Chapter 4 Heat and Temperature An introduction to Thermodynamics

Thermodynamics Study of heat and its relationship to mechanical energy Collection of assumptions that all matter is made up of tiny atoms and molecules that interact physically Kinetic Molecular Theory (Kinetic means “motion”).

Molecules 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 4.1 Fig 4.3 Three Phases of Matter Fluids: Ability to Flow. Comparison of Characteristics

Temperature and Heat are different concepts Heat (Q) Heat (Q) Measure of the internal energy (U) that has been absorbed or transferred from one body to another.Measure of the internal energy (U) that has been absorbed or transferred from one body to another. Temperature (T) A measure of the average kinetic energy of the moleculesA measure of the average kinetic energy of the molecules Three Scales ( °F, °C, K ) (See Fig 4.7)Three Scales ( °F, °C, K ) (See Fig 4.7) Fig 4.10

Heat: “Energy in transit” Total PE + KE External energy versus Internal energy (U) Fig 4.9 External energy can be transferred to Internal energyExternal energy can be transferred to Internal energy –Result: a temperature increase –See “Friction” in Fig 4.8.

Heat (Q) (page 100) Heating methods: gain of internal energy 1.Temperature difference between objects  Naturally from “higher” to “lower” 2.Energy-form conversion (thru work)  Mechanical, radiant, electrical. “Cooling” = decreasing internal energy “Heating” = increasing internal energy Think: “Process” ; “Direction of Flow”

Measures of Heat (know the definitions) Metric units calorie (cal)calorie (cal) kilocalorie (kcal)kilocalorie (kcal) Nutrition: Calorie, (Cal)Nutrition: Calorie, (Cal) English system British thermal unit (Btu)British thermal unit (Btu) Mechanical equivalence (energy-form conversion) J = 1 cal ; 4184 J = 1 kcal Example 4.4 Example 4.4

Mechanical work and heat are different forms of the same thing

Variables involved in finding Q Mass (m) Specific Heat (c) Temperature change (ΔT) Fig 4.12 Table 4.2

Calculating Amount of Heat A Key Point!

Determining Specific Heat

Heat flow (energy transfer processes) Energy (heat) transfers take place because of a temperature difference by:Energy (heat) transfers take place because of a temperature difference by:  Conduction Conduction  Convection Convection  Radiation Radiation

Conduction Heat flowing through matter –Occurs easily in solids Mechanism –Molecule-to-molecule contact –Warmer object to colder object Poor conductors are good insulators (Air, styrofoam, wool, …) Fig 4.13

Conductivities: Table 4.3 on p 104

Insulation and R-Value

Convection: a “Vertical Current” Fig 4.16

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 Changes Fig 4.17 Heat that is not associated with a Temp changeHeat that is not associated with a Temp change Energy Absorbed or ReleasedEnergy Absorbed or Released Latent HeatLatent Heat –“Hidden” energy –“Internal PE”

Heat, Temperature, and Phase Changes Fig 4.20 Must determine Q for: Changes in Temperature Changes in Phase

Example 4.7: Energy and Making Ice Refer 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 mechanical and other forms of energyThe study of heat and its relationship to mechanical and other forms of energy Thermodynamic analysis includes:Thermodynamic analysis includes: –System and the Surroundings (everything else) –Internal energy (U)

The First Law of Thermodynamics Energy supplied to a thermodynamic system in the form of heat, minus the work done by the system, is equal to the change in internal energyEnergy supplied to a thermodynamic system in the form of heat, minus the work done by the system, is 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.

Equation 4.8

The Second Law of Thermodynamics Heat naturally flows from a warm object to a cold objectHeat naturally flows from a warm object to a cold object Another way to state it: It is impossible to convert heat completely into mechanical energy.It is impossible to convert heat completely into mechanical energy.

Next: Chapter 5 Waves and Vibrations