance is:L=latent heat depends on substance and nature of phase transition+()sign used if energy is added(removed)mLQ12Phase Transitions All phase changes can go in either direction Heat flowing

18、into a substance can cause melting(solid to liquid)or boiling(liquid to gas)Heat flowing out of a substance can cause freezing(liquid to solid)or condensation(gas to liquid)Latent heat of fusion Lf is used for melting or freezing Latent heat of vaporization Lv is used for boiling or condensing(somew

19、hat larger for lower pressures)Table 11.2 gives the latent heats for various substances Large Lf of water is partly why spraying fruit trees with water can protect the buds from freezing In process of freezing,water gives up a large amount of energy and keeps bud temperature from going below 0C 13 T

20、 vs.Q for Transition from Ice to Steam Part A:Temperature of ice changes from 30C to 0C Q=mcice DT=(1.00 103 kg)(2090 J/kgC)(30.0C)=62.7 JPart B:Ice melts to water at 0C Q=mLf=(1.00 103 kg)(3.33 105 J/kg)=333 JPart C:Temperature of water changes from 0C to 100C Q=mcwater DT=(1.00 103 kg)(4.19 103 J/

21、kgC)(100C)=419 JPart D:Water changes to steam at 100C Q=mLv=(1.00 103 kg)(2.26 106 J/kg)=2.26 103 JPart E:Temperature of steam changes from 100C to 120C Q=mcsteam DT=(1.00 103 kg)(2.01 103 J/kgC)(20C)=40.2 J Initial state:1 g of ice at 30CFinal state:1 g of steam at 120CQtot=3.11 103 J14Evaporation

22、and Condensation The previous example shows why a burn caused by 100C steam is much more severe than a burn caused by 100C water Steam releases large amount of energy through heat as it condenses to form water on the skin Much more energy is transferred to the skin than would be the case for same am

23、ount of water at 100C Evaporation is similar to boiling Molecular bonds are being broken by the most energetic molecules Average kinetic energy is lowered as a result,which is why evaporation is a cooling process Approximately the same latent heat of vaporization applies Reason why you feel cool aft

24、er stepping out from a swimming pool15Example Problem#11.31Solution(details given in class):16CA 40-g block of ice is cooled to 78C and is then added to 560 g of water in an 80-g copper calorimeter at a temperature of 25C.Determine the final temperature of the system consisting of the ice,water,and

25、calorimeter.(If not all the ice melts,determine how much ice is left.)Remember that the ice must first warm to 0C,melt,and then continue warming as water.The specific heat of ice is 0.500 cal/gC=2090 J/kgC.16Conduction Energy can be transferred via heat in one of three ways:conduction,convection,rad

26、iation Conduction occurs with temperature differences Transfer by conduction can be understood on an atomic scale It is an exchange of energy between microscopic particles by collisions Less energetic particles gain energy during collisions with more energetic particles Net result is heat flow from

27、higher temperature region to lower temperature region Rate of conduction depends upon the characteristics of the substance Metals are good conductors due to loosely-bound electrons17Conduction Consider the flow of heat by conduction through a slab of cross-sectional area A and width L The rate of en

28、ergy transfer(power)is given by:Assumes that slab is insulated so that energy cannot escape by conduction from its surface except at the ends k is the thermal conductivity and depends on the material Substances that are good(poor)conductors have large(small)thermal conductivities(see Table 11.3)P is

29、 in Watts when Q is in Joules and Dt is in secondsLTTkAtQPchDL18Home Insulation In engineering,the insulating quality of materials are rated according to their R value:R=L/k R values have strange units:Fft2/(Btu/h)Thats why units are not usually given!Substances with larger R value are better insula

30、tors For multiple layers,the total R value is the sum of the R values of each layer Still air provides good insulation,but moving air increases the energy loss by conduction in a home Much of the thermal resistance of a window is due to the stagnant air layers rather than to the glass 19Convection C

31、onvection is heat flow by the movement of a fluid When the movement results from differences in density,it is called natural convection(fluid currents are due to gravity)Air currents at the beach Water currents in a saucepan while heating When the movement is forced by a fan or a pump,it is called f

32、orced convection(fluid is pushed around by mechanical means fan or pump)Forced-air heating systems Hot-water baseboard heating Blood circulation in the body(although air currents move under natural convection)20Thermal Radiation Thermal radiation transfers energy through emission of electromagnetic

33、waves does not require physical contact All objects radiate energy continuously in the form of electromagnetic waves due to thermal vibrations of the molecules At ordinary temperatures(20C)nearly all the radiation is in the infrared(wavelengths longer than visible light)At 800C a body emits enough v

34、isible radiation to be self-luminous and appears“red-hot”At 3000C(incandescent lamp filament)the radiation contains enough visible light so the body appears“white-hot”An ideal emitter and absorber of radiation is called a blackbody(would appear black)21Thermal Radiation The rate at which energy is r

35、adiated is given by Stefans Law:P is the rate of energy transfer(power),in Watts =Stefan-Boltzmann constant=5.6696 x 108 W/m2K4 A is the surface area of the object e is a constant called the emissivity,and ranges from 0 to 1 depending on the properties of the objects surface T is the temperature in

36、Kelvin Objects absorb radiation as well Net rate of energy gained or lost given by:T0=temperature of environment4AeTP404netTTAeP22Applications of Thermal Radiation Choice of clothing Black fabric acts as a good absorber,so about half of the emitted energy radiates toward the body White fabric reflec

37、ts thermal radiation well Thermography as medical diagnostic tool Measurement of emitted thermal energy using infrared detectors,producing a visual display(see Fig.11.13)Areas of high temperature are indicated,showing regions of abnormal cellular activity Measuring body temperature Radiation thermom

38、eter measures the intensity of the infrared radiation from the eardrum(see Fig.11.14)Eardrum is good location to measure temperature since it is near hypothalamus(bodys temperature control center)23Resisting Energy Transfer Dewar flask/thermos bottle Designed to minimize energy transfer to surroundi

39、ngs Space between walls is evacuated to minimize conduction and convection Silvered surface minimizes energy transfer by radiation Neck size is reduced Same principle behind dressing in coats and sweaters to keep warm Warmer air is trapped close to our bodies,reducing energy loss by convection and c

40、onduction24Global Warming Analogous to a greenhouse Visible light and short-wavelength infrared radiation are absorbed by contents of greenhouse,resulting in the emission of longer-wavelength infrared radiation(IR)Longer-wavelength IR absorbed by glass Glass emits IR,half of which is emitted back in

41、side the greenhouse Convection currents are inhibited by the glass(although this is not mirrored in Earths atmosphere)Earths atmosphere fills role of glass roof in greenhouse“Greenhouse gasses”like CO2 are particularly good absorbers of IR More greenhouse gasses in the atmosphere means more IR is absorbed and Earths surface becomes warmer