Transcript Diapositiva 1

Thermodynamics I
Temperature
•
Thermal Equilibrium and Temperature. Temperature scales
•
Absolute Temperature Scale. The Ideal-Gas Law
•
The Kinetic Theory of Gases. Pressure and Temperature
Heat
•
Heat. Heat capacity and Specific Heat
•
Change of Phase and Latent Heat
•
Thermal expansion and Phase Diagrams
•
Heat Transfer
•
Transport Laws
References: Tipler; wikipedia, Britannica
Thermodynamics II
The First Law of Thermodynamics
•
Heat and Work. First Law of Thermodynamics
•
Heat and Work on Quasi-Static Processes for a Gas.
The Second Law of Thermodynamics
•
Heat Engines and the Second Law of Thermodynamics
•
Refrigerators and the Second Law of Thermodynamics
•
The Carnot Engine
•
Heat Pumps
•
Irreversibility and disorder. Entropy
References: Tipler; wikipedia,…
Temperature
Thermal Equilibrium and Temperature. Temperature scales
Our sense of touch can usually tell us if an object is
hot or cold. Usually we need get in touch –physical
contact- to appreciate if a body is hot or cold.
But our perception is very subjective.
Temperature:
measure of hotness
and coldness in
terms of any
arbitrary scales and
indicating the
direction which
energy
spontaneously flows
(from a hotter body
to a colder one)
A thermometer is any of class of instrument that measures the
temperature. Temperature is the physical magnitude that is
measured by thermometers.
A physical property that changes with the temperature is called
a thermometric property
- most solids an liquids expand when they are heated
- electrical resistance change when is heated
- in a gas pressure and volume change when it is heated
- radiation from the surface of a body
-……
References: Tipler; Britannica
Temperature
• Thermal Equilibrium and Temperature. Temperature scales
Thermal contact
Thermal equilibrium
If two objects are in thermal equilibrium with
a third, then they are in thermal equilibrium
each other
(Zeroth Law of thermodynamics)
Two objects are defined to have the same
temperature if they are in thermal equilibrium
with each other. Temperature may be defined
as the property of a system that determines
whether it is in thermal equilibrium with other
system.
Temperature is one of the seven basic physical
quantities in term of which all other physical
quantities are defined. It is an “intensive”
property, as pressure or density. Length, mass
are “extensive”
Temperature
• Thermal Equilibrium and Temperature. Temperature scales
Thermal contact
Thermal equilibrium
If two objects are in thermal
equilibrium with a third, then they
are in thermal equilibrium each other
(Zeroth Law of thermodynamics)
Two objects are defined to have the
same temperature if they are in
thermal equilibrium with each other
Thermodynamics. Temperature
• Temperature scales
Calibration of thermometer:
Reproducibility and Reliability.
When the thermometric property changes
lineally with the temperature, two fixed
points can be used to calibrate.
Ice point temperature (normal
freezing point of water)
Steam-point temperature : normal
boiling point of water
Centigrade Temperature Scale
(Celsius scale)
Fahrenheit Temperature Scale
Absolute Temperature Scale
t F  95 (tC  32)
tC  95 (t F  32)
T  tC  273.15
Derive the expression to convert Fahrenheit temperature and centigrade temperature and the
inverese relationship. Apply to obtain the Fahrenheit normal human temperature if it is 36.5
centigrades degrees
Thermodynamics. Absolute Temperature Scale. Kelvin Scale.
It is possible to define a temperature scale in a
independent way of the used thermometric substance
A constant-volume gas
thermometer
Temperature of the boiling point of sulfur measured with
constant-volume gas thermometers . P100 is the pressure
of the gas at 100ºC
Plot of pressure versus
temperature for a gas,
as measured by a
constant-volume gas
thermometer. When
extrapolated to zero
pressure, the plot
intersects the
temperature axis at the
showed value of 273.25 ºC
The ideal-gas temperature scale
is defined so that the temperature
of the triple point state is 273.16
kelvins, K.
