Thermodynamics. Some History

Transcription

1 Thermodynamics Some History! " # $ %& ' (! ) * " +, ' (! ) * " + * -.! / #! % " * -0! "! %1 +!, 2 3, &, #! (, ! % "! 6 7 & 8! 9 # 2 " :, -* 9, ; ( 2 # +5 * #! 6 7 & ( 2 # +2 " 2 3! -!, # +* <! #,. = -* "! #, * % 2 $ " 6 # )!, $ " 0 > * " 6?! " # $ % +!, ' # )! % ( 2 6 & " * ( +, ' + " #! % " * -! "! %1 & 2 3, &, #! (, * " 6 +#, %! -* # +2 " # ; A" 6 $, # % +* - %! 5 2 -$ # +2 " B C -! # % + +# & * " 6 ( * 1 "! # +, ( ' 2 "! = #, 2 3 ) * %1! 4 3 +! -6 4 % * 6 +* # +2 " B D E! " # $ %& ' %! -* # +5 +# & * " 6 F $ * " # $ ( (! ) * " +, ; # )! -* 9, 2 3 # )! * # 2 ( % -6

2 # # ) +, = 2 +" # 4 * -- # ) +,, 2 $ " 6, = %! # # & * 7, # % * # 4 # ) 2 $ 1 ) We know what work is, but What is heat?

3 Work: mechanical energy Heat: thermal energy But what is energy? We have no knowledge of what energy is it is an abstract thing (Richard Feynman) Definition 1: a scalar quantity that describes the amount of work that can be performed by a force Definition 2: Energy is a property or characteristic (or trait or aspect?) of matter that makes things happen, or, in the case of stored or potential energy, has the "potential" to make things happen. By "happen", we mean to make things move or change condition. Examples of changes in condition are changes in shape, volume, and chemical composition (results of a chemical reaction). There are also changes in pressure, temperature, and density which we call a "change of state" in thermodynamics. Phase changes, such as changing from solid to liquid, or liquid to vapor, or back the other way, are also good examples of condition changes. Something happened! Good, but what about radiation?

4 My definition of energy Definition 3: a scalar quantity conveniently defined so that it is conserved in all physical processes taking place in a closed system, and that obeys certain symmetry principles. Energy comes in many inter convertible forms: -internal (atomic motion in solids, liquids & gases) -electrical & magnetic -chemical - in molecular bonds (coal power) -kinetic (wind power) -potential gravitational (hydropower) -radiant (solar power) -nuclear in proton-neutron bonds (nuclear power) ENERGY obeys conservation laws!!! What is work? Mechanical work: scalar quantity describing the amount of energy transferred by a force acting through a distance W = F. d Units: [W] = [F].[d] = N.m (Newton.meter) = J = Joule

5 Kinetic-Molecular Theory! " # $ % & Internal energy ' ( ) % *

6 Internal Energy (contd.) ( ) % % % ( % % ) Temperature vs. Internal Energy! " #! $ % & % ' ( ) ) ) $ * $ ) ) $ ) $ +, -. ) / ) $ ) $

7 Internal Energy vs. Heat % ( ) Summary of concepts

8 CPS question: Which one is colder (has lower temperature?) ICE CUBE ICEBERG a) The iceberg b) The ice cube c) They both have the same temperature CPS question: Which one has more internal energy ICE CUBE ICEBERG a) The iceberg b) The ice cube c) They both have the same energy

9 What is Heat? What causes it? The Concept of Temperature Without realizing its significance, Galileo (ca 1630) developed a crude thermometer Fahrenheit (1715); measured temperature by expansion of a fluid (mercury) Celsius (1742) defined 0 o C as the melting point of ice; 100 o C as the boiling point of water; with a scale in between linear with expansion of fluid Lavoisier (1780) realized that matter is composed of discrete atoms and molecules Dalton (1808), temperature interpreted as a measure of particle speed (gas) or vibration (solid) Kelvin (ca 1885) introduced the notion of the absolute zero temperature, where all atomic motion stops

10 Measuring temperature (cont) % % ' CPS question The illustration shows a thermometer that uses a column of liquid (usually mercury or ethanol) to measure air temperature. In thermal equilibrium, this thermometer measures the temperature of A. the column of liquid. B. the glass that encloses the liquid. C. the air outside the thermometer. D. both A. and B. E. all of A., B., and C.

