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THERMODYNAMICS

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mechanics: mechanical (external) energies of systems, governed by Newton's laws; ... Hooke, Huygens, Boyle (1665): 'fixed points' -freezing or boiling point of water; ... – PowerPoint PPT presentation

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Title: THERMODYNAMICS


1
THERMODYNAMICS
  • Thermodynamics
  • (from Greek therme heat, dynamis strength,
    power) branch of physic dealing with energy
    transformations from and into thermal energy
  • mechanics mechanical (external) energies of
    systems, governed by Newton's laws
  • thermodynamics internal energy of systems and
    its relation to work
  • keywords of thermodynamics temperature, heat,
    internal/thermal energy, entropy
  • four laws of thermodynamics
  • heat transfer, thermal equilibrium
  • energy conservation
  • not all thermal energy is useful
  • impossibility to reach absolute zero temperature
  • topics to be discussed
  • thermal energy, temperature, heat
  • 0th law
  • temperature scales
  • thermal expansion
  • heat capacity, specific heat
  • heat transfer conduction, convection, radiation
  • 1st law
  • heat engines, efficiency

2
thermal energy, temperature, heat
  • Brownian motion
  • Robert Brown observed burlap seeds dancing in
    water (1827) explained by A. Einstein (1905)
    calculated mea net distance travelled by random
    motion
  • experimental verification by Jean Perrin (1908).
  • thermal motion
  • disorganized random motion of constituent atoms
    and molecules within body of matter
  • thermal energy
  • kinetic energy of thermal motion (translational,
    rotational, vibrational) associated with ensemble
    of particles
  • temperature
  • is measure of average value of thermal energy of
    atoms and molecules (not total amount of thermal
    energy)
  • (temperature of a substance is independent of
    total number of atoms/molecules)
  • is a measure of the ability of randomly moving
    particles to impart thermal energy to a
    thermometer
  • heat
  • thermal energy transferred from a region of
    high temperature to region of lower temperature
  • body stores thermal energy (internal energy)
  • heat thermal energy in transit

3
0th law of thermodynamics
  • between bodies of different temperature (i.e. of
    different average internal thermal energy), heat
    will flow from the body of higher temperature to
    the body of lower temperature until the
    temperatures of the two bodies are the same
  • then the bodies are in thermal equilibrium
  • two bodies are in thermal equilibrium (at same
    temperature) if there is no heat flow between
    them
  • corollary if two bodies are in thermal
    equilibrium with a third body, then they are in
    thermal equilibrium with each other.
  • ? can use thermometer to compare temperature
  • note
  • observation only shows that temperatures equalize
    - heat flow is hypothesis

4
TEMPERATURE SCALES
  • Temperature
  • was measured long before it was understood
  • Galilei (around 1592) device to measure degree
    of hotness inverted narrow-necked flask,warmed
    inhand, put upside down into liquid liquid level
    indicates temperature OK, but not calibrated.
  • Hooke, Huygens, Boyle (1665) fixed points
    -freezing or boiling point of water
  • C. Renaldini (1694) use both freezing and
    boiling point.
  • Fahrenheit scale
  • Gabriel Daniel Fahrenheit (Danzig, 1686-1736),
    glassblower and physicist
  • reproducible thermometer using mercury (liquid
    throughout range) (around 1715)
  • 0 point lowest temperature of winter of 1709,
    (using mix of water, ice, salt)
  • 96o body temperature (96 divisible by 12, 8),
  • water freezes at 32oF, boils at 212oF
  • Celsius scale
  • Anders Celsius (Swedish astronomer, 1701 - 1744)
  • 0o C ice point (mixture of water and ice at 1
    atm)
  • 100o C boiling point of water at 1 atm. (1742)
  • relation between Fahrenheit and Celsius degrees
  • TC (5/9)(TF - 32 ) , TF (9/5)TC 32

5
Temperature, contd
  • thermodynamic temperature scale
  • (absolute, Kelvin scale)
  • pressure vs temperature of gas at constant volume
    and volume vs temperature of gas at constant
    pressure extrapolate to zero at - 273.15o C
  • this is absolute zero
  • unit Kelvin
  • Range of temperatures
  • highest temperature in core of stars, ?4?109 K
    seems maximum
  • hydrogen bomb ignites at , ?4?107K
  • interior of Sun , ?1.5?106K
  • plasma ?105K
  • ?105K clouds of atoms, ions, e, occasional
    molecule
  • 5800 K surface of the Sun 5000 K cool spots
    at surface of the Sun evidence for some
    molecules
  • 3000 K water steam about 1/4 of water
    molecules ruptured into atoms
  • 2800 K W light bulb filament
  • 2000 K molten lava
  • 1520 oC iron melts 327 o C lead melts
  • 100oC (373 K) water boils
  • 252 K temp. of salt-ice mix
  • 234 K mercury freezes 194 K dry ice freezes

6
THERMAL EXPANSION
  • solids, liquids and gases
  • expand when heated
  • knowledge about this is old e.g. red-hot iron
    rims put on wagon wheels
  • thermometers are based on this
  • heating ? internal energy rises ? vibrations
    have larger amplitude, equilibrium positions move
    farther apart.
  • typical metal expands by about 7 between 0 K and
    melting point.
  • ?L/L0 ? ?T, ? coefficient of thermal
    expansion
  • examples for values of ? (in units of 10-6)
  • iron 10,
  • brass 19,
  • lead 30,
  • Pyrex glass 3,
  • ordinary glass 5 to 10,
  • concrete 10 to 14
  • mercury 60,
  • ethanol 250
  • have to account for this in construction, e.g.
    expansion joints at end of bridge, gaps in rails
    also in dental fillings
  • uses thermostats, thermometers (bimetal strips)
  • anomaly of water maximum of density at 4oC.

7
HEAT CAPACITY
  • Heat capacity
  • measure of ability of a substance to absorb
    thermal energy
  • specific heat capacity heat capacity per unit
    mass
  • Q c m ?T,
    Q amount of thermal
    energy added, c
    specific heat capacity,
    ?T raise in
    temperature
  • 1 calorie 1 cal ( 4.186 J) thermal energy
    necessary to raise temperature of 1 gram of water
    by 1 degree Celsius
  • 1 kcal 1 Cal thermal energy necessary to
    raise temperature of 1 kilogram of water by 1
    degree Celsius
    called calorie
    in nutrition
  • water has high specific heat capacity
    ?moderating influence on climate
  • some values of specific heat capacity
  • aluminum 0.21
  • clay 0.22
  • glass 0.20
  • marble 0.21
  • iron 0.11
  • air 0.24
  • water 1.00

8
HEAT TRANSFER
  • Conduction
  • heat transfer by atomic/molecular collisions
  • thermal conductivity ability of substance to
    transmit heat, depends on atomic/molecular
    structure
  • metals typically 400 times better than other
    solids
  • most solids little better than liquids
  • liquids about 10 times better than gases
  • good heat conductor usually good electric
    conductor
  • Convection
  • heat transfer by motion of hot matter change of
    density of fluid (liquid or gas) due to heating
  • flow of fluid up, away from heat source
  • dominant mechanism for many heat loss processes
    in air
  • examples household radiator, hurricanes
  • purpose of fur, feathers, clothing,
    blanketsprevent convection
  • chill-factor
  • Radiation
  • heat transfer by emission and absorption of
    electromagnetic radiation e.g. Earth receives
    1.4kW/m2 by radiation from the Sun.
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