Transcript THERMODYNAMICS - FSU High Energy Physics

THERMODYNAMICS
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Thermodynamics
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(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:
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thermal energy, temperature, heat
0th law
temperature scales
thermal expansion
heat capacity, specific heat
heat transfer: conduction, convection, radiation
1st law
heat engines, efficiency
2nd law, entropy
thermal energy, temperature, heat
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Brownian motion:
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thermal motion:
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kinetic energy of thermal motion (translational,
rotational, vibrational) associated with ensemble of
particles
temperature:
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disorganized random motion of constituent atoms
and molecules within body of matter;
thermal energy:
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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).
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
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= 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”
0th law of thermodynamics
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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
TEMPERATURE SCALES
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Temperature:
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Fahrenheit scale:
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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)
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 96 = body temperature (96 divisible by 12, 8),
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 water freezes at 32 F, boils at 212 F
Celsius scale:
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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.
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:
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TC = (5/9)(TF - 32 ) , TF = (9/5)TC + 32
Temperature, cont’d
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thermodynamic temperature scale
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(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
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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;
77 K: nitrogen boils
4 K: helium boils.
THERMAL EXPANSION
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solids, liquids and gases:
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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.
HEAT CAPACITY
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Heat capacity =
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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
HEAT TRANSFER
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Conduction:
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Convection:
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= 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
= 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, blankets:prevent
convection
“chill-factor”
Radiation
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= heat transfer by emission and absorption of
electromagnetic radiation; e.g. Earth receives
1.4kW/m2 by radiation from the Sun.