11 Temperature scales Values on the temperatures scales (Fahrenheit, Centigrade/Celsius, and Kelvin) may be readily interconverted. Physics professors will want values to eventually be in Kelvins because that s the form in SI units. Temperature conversions from Fahrenheit ( (! " #! " $% %! " '& ) $ / ( You only need to remember: 1K = 1 C

12 Absolute Zero & the Kelvin Scale 1 1 $ 1 " / ( $!. 1 $ / 0 ) 1 1 $ ) ) ) Thermal Equilibrium ) ) ) $ ) ) $ $ ) ) & % & ' heat & %! %! % '& $ ) ) )

13 The 0th law of thermodynamics Effects of heat: expansion of a body producing mechanical work increasing the temperature of a body melting a body vaporizing a liquid Phase transitions Liquefying a gas

14 Thermal expansion linear A change in length will accompany a change in temperature. The size of the change will depend on the material. The change in length is proportional to the temperature change and the initial length: 0 L = αl T α is the linear expansion coefficient and is material dependent Nice property for building thermometers! (e.g. mercury) CPS question A solid object has a hole in it. Which of these illustrations more correctly shows how the size of the object and the hole change as the temperature increases? A. illustration #1 #1 #2 B. illustration #2 C. The answer depends on the material of which the object is made. D. The answer depends on how much the temperature increases. E. Both C. and D. are correct.

15 Measuring Heat Quantity of Heat Calorie: amount of heat needed to raise the temperature of 1 gram of water from 14.5 C to 15.5 C Since heat is another form of energy, there must be a relationship between these units and the familiar mechanical energy units. Experimentally, one finds: 1 cal = J The calorie is NOT a fundamental SI unit. The Joule IS the standard unit of energy

16 Specific heat ) ) ) ) - Q = mc T + -. ) ) ) ) / ) / ( - For water: c = 1 cal/(g C )=4190 J/(kg K) SI units Definitions: Specific Heat is the amount of heat required to change temperature of one kilogram of a substance by one degree. Heat Capacity of a system is the amount of heat required to change the temperature of the whole system by one degree. It s an extensive quantity. Heat capacity = Specific Heat x Mass. C=c.m Molar Heat Capacity specific heat in units of Moles, a certain number of atoms (Avogadro constant) Avogadro constant

17 Specific heat values Small!!! Small!!!? LARGE!!! (Dulong and Petit) Phases of matter Water, Steam, Ice

18 Liquid Solid (ice) Gas (steam) Water exists in a few phases Gas

19 Liquid Solid

20 Generally: Liquids denser than gas Solids denser than liquids Not water! Water most dense at 4 C + ) ) * Phase equilibrium

21 Phase transitions Transformation from one phase to another Absorbs/releases latent heat (energy in bonds) Represents a change in order Whenever a substance undergoes a phase transition, energy is transferred into or out of the substance WITHOUT causing a change in temperature. Example: Phase transition - melting heat

22 sublimation freezing condensation melting evaporation deposition Latent heat ) ), - ) 1 Video demo:

23 Phase transitions: Example Energy required to convert 1 g of ice, initially at -30 C to steam at 120 C. Specific Heat of: (in J/g K) Water: 4.19; Steam: 2.01; Ice: 2.06 Latent heat of fusion: 334 J/g Latent heat of vaporization: 2260 J/g Phase transitions can be affected by pressure

24 Summary: Phase changes and temperature behavior A solid will absorb heat according to its heat capacity, becoming a hotter solid. At the melting point, a solid will absorb its heat of fusion and become a liquid. An equilibrium mixture of a substance in both its liquid and solid phases will have a constant temperature. A cold liquid will absorb heat according to its heat capacity to become a hotter liquid. At the boiling point, a liquid will absorb its heat of vaporization and become a gas. An equilibrium mixture of liquid and gas will have a constant temperature. A cold gas can absorb heat according to its heat capacity and become a hotter gas. CPS Question A pitcher contains 0.50 kg of liquid water and 0.50 kg of ice at 0 C. You let heat flow into the pitcher until there is 0.75 kg of liquid water and 0.25 kg of ice. During this process, A. the temperature of the ice-water mixture increases slightly. B. the temperature of the ice-water mixture decreases slightly. C. the temperature of the ice-water mixture remains the same. D. The answer depends on the rate at which heat flows.

25 Heats of Fusion and Heats of Vaporization Heat Transfer Processes

26 How does heat travel? ( ) ) ) ) ) ( ) + - ) ) * ) Conduction %! " & '! '! $" & % % & & &! $% $!

27 Convection +) ) $ ) $ ( Radiation

28 Thermal Conductivity, k k k T 2 T 1 heat Heat current: H = dq dt = ka L ( T ) 2 T1 ) A = ) L = ) k = ) ) T 2 - T 1 $ + H = dq dt = ka ( T ) 2 T1 L

29 k SI Units for Thermal Conductivity ka H = ( T ) 2 T1 L $ ) - $ ( $ - ) k k *,

30 CPS question A chair has a wooden seat but metal legs. The chair legs feel colder to the touch than does the seat. Why is this? A. The metal is at a lower temperature than the wood. B. The metal has a higher specific heat than the wood. C. The metal has a lower specific heat than the wood. D. The metal has a higher thermal conductivity than the wood. E. The metal has a lower thermal conductivity than the wood. Radiation: Power & Temperature H H H = Aσe T e: emissivity (dimensionless number between 0 and 1) σ: Stefan-Boltzmann constant = 5.67x10-8 W/m 2 K 4

31 Radiation and Absortion % H! " # $ H = Aσe (T -T % ) T env : Temperature of the environment surrounding the body