An introduction to Thermodynamics

Chapter 4Chapter 4Heat and TemperatureHeat and Temperature

An introduction to An introduction to ThermodynamicsThermodynamics

Molecules InteractMolecules Interact• Usually attractive ; materials cling together

– Cohesion • Attractive forces between “like” molecules

– Adhesion • Attractive forces between “unlike” molecules

• Interactions can also be repulsive– Water beading on wax ; Think about “Gortex”.

Table 4.1Fig 4.3

Three Phases of MatterThree Phases of Matter

Fluids:Ability

to Flow.

Comparison of CharacteristicsComparison of Characteristics

Temperature and HeatTemperature and Heatare different conceptsare different concepts

Heat (Q)Heat (Q)• Measure of the internal energy (U) that has been Measure of the internal energy (U) that has been

absorbed or transferred from one body to another.absorbed or transferred from one body to another.

Temperature (T)Temperature (T)• A measure of the average kinetic energy of the moleculesA measure of the average kinetic energy of the molecules• Three Scales ( Three Scales ( °F, °C, K )°F, °C, K )

(See Fig 4.7)(See Fig 4.7)

Fig 4.10Fig 4.10

Heat: “Energy in transit” Heat: “Energy in transit” Total PE + KETotal PE + KEExternal energyExternal energy

versusversusInternal energy (U)Internal energy (U)

Fig 4.9Fig 4.9

• External energy can be External energy can be transferred to Internal energytransferred to Internal energy– Result: a temperature increaseResult: a temperature increase– See “Friction” in Fig 4.8.See “Friction” in Fig 4.8.

Heat (Q) Heat (Q) (page 100)(page 100)

Heating methods: gain of internal energy

1.1. Temperature difference between objectsTemperature difference between objects Naturally from “higher” to “lower”Naturally from “higher” to “lower”

2.2. Energy-form conversion (thru work)Energy-form conversion (thru work) Mechanical, radiant, electrical.Mechanical, radiant, electrical.

“Cooling” = decreasing internal energy

“Heating” = increasing internal energy

Think: “Process” ; “Direction of Flow”Think: “Process” ; “Direction of Flow”

Measures of HeatMeasures of Heat(know the definitions)(know the definitions)

Metric unitsMetric units• calorie (cal) calorie (cal) • kilocalorie (kcal)kilocalorie (kcal)• Nutrition: Calorie, (Cal)Nutrition: Calorie, (Cal)

English systemEnglish system• British thermal unit British thermal unit

(Btu)(Btu)

Mechanical equivalence Mechanical equivalence (energy-form conversion)(energy-form conversion)

4.184 J = 1 cal ; 4184 J = 1 kcal4.184 J = 1 cal ; 4184 J = 1 kcalExample 4.4Example 4.4

Mechanical work and heat are different forms of the same thing

Variables involved in finding QVariables involved in finding Q

• Mass (m)• Specific Heat (c)• Temperature change (ΔT)

Fig 4.12Fig 4.12

Table 4.2

Calculating Amount of HeatCalculating Amount of Heat

A Key Point!

Determining Specific HeatDetermining Specific Heat

Heat flowHeat flow(energy transfer processes)(energy transfer processes)

• Energy (heat) transfers take place because Energy (heat) transfers take place because of a temperature difference by:of a temperature difference by:

Conduction ConvectionConvection RadiationRadiation

Conduction• Heat flowing through matter– Occurs easily in solidsOccurs easily in solids

• Mechanism– Molecule-to-molecule contactMolecule-to-molecule contact– Warmer object to colder object Warmer object to colder object

• Poor conductors are good insulators (Air, styrofoam, wool, …)

Fig 4.13Fig 4.13

Conductivities: Table 4.3 on p 104Conductivities: Table 4.3 on p 104

Insulation and R-ValueInsulation and R-Value

Convection: a “Vertical Current” Convection: a “Vertical Current” Fig 4.16

Radiation(Radiant Energy)

Radiation(Radiant Energy)

• Energy associated with EM waves

• Emitted, absorbed, or reflected (scattered)

• Can operate through space (or a vacuum)– Does not require matter for transfer

Phase ChangesPhase ChangesFig 4.17 • Heat that is Heat that is not not

associatedassociated with a with a Temp changeTemp change

• Energy Absorbed Energy Absorbed or Releasedor Released

• Latent HeatLatent Heat– ““Hidden” energyHidden” energy– ““Internal PE”Internal PE”

Heat, Temperature, and Phase ChangesFig 4.20

Must determine Q for:

Changes in TemperatureChanges in Temperature

Changes in PhaseChanges in Phase

Example 4.7: Example 4.7: Energy and Making IceEnergy and Making Ice

Refer to Table 4.4Refer to Table 4.4

Example 4.7 Solution

Evaporation and Condensation

• Individual molecules can change phase any time

• Evaporation: liquid to gas (vapor) phase change – Higher energy molecules near the liquid surface can

escape– Increases if?

• Condensation: gas (vapor) to liquid phase change – Gas molecules near the surface lose KE to liquid

molecules and return– Primarily occurs through?

Thermodynamics • The study of heat and its relationship to The study of heat and its relationship to

mechanical and other forms of energymechanical and other forms of energy

• Thermodynamic analysis includes: Thermodynamic analysis includes: – System and the Surroundings (everything else)System and the Surroundings (everything else)

– Internal energy (U)Internal energy (U)

The First Law of Thermodynamics• Energy supplied to a thermodynamic system in the Energy supplied to a thermodynamic system in the

form of heat, minus the work done by the system, is form of heat, minus the work done by the system, is equal to the change in internal energy equal to the change in internal energy

• Application of the Law of Conservation of EnergyApplication of the Law of Conservation of Energy– Internal Energy in this case.Internal Energy in this case.

Equation 4.8

The Second Law of The Second Law of ThermodynamicsThermodynamics

• Heat naturally flows from a warm object to a Heat naturally flows from a warm object to a cold object cold object

Another way to state it: Another way to state it: • It is impossible to convert heat completely into It is impossible to convert heat completely into

mechanical energy.mechanical energy.

Next: Next:

Chapter 5Chapter 5

Waves and VibrationsWaves and Vibrations