Chapter 11Chapter 11

Energy in Thermal Energy in Thermal ProcessesProcesses

Heat Compared to Heat Compared to Internal EnergyInternal Energy

Important to distinguish between Important to distinguish between themthem

They mean very different things They mean very different things when used in physicswhen used in physics

Internal EnergyInternal Energy

Internal EnergyInternal Energy, U, is the energy , U, is the energy associated with the microscopic associated with the microscopic components of the systemcomponents of the system Includes kinetic and potential energy Includes kinetic and potential energy

associated with the random associated with the random translational, rotational and translational, rotational and vibrational motion of the atoms or vibrational motion of the atoms or moleculesmolecules

Also includes the intermolecular Also includes the intermolecular potential energypotential energy

HeatHeat

HeatHeat is a mechanism by which is a mechanism by which energy is transferred between a energy is transferred between a system and its environment system and its environment because of a temperature because of a temperature difference between themdifference between them The system Q is used to represent the The system Q is used to represent the

amount of energy transferred by heat amount of energy transferred by heat between a system and its environmentbetween a system and its environment

Units of HeatUnits of Heat

CalorieCalorie An historical unit, before the An historical unit, before the

connection between thermodynamics connection between thermodynamics and mechanics was recognizedand mechanics was recognized

A A caloriecalorie is the amount of energy is the amount of energy necessary to raise the temperature of necessary to raise the temperature of 1 g of water from 14.5° C to 15.5° C .1 g of water from 14.5° C to 15.5° C .

A Calorie (food calorie) is 1000 calA Calorie (food calorie) is 1000 cal 1 cal = 4.186 J1 cal = 4.186 J

This is called the This is called the Mechanical Mechanical Equivalent of HeatEquivalent of Heat

Units of Heat, cont.Units of Heat, cont.

US Customary Unit – BTUUS Customary Unit – BTU BTU stands for British Thermal UnitBTU stands for British Thermal Unit

A A BTUBTU is the amount of energy is the amount of energy necessary to raise the temperature of necessary to raise the temperature of 1 lb of water from 63° F to 64° F 1 lb of water from 63° F to 64° F

Specific HeatSpecific Heat

Every substance requires a unique Every substance requires a unique amount of energy per unit mass to amount of energy per unit mass to change the temperature of that change the temperature of that substance by 1° Csubstance by 1° C

The The specific heat, c,specific heat, c, of a substance of a substance is a measure of this amountis a measure of this amount

Tm

Qc

Units of Specific HeatUnits of Specific Heat

SI unitsSI units J / kg °CJ / kg °C

Historical unitsHistorical units cal / g °Ccal / g °C

Heat and Specific HeatHeat and Specific Heat

Q = m c ΔTQ = m c ΔT ΔT is always the final temperature ΔT is always the final temperature

minus the initial temperatureminus the initial temperature When the temperature increases, ΔT When the temperature increases, ΔT

and ΔQ are considered to be positive and ΔQ are considered to be positive and energy flows into the systemand energy flows into the system

When the temperature decreases, ΔT When the temperature decreases, ΔT and ΔQ are considered to be negative and ΔQ are considered to be negative and energy flows out of the systemand energy flows out of the system

Consequences of Different Consequences of Different Specific HeatsSpecific Heats

Water has a high Water has a high specific heat specific heat compared to landcompared to land

On a hot day, the On a hot day, the air above the land air above the land warms fasterwarms faster

The warmer air The warmer air flows upward and flows upward and cooler air moves cooler air moves toward the beachtoward the beach

CalorimeterCalorimeter

One technique for determining the One technique for determining the specific heat of a substancespecific heat of a substance

A A calorimetercalorimeter is a vessel that is a is a vessel that is a good insulator that allows a good insulator that allows a thermal equilibrium to be achieved thermal equilibrium to be achieved between substances without any between substances without any energy loss to the environmentenergy loss to the environment

CalorimetryCalorimetry

Analysis performed using a calorimeterAnalysis performed using a calorimeter Conservation of energy applies to the Conservation of energy applies to the

isolated systemisolated system The energy that leaves the warmer The energy that leaves the warmer

substance equals the energy that enters substance equals the energy that enters the waterthe water QQcoldcold = -Q = -Qhothot Negative sign keeps consistency in the sign Negative sign keeps consistency in the sign

convention of ΔTconvention of ΔT

Phase ChangesPhase Changes

A A phase changephase change occurs when the occurs when the physical characteristics of the physical characteristics of the substance change from one form substance change from one form to anotherto another

Common phases changes areCommon phases changes are Solid to liquid – meltingSolid to liquid – melting Liquid to gas – boilingLiquid to gas – boiling

Phases changes involve a change Phases changes involve a change in the internal energy, but in the internal energy, but no no change in temperaturechange in temperature

Latent HeatLatent Heat During a phase change, the During a phase change, the

amount of heat is given asamount of heat is given as Q = m LQ = m L

L is the L is the latent heatlatent heat of the of the substancesubstance Latent means hidden or concealedLatent means hidden or concealed

Choose a positive sign if you are Choose a positive sign if you are adding energy to the system and a adding energy to the system and a negative sign if energy is being negative sign if energy is being removed from the systemremoved from the system

Latent Heat, cont.Latent Heat, cont.