T
The triple point of water is the
unique temperature and pressure at
which water, water vapor and ice
coexist in equilibrium. [0.01 ºC and
4.58 mmHg]
Thermodynamics. Ideal Gas Law
The properties of gas samples that have low densities led to the
definition of the ideal-gas temperature scales. The behavior of
gases at this low densities was described
(1) by Boyle´s Law (1661)
PV = constant
(for a constant temperature)
(2) by Charles and Gay-Lussac Law (about 1800)
P = C1 T
(for a constant volume)
V = C2 T
(for a constant pressure)
T absolute temperatures; C1 and C2 constants
Ideal Gas-Law
PV  n R T
Equation of state of ideal gas
n = amount of gas expressed in moles
R : Universal gas constant
R = 8.314 J/(mol • K) =
= 0.082 atm • L/(mol • K)
The temperature of 0º (273 K) and the
pressure of 1 atm are often referred as
standard condition.
A mole (mol) of any substance is the
amount of substance that contains the
Avogadro number, NA, of atoms or
molecules, defined as the number of
carbon atoms in 12 g of 12C.
Thermodynamics. Dealing with the Ideal Gas Law
PV  n R T
Ideal Gas-Law
Equation of state of ideal gas
n = m/ M [mass of the substance in g/molecular mass] mol
R = 8.314 J/(mol • K) = 0.082 atm • L/(mol • K)
P

 RT
  R T
M
density
The mass per mole of a substance is called its
molar mass. (The terms molecular mass or
molecular weight are sometimes used
A gas has a volume of 2 L, a temperature of 30ºC, and a pressure of 1 atm. When
the gas is heated to 60ºC and compressed to a volume of 1.5 L, what is the new
pressure
What is the density of dry air at standard conditions of pressure and temperature?.
The same at 20ºC; The same at 20ºC and 933 mb.
Molecular mass: 28.97
g.Answer: 1.292 kg/m3; 1.204 kg/m3; 1.103 kg/m3
An automobile tire is filled to a gauge pressure of 200 kPa when its temperature is
20ºC. After the car has been driven at high speeds, the tire temperature increases to
50ºC. (a) Assuming that the tire volumen does not change, find the gauge pressure
in the tire (b) Calculate the gauge pressure if the volume of the tire expands by 10%.
Thermodynamics. The Kinetic Theory of Gases.
Molecular Interpretation of Pressure and Temperature
Goal : To relate macroscopic point of view abot pressure
and temperature with the microscopic motion.
For a solid, these microscopic motions are principally the
vibrations of its atoms about their sites in the solid. For an
ideal monatomic gas, the microscopic motions are the
translational motions of the constituent gas particles. For
a multiatomic gas, vibrational and rotational motion should
be included too.
The kinetic theory of gases is able us to establish
quantitatively this relationship for gases
The pressure that a gas exerts on its container is due to collisions between gas molecules and
the container walls. This pressure is a force per unit of area and, by Newton´s second law, this
force is the rate of change of momentum of the gas molecules colliding with the walls.
The absolute temperature is a measure of the average translational kinetic energy of the
molecules.
References
Tipler, Chapter 17
http://en.wikipedia.org/wiki/Kinetic_theory#Pressure
http://en.wikipedia.org/wiki/Temperature
http://en.wikipedia.org/wiki/Image:Translational_motion.gif
Thermodynamics I
Temperature
•
Thermal Equilibrium and Temperature. Temperature scales
•
Absolute Temperature Scale. The Ideal-Gas Law
•
The Kinetic Theory of Gases. Pressure and Temperature
Heat
• Heat. Heat capacity and Specific Heat
• Change of Phase and Latent Heat
• Thermal expansion and Phase Diagrams
• Heat Transfer
• Transport Laws
References: Tipler; wikipedia, Britannica
Thermodynamics. Heat. Heat capacity and Specific Heat
Heat is the energy that is being transferred from one system to another as a
result of difference in temperature.
If two bodies at different temperature are brought together, energy is
transferred –i.e. heat flows- from the hotter body to the colder. The effect of
this transfer of energy usually, but non always*, is an increase in the
temperature of the colder body and an decrease of the hotter body; the
amount of heat that leaves one equals the amount that enters the other.
Heat Capacity and Specific Heat
Q  CT  m c T
C heat capacity; c specific heat
cC
m
Units of heat: Calorie [cal] : the amount of
energy to be transferred to raise the temperature
of one gram of water one centigrade degree.
1cal = 4.184 J
The amount of heat energy Q needed to
raise the temperature of a substance is
proportional to the temperature change
and to the mass of substance.
cwater: 1 cal/(g•ºC)= 1kcal/(kg•ºC)=
4.184 kJ/(kg•ºC) = 4.184 kJ/(kg•K)
The heat capacity per mole is called
the molar specific heat
* The exceptions occurs during a change of phase
Thermodynamics. Heat. Heat capacity and Specific Heat
Heat is the energy that is being transferred from one system to another as a
result of difference in temperature.
