Thermodynamics - - PDF document

Thermodynamics

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Some History

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We know what work is, but… What is heat?

• Work: mechanical energy
• Heat: thermal energy…
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But what is energy? But what is energy? But what is 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?

My definition of energy My definition of energy My definition of energy 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

• beys 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)

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ENERGY obeys conservation laws!!!

What is work?

Mechanical work: scalar quantity describing the amount of energy transferred by a force acting through a distance

d F W . =

Units: [W] = [F].[d] = N.m (Newton.meter) = J = Joule

Kinetic-Molecular Theory

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Internal energy

Internal Energy (contd.)

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http://mutuslab.cs.uwindsor.ca/schurko/animations/particlesinmetals/eqilibrium-v1.htm

Temperature vs. Internal Energy

http://www.absorblearning.com/media/item.action?quick=ad

Internal Energy vs. Heat

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Summary of concepts

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http://mutuslab.cs.uwindsor.ca/schurko/animations/particlesinmetals/eqilibrium-v1.htm

CPS question: Which one is “colder” (has lower temperature?)

ICEBERG ICE CUBE a) The iceberg b) The ice cube c) They both have the same temperature

CPS question: Which one has more “internal energy”

ICEBERG ICE CUBE a) The iceberg b) The ice cube c) They both have the same energy

What is Heat? What causes it?

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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 0oC as the melting point of ice; 100oC 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

Measuring temperature (cont)

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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.

CPS question

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

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You only need to remember:

1K = 1 ° C

Absolute Zero & the Kelvin Scale

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Thermal Equilibrium

heat

http://jersey.uoregon.edu/vlab/Thermodynamics/index.html

The 0th law of thermodynamics

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Effects of heat:

• expansion of a body
• producing mechanical work
• increasing the temperature of a body
• melting a body
• vaporizing a liquid
• Liquefying a gas

Phase transitions

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:

T L L ∆ = ∆ α

α is the linear expansion coefficient and is material dependent

Nice property for building thermometers! (e.g. mercury)

• A. illustration #1
• 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.

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? #1 #2

http://freedrive.com/file/831762

Measuring Heat

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Quantity of Heat

Calorie: amount of heat needed to raise the temperature

• f 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 = 4.186 J

The calorie is NOT a fundamental SI unit. The Joule IS the standard unit of energy

Specific heat

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∆

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T mc Q ∆ =

For water: c = 1 cal/(g C

)=4190 J/(kg K)

http://www.chem.iastate.edu/group/Greenbowe/sections/projectfolder/flashfiles/thermochem/heat_metal.html

SI units

Definitions:

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.

Specific Heat is the amount of heat required to change temperature of one kilogram of a substance by one degree.

Heat capacity = Specific Heat x Mass. C=c.m

Avogadro constant Molar Heat Capacity specific heat in units of “Moles”, a certain number of atoms (Avogadro constant)

Specific heat values

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LARGE !!! Small !!! Small !!! (Dulong and Petit)

Phases of matter Phases of matter

Water, Steam, Ice

Water exists in a few phases Water exists in a few phases

• Liquid
• Solid (ice)
• Gas (steam)
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Gas Gas

Liquid Liquid

Solid Solid

Generally: Generally:

• Liquids denser than gas
• Solids denser than liquids

Not water! Not water!

Water most dense at 4 C Water most dense at 4 C Phase equilibrium Phase equilibrium

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Phase transitions 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 Example: Phase transition -

• melting

melting heat

melting freezing evaporation condensation sublimation deposition

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Latent heat

http://www.absorblearning.com/media/item.action?quick=zw Video demo: http://www.youtube.com/watch?gl=GB&hl=en-GB&v=1PcnCWZP7l0

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 Phase transitions can be affected by can be affected by pressure pressure

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
• f 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
• f 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.

A pitcher contains 0.50 kg of liquid water and 0.50 kg of ice at 0°

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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.

CPS Question

Heats of Fusion and Heats of Vaporization

Heat Transfer Processes

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http://www.wisc-online.com/ViewObject.aspx?ID=SCE304 http://www.kangwon.ac.kr/~sericc/sci_lab/physics/conduction/conduction.html

How does heat travel?

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Conduction

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Convection

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Radiation

Thermal Conductivity, k

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T2 T1

heat A =

L =

k =

T2 - T1

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T T L kA dt dQ H − = =

Heat current:

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T T L kA dt dQ H − = =

SI Units for Thermal Conductivity

k

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T T L kA H − =

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.

CPS question

Radiation: Power & Temperature

H

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H = AσeT

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e: emissivity (dimensionless number between 0 and 1) σ: Stefan-Boltzmann constant = 5.67x10-8 W/m2K4

Radiation and Absortion

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Tenv: Temperature of the environment surrounding the body