Latent heat of fusionLatent heat of fusion is used for is used for melting or freezingmelting or freezing

Latent heat of vaporizationLatent heat of vaporization is used is used for boiling or condensingfor boiling or condensing

Table 11.2 gives the latent heats Table 11.2 gives the latent heats for various substancesfor various substances

Graph of Ice to SteamGraph of Ice to Steam

Warming IceWarming Ice

Start with one Start with one gram of ice at –gram of ice at –30.0º C30.0º C

During A, the During A, the temperature of temperature of the ice changes the ice changes from –30.0º C to from –30.0º C to 0º C0º C

Use Q = m c ΔTUse Q = m c ΔT

Melting IceMelting Ice

Once at 0º C, the Once at 0º C, the phase change phase change (melting) starts(melting) starts

The temperature The temperature stays the same stays the same although energy although energy is still being is still being addedadded

Use Q = m LUse Q = m Lff

Warming WaterWarming Water

Between 0º C and Between 0º C and 100º C, the 100º C, the material is liquid material is liquid and no phase and no phase changes take placechanges take place

Energy added Energy added increases the increases the temperaturetemperature

Use Q = m c ΔTUse Q = m c ΔT

Boiling WaterBoiling Water

At 100º C, a At 100º C, a phase change phase change occurs (boiling)occurs (boiling)

Temperature does Temperature does not changenot change

Use Q = m LUse Q = m Lvv

Heating SteamHeating Steam

After all the water is After all the water is converted to steam, converted to steam, the steam will heat upthe steam will heat up

No phase change No phase change occursoccurs

The added energy The added energy goes to increasing the goes to increasing the temperaturetemperature

Use Q = m c ΔTUse Q = m c ΔT

Problem Solving StrategiesProblem Solving Strategies

Use consistent unitsUse consistent units Transfers in energy are given as Transfers in energy are given as

Q=mcΔT for processes with no Q=mcΔT for processes with no phase changesphase changes

Use Q = m LUse Q = m Lff or Q = m L or Q = m Lvv if there is if there is a phase changea phase change

In QIn Qcoldcold = - Q = - Qhothot be careful of sign be careful of sign ΔT is TΔT is Tff - T - Tii

Methods of Heat TransferMethods of Heat Transfer

Need to know the rate at which Need to know the rate at which energy is transferredenergy is transferred

Need to know the mechanisms Need to know the mechanisms responsible for the transferresponsible for the transfer

Methods includeMethods include ConductionConduction ConvectionConvection RadiationRadiation

ConductionConduction

The transfer can be viewed on an The transfer can be viewed on an atomic scaleatomic scale It is an exchange of energy between It is an exchange of energy between

microscopic particles by collisionsmicroscopic particles by collisions Less energetic particles gain energy Less energetic particles gain energy

during collisions with more energetic during collisions with more energetic particlesparticles

Rate of conduction depends upon the Rate of conduction depends upon the characteristics of the substancecharacteristics of the substance

Conduction exampleConduction example The molecules vibrate The molecules vibrate

about their equilibrium about their equilibrium positionspositions

Particles near the Particles near the flame vibrate with flame vibrate with larger amplitudeslarger amplitudes

These collide with These collide with adjacent molecules adjacent molecules and transfer some and transfer some energyenergy

Eventually, the energy Eventually, the energy travels entirely through travels entirely through the rodthe rod

Conduction, cont.Conduction, cont.

In general, metals are good In general, metals are good conductorsconductors They contain large numbers of electrons They contain large numbers of electrons

that are relatively free to move through that are relatively free to move through the metalthe metal

They can transport energy from one They can transport energy from one region to anotherregion to another

Conduction can occur only if there is a Conduction can occur only if there is a difference in temperature between difference in temperature between two parts of the conducting mediumtwo parts of the conducting medium

Conduction, equationConduction, equation

The slab allows The slab allows energy to transfer energy to transfer from the region of from the region of higher higher temperature to temperature to the region of the region of lower lower temperaturetemperature

L

TTkA

t

QP ch

Conduction, equation Conduction, equation explanationexplanation

A is the cross-sectional areaA is the cross-sectional area L = Δx is the thickness of the slab or the L = Δx is the thickness of the slab or the

length of a rodlength of a rod P is in Watts when Q is in Joules and t is P is in Watts when Q is in Joules and t is

in secondsin seconds k is the k is the thermal conductivitythermal conductivity of the of the

materialmaterial See table 11.3See table 11.3 Good conductors have high k values and Good conductors have high k values and

good insulators have low k valuesgood insulators have low k values

Home InsulationHome Insulation

Substances are rated by their Substances are rated by their R R valuesvalues R = L / kR = L / k