If two bodies at different temperature are brought together, energy is
transferred –i.e. heat flows- from the hotter body to the colder. The effect of
this transfer of energy usually, but non always*, is an increase in the
temperature of the colder body and an decrease of the hotter body; the
amount of heat that leaves one equals the amount that enters the other.
Heat Capacity and Specific Heat
Q  CT  m c T
C heat capacity; c specific heat
cC
m
Units of heat: Calorie [cal] : the amount of
energy to be transferred to raise the temperature
of one gram of water one centigrade degree.
1cal = 4.184 J
The amount of heat energy Q needed to
raise the temperature of a substance is
proportional to the temperature change
and to the mass of substance.
cwater: 1 cal/(g•ºC)= 1kcal/(kg•ºC)=
4.184 kJ/(kg•ºC) = 4.184 kJ/(kg•K)
The specific heat of a substance depends of the way as
the heat is transferred. The most commonly determined
specific heats are the specific heat at constant pressure
and the specific heat at constant volume
The heat capacity per mole is
called the molar specific heat
* The exceptions occurs during
a change of phase
Thermodynamics. Change of Phase and Latent Heat
Common types of phase change include fusion (liquid to solid), melting
(solid to liquid), vaporization (liquid to vapor or gas); condensation (gas
or vapor to liquid), and sublimation (solid directly to vapor).
When a phase change appears there is no temperature change when the
thermal energy is being transferred to the body in which the phase
change is occurring. In the case of a phase change the specific heat (or
capacity) is infinitum.
Latent Heat
Qf  m Lf
QV  m LV
Latent heat of fusion, Lf. At a pressure of 1 atm, the
latent heat of fusion for water
Lf =333.5 KJ/kg
Latent heat of vaporization, LV . For water at a
pressure of 1 atm, the latent heat of vaporization is
Lf = 2.25 MJ/kg.
Latent heat depends on the temperature. The value
of
Thermodynamics. Change of Phase and Latent Heat
Common types of phase change include fusion (liquid to solid), melting
(solid to liquid), vaporization (liquid to vapor or gas); condensation (gas
or vapor to liquid), and sublimation (solid directly to vapor).
http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/phase.html
Thermodynamics. Change of Phase and Latent Heat
Common types of phase change include fusion (liquid to solid), melting
(solid to liquid), vaporization (liquid to vapor or gas); condensation (gas
or vapor to liquid), and sublimation (solid directly to vapor).
Evaporation
Ordinary evaporation is a surface phenomenon - some molecules have
enough kinetic energy to escape. If the container is closed, an equilibrium is
reached where an equal number of molecules return to the surface. The
pressure of this equilibrium is called the saturation vapor pressure.
In order to evaporate, a mass of water must collect the large heat of
vaporization, so evaporation is a potent cooling mechanism. Evaporation heat
loss is a major climatic factor and is crucial in the cooling of the human body.
http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/phase.html
Thermodynamics. Heat. Heat capacity and Specific Heat
Evaporation vs Boiling
Ordinary evaporation is a surface phenomenon - since the
vapor pressure is low and since the pressure inside the
liquid is equal to atmospheric pressure plus the liquid
pressure, bubbles of water vapor cannot form. But at the
boiling point, the saturated vapor pressure is equal to
atmospheric pressure, bubbles form, and the vaporization
becomes a volume phenomena.
http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/phase.html
Thermodynamics. Heat Transfer
Heat Transfer
The transfer of heat is normally from a high temperature object to a lower
temperature object. Heat transfer changes the internal energy of both systems
involved according to the First Law of Thermodynamics.
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
Thermodynamics. Heat Transfer
Heat Conduction
Conduction is heat transfer by means of molecular agitation within a material
without any motion of the material as a whole. If one end of a metal rod is at a
higher temperature, then energy will be transferred down the rod toward the
colder end because the higher speed particles will collide with the slower ones
with a net transfer of energy to the slower ones. For heat transfer between two
plane surfaces, such as heat loss through the wall of a house, the rate of
conduction heat transfer is:
Calculation
= area
= thermal conductivity of the barrier
= temperature
= heat transferred in time =
= thickness of
barrier
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html