Heat and Internal EnergyInternal Energy U is the total energy associated with the microscopic components of the systemIncludes kinetic and potential energy associated with the random translational, rotational and vibrational motion of the atoms or moleculesAlso includes the intermolecular potential energyDoes not include macroscopic kinetic energy or external potential energyHeat refers to the transfer of energy between a system and its environment due to a temperature difference between themAmount of energy transferred by heat designated by symbol QA system does not have heat, just like it does not have work (heat and work speak to transfer of energy)

Units of HeatThe historical unit of heat was the calorieA calorie is the amount of energy necessary to raise the temperature of 1 g of water from 14.5C to 15.5CA Calorie (food calorie, with a capital C) is 1000 calSince heat (like work) is a measure of energy transfer, its SI unit is the joule1 cal = 4.186 J (Mechanical Equivalent of Heat)New definition of the calorieThe 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 63F to 64F

More About HeatHeat is a microscopic form of energy transfer involving large numbers of particlesEnergy exchange occurs due to individual interactions of the particlesNo macroscopic displacements or forces involvedHeat flow is from a system at higher temperature to one at lower temperatureFlow of heat tends to equalize average microscopic kinetic energy of moleculesWhen 2 systems are in thermal equilibrium, they are at the same temperature and there is no net heat flowEnergy transferred by heat does not always mean there is a temperature change (see phase changes)

Heat Transfer SimulationSimulation presented in class.(ActivPhysics Online Exercise #8.6, copyright Addison Wesley publishing)

Specific HeatEvery substance requires a unique amount of energy per unit mass to change the temperature of that substance by 1CThe specific heat c of a substance is a measure of this amount, defined as:Or DT is always the final temperature minus the initial temperatureWhen the temperature increases, DT and Q are considered to be positive and energy flows into the systemWhen the temperature decreases, DT and Q are considered to be negative and energy flows out of the system c varies slightly with temperature (units of J / kgoC)

Consequences of Different Specific HeatsAir circulation at the beachWater has a high specific heat compared to landOn a hot day, the air above the land warms fasterThe warmer air flows upward and cooler air moves toward the beach, creating air circulation patternModerate winter temperatures in regions near large bodies of waterWater 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 glidersSections of land are at higher temp. than other areas

CalorimetryCalorimetry means measuring heatIn practice, it is a technique used to measure specific heat Technique involves:Raising temperature of object(s) to some valuePlace object(s) in vessel containing cold water of known mass and temperatureMeasure temperature of object(s) + water after equilibrium is reachedA 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:(Q > 0 (< 0) when energy is gained (lost))

Example 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 it reaches its final equilibrium temperature of 32C. Calculate the mass of the aluminum cup.

CQ1: 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

CQ2: Interactive 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

Phase TransitionsA phase transition occurs when the physical characteristics of the substance change from one form to anotherCommon phase transitions areSolid liquid (melting)Liquid gas (boiling)Phase transitions involve a change in the internal energy, but no change in temperatureKinetic energy of molecules (which is related to temperature) is not changing, but their potential energy changes as work is done to change their positionsEnergy 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)

Phase TransitionsAll phase changes can go in either directionHeat flowing 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 freezingLatent heat of vaporization Lv is used for boiling or condensing (somewhat larger for lower pressures)Table 11.2 gives the latent heats for various substancesLarge Lf of water is partly why spraying fruit trees with water can protect the buds from freezingIn process of freezing, water gives up a large amount of energy and keeps bud temperature from going below 0C

T 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/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 J

Evaporation and CondensationThe previous example shows why a burn caused by 100C steam is much more severe than a burn caused by 100C waterSteam releases large amount of energy through heat as it condenses to form water on the skinMuch more energy is transferred to the skin than would be the case for same amount of water at 100CEvaporation is similar to boilingMolecular bonds are being broken by the most energetic moleculesAverage kinetic energy is lowered as a result, which is why evaporation is a cooling processApproximately the same latent heat of vaporization appliesReason why you feel cool after stepping out from a swimming pool

Example 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 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.

ConductionEnergy can be transferred via heat in one of three ways: conduction, convection, radiationConduction occurs with temperature differencesTransfer by conduction can be understood on an atomic scaleIt is an exchange of energy between microscopic particles by collisionsLess energetic particles gain energy during collisions with more energetic particlesNet result is heat flow from higher temperature region to lower temperature regionRate of conduction depends upon the characteristics of the substanceMetals are good conductors due to loosely-bound electrons

ConductionConsider the flow of heat by conduction through a slab of cross- sectional area A and width LThe rate of energy 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 materialSubstances that are good (poor) conductors have large (small) thermal conductivities (see Table 11.3) P is in Watts when Q is in Joules and Dt is in secondsL

Home InsulationIn 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 insulators For multiple layers, the total R value is the sum of the R values of each layerStill air provides good insulation, but moving air increases the energy loss by conduction in a homeMuch of the thermal resistance of a window is due to the stagnant air layers rather than to the glass

ConvectionConvection is heat flow by the movement of a fluidWhen the movement results from differences in density, it is called natural convection (fluid currents are due to gravity)Air currents at the beachWater currents in a saucepan while heatingWhen the movement is forced by a fan or a pump, it is called forced convection (fluid is pushed around by mechanical means fan or pump)Forced-air heating systemsHot-water baseboard heatingBlood circulation in the body (although air currents move under natural convection)

Thermal RadiationThermal radiation transfers energy through emission of electromagnetic waves does not require physical contactAll objects radiate energy continuously in the form of electromagnetic waves due to thermal vibrations of the moleculesAt ordinary temperatures (~20C) nearly all the radiation is in the infrared (wavelengths longer than visible light)At 800C a body emits enough visible radiation to be self-luminous and appears red-hotAt 3000C (incandescent lamp filament) the radiation contains enough visible light so the body appears white-hotAn ideal emitter and absorber of radiation is called a blackbody (would appear black)

Thermal RadiationThe rate at which energy is radiated 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 KelvinObjects absorb radiation as wellNet rate of energy gained or lost given by:

T0 = temperature of environment

Applications of Thermal RadiationChoice of clothingBlack fabric acts as a good absorber, so about half of the emitted energy radiates toward the bodyWhite fabric reflects thermal radiation wellThermography as medical diagnostic toolMeasurement 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 activityMeasuring body temperatureRadiation thermometer 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)

Resisting Energy TransferDewar flask/thermos bottleDesigned to minimize energy transfer to surroundingsSpace between walls is evacuated to minimize conduction and convectionSilvered surface minimizes energy transfer by radiationNeck size is reducedSame principle behind dressing in coats and sweaters to keep warmWarmer air is trapped close to our bodies, reducing energy loss by convection and conduction

Global WarmingAnalogous 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 glassGlass emits IR, half of which is emitted back inside the greenhouseConvection currents are inhibited by the glass (although this is not mirrored in Earths atmosphere)Earths atmosphere fills role of glass roof in greenhouseGreenhouse gasses like CO2 are particularly good absorbers of IRMore greenhouse gasses in the atmosphere means more IR is absorbed and Earths surface becomes warmer