More multiple layers, the total R More multiple layers, the total R value is the sum of the R values of value is the sum of the R values of each layereach layer

Wind increases the energy loss by Wind increases the energy loss by conduction in a homeconduction in a home

ConvectionConvection

Energy transferred by the Energy transferred by the movement of a substancemovement of a substance When the movement results from When the movement results from

differences in density, it is called differences in density, it is called natural conductionnatural conduction

When the movement is forced by a When the movement is forced by a fan or a pump, it is called fan or a pump, it is called forced forced convectionconvection

Convection exampleConvection example

Air directly above Air directly above the flame is the flame is warmed and warmed and expandsexpands

The density of the The density of the air decreases, and air decreases, and it risesit rises

The mass of air The mass of air warms the hand as warms the hand as it moves byit moves by

Convection applicationsConvection applications

RadiatorsRadiators UpwellingUpwelling Cooling automobile enginesCooling automobile engines Algal blooms in ponds and lakesAlgal blooms in ponds and lakes

RadiationRadiation

Radiation does not require physical Radiation does not require physical contactcontact

All objects radiate energy All objects radiate energy continuously in the form of continuously in the form of electromagnetic waves due to electromagnetic waves due to thermal vibrations of the thermal vibrations of the moleculesmolecules

Rate of radiation is given by Rate of radiation is given by Stefan’s LawStefan’s Law

Radiation exampleRadiation example

The electromagnetic waves carry the The electromagnetic waves carry the energy from the fire to the handsenergy from the fire to the hands

No physical contact is necessaryNo physical contact is necessary

Radiation equationRadiation equation

P = σAeTP = σAeT44

P is the rate of energy transfer, in P is the rate of energy transfer, in WattsWatts

σ = 5.6696 x 10σ = 5.6696 x 10-8-8 W/m W/m22 K K44

A is the surface area of the objectA is the surface area of the object e is a constant called the e is a constant called the emissivityemissivity

e varies from 0 to 1e varies from 0 to 1 T is the temperature in KelvinsT is the temperature in Kelvins

Energy Absorption and Energy Absorption and Emission by RadiationEmission by Radiation

With its surroundings, the rate at With its surroundings, the rate at which the object at temperature T which the object at temperature T with surroundings at Twith surroundings at Too radiates is radiates is PPnetnet = σAe(T = σAe(T44 – T – T44

oo)) When an object is in equilibrium with When an object is in equilibrium with

its surroundings, it radiates and its surroundings, it radiates and absorbs at the same rateabsorbs at the same rate

Its temperature will not changeIts temperature will not change

Ideal AbsorbersIdeal Absorbers

An An ideal absorberideal absorber is defined as an is defined as an object that absorbs all of the object that absorbs all of the energy incident on itenergy incident on it e = 1e = 1

This type of object is called a This type of object is called a black black bodybody

An ideal absorber is also an ideal An ideal absorber is also an ideal radiator of energyradiator of energy

Ideal ReflectorIdeal Reflector

An ideal reflector absorbs none of An ideal reflector absorbs none of the energy incident on itthe energy incident on it e = 0e = 0

Applications of RadiationApplications of Radiation

ClothingClothing Black fabric acts as a good absorberBlack fabric acts as a good absorber White fabric is a better reflectorWhite fabric is a better reflector

ThermographyThermography The amount of energy radiated by an object The amount of energy radiated by an object

can be measured with a thermographcan be measured with a thermograph Body temperatureBody temperature

Radiation thermometer measures the Radiation thermometer measures the intensity of the infrared radiation from the intensity of the infrared radiation from the eardrumeardrum

Resisting Energy TransferResisting Energy Transfer

Dewar flask/thermos bottleDewar flask/thermos bottle Designed to minimize Designed to minimize

energy transfer to energy transfer to surroundingssurroundings

Space between walls is Space between walls is evacuated to minimize evacuated to minimize conduction and convectionconduction and convection

Silvered surface minimizes Silvered surface minimizes radiationradiation

Neck size is reducedNeck size is reduced

Global WarmingGlobal Warming

Greenhouse exampleGreenhouse example Visible light is absorbed and re-Visible light is absorbed and re-

emitted as infrared radiationemitted as infrared radiation Convection currents are inhibited by Convection currents are inhibited by

the glassthe glass Earth’s atmosphere is also a good Earth’s atmosphere is also a good

transmitter of visible light and a transmitter of visible light and a good absorber of infrared radiationgood absorber of infrared radiation