Definition of Thermodynamic & Laws |
What are Thermodynamics?
Heat is
energy can be converted from one form to another or transferred from one object
to another. For example, a stove burner converts electrical energy to heat and conducts
that energy through the pot to the water. This increases the kinetic energy of
the water molecules, causing them to move faster and faster. At a certain
temperature (the boiling point), the atoms have gained enough energy to break
free of the molecular bonds of the liquid and escape as vapor.
Thermodynamics
is the branch of physics that deals with the relationships between heat and
other forms of energy. In particular, it describes how thermal energy is
converted to and from other forms of energy and how it affects matter.
Thermal
energy is the energy a substance or system has due to its temperature, i.e.,
the energy of moving or vibrating molecules, according to the Energy
Education website of the Texas Education Agency. Thermodynamics involves
measuring this energy, which can be "exceedingly complicated,"
according to David McKee, a professor of physics at Missouri Southern State
University. "The systems that we study in thermodynamics … consist of very
large numbers of atoms or molecules interacting in complicated ways. But, if
these systems meet the right criteria, which we call equilibrium, they can be
described with a very small number of measurements or numbers. Often this is
idealized as the mass of the system, the pressure of the system, and the volume
of the system, or some other equivalent set of numbers. Three numbers describe
1026 or 1030 nominal independent variables."
Heat
Thermodynamics,
then, is concerned with several properties of matter; foremost among these is
heat. Heat is energy transferred between substances or systems due to a
temperature difference between them, according to Energy Education. As a form
of energy, heat is conserved, i.e., it cannot be created or destroyed. It can,
however, be transferred from one place to another. Heat can also be converted
to and from other forms of energy. For example, a steam turbine can convert
heat to kinetic energy to run a generator that converts kinetic energy to
electrical energy. A light bulb can convert this electrical energy to
electromagnetic radiation (light), which, when absorbed by a surface, is
converted back into heat.
Temperature
The
amount of heat transferred by a substance depends on the speed and number of
atoms or molecules in motion, according to Energy Education. The faster
the atoms or molecules move, the higher the temperature, and the more atoms or
molecules that are in motion, the greater the quantity of heat they transfer.
Temperature
is "a measure of the average kinetic energy of the particles in a sample
of matter, expressed in terms of units or degrees designated on a standard
scale," according to the American Heritage Dictionary. The most
commonly used temperature scale is Celsius, which is based on the freezing and
boiling points of water, assigning respective values of 0 degrees C and 100
degrees C. The Fahrenheit scale is also based on the freezing and boiling
points of water which have assigned values of 32 F and 212 F,
respectively.
Scientists
worldwide, however, use the Kelvin (K with no degree sign) scale, named
after William Thomson, 1st Baron Kelvin, because it works in calculations.
This scale uses the same increment as the Celsius scale, i.e., a temperature
change of 1 C is equal to 1 K. However, the Kelvin scale starts at absolute
zero, the temperature at which there is a total absence of heat energy and all
molecular motion stops. A temperature of 0 K is equal to minus 459.67 F or
minus 273.15 C.
Specific heat
The
amount of heat required to increase the temperature of a certain mass of a
substance by a certain amount is called specific heat, or specific heat
capacity, according to Wolfram Research. The conventional unit for this is
calories per gram per kelvin. The calorie is defined as the amount of heat
energy required to raise the temperature of 1 gram of water at 4 C by 1
degree.
The
specific heat of a metal depends almost entirely on the number of atoms in the
sample, not its mass. For instance, a kilogram of aluminum can absorb
about seven times more heat than a kilogram of lead. However, lead atoms can
absorb only about 8 percent more heat than an equal number of aluminum atoms. A
given mass of water, however, can absorb nearly five times as much heat as an
equal mass of aluminum. The specific heat of a gas is more complex and depends
on whether it is measured at constant pressure or constant volume.
Thermal conductivity
Thermal
conductivity (k) is “the rate at which heat passes through a specified
material, expressed as the amount of heat that flows per unit time through a
unit area with a temperature gradient of one degree per unit distance,”
according to the Oxford Dictionary. The unit for k is watts
(W) per meter (m) per kelvin (K). Values of k for metals such as
copper and silver are relatively high at 401 and 428 W/m·K, respectively. This
property makes these materials useful for automobile radiators and cooling fins
for computer chips because they can carry away heat quickly and exchange it
with the environment. The highest value of k for any natural
substance is diamond at 2,200 W/m·K.
Other
materials are useful because they are extremely poor conductors of heat; this
property is referred to as thermal resistance, or R-value, which
describes the rate at which heat is transmitted through the material. These
materials, such as rock wool, goose down and Styrofoam, are used for insulation
in exterior building walls, winter coats and thermal coffee mugs. R-value
is given in units of square feet times degrees Fahrenheit times hours
per British thermal unit (ft2·°F·h/Btu) for a 1-inch-thick slab.
Newton's Law of Cooling
In
1701, Sir Isaac Newton first stated his Law of Cooling in a short
article titled "Scala graduum Caloris" ("A Scale of the Degrees
of Heat") in the Philosophical Transactions of the Royal Society. Newton's
statement of the law translates from the original Latin as, "the excess of
the degrees of the heat ... were in geometrical progression when the times are
in an arithmetical progression." Worcester Polytechnic
Institute gives a more modern version of the law as "the rate of
change of temperature is proportional to the difference between the temperature
of the object and that of the surrounding environment."
This
results in an exponential decay in the temperature difference. For
example, if a warm object is placed in a cold bath, within a certain length of
time, the difference in their temperatures will decrease by half. Then in that
same length of time, the remaining difference will again decrease by half. This
repeated halving of the temperature difference will continue at equal time
intervals until it becomes too small to measure.
Heat transfer
Heat can
be transferred from one body to another or between a body and the environment
by three different means: conduction, convection and radiation. Conduction is
the transfer of energy through a solid material. Conduction between
bodies occurs when they are in direct contact, and molecules transfer their
energy across the interface.
Convection
is the transfer of heat to or from a fluid medium. Molecules in a gas or liquid
in contact with a solid body transmit or absorb heat to or from that body and
then move away, allowing other molecules to move into place and repeat the
process. Efficiency can be improved by increasing the surface area to be heated
or cooled, as with a radiator, and by forcing the fluid to move over the
surface, as with a fan.
Radiation
is the emission of electromagnetic (EM) energy, particularly infrared photons
that carry heat energy. All matter emits and absorbs some EM radiation, the net
amount of which determines whether this causes a loss or gain in heat.
The Carnot cycle
In
1824, Nicolas Léonard Sadi Carnot proposed a model for a heat engine
based on what has come to be known as the Carnot cycle. The cycle exploits
the relationships among pressure, volume and temperature of gasses and how an
input of energy can change form and do work outside the system.
Compressing
a gas increases its temperature so it becomes hotter than its environment. Heat
can then be removed from the hot gas using a heat exchanger. Then,
allowing it to expand causes it to cool. This is the basic principle behind
heat pumps used for heating, air conditioning and refrigeration.
Conversely,
heating a gas increases its pressure, causing it to expand. The expansive
pressure can then be used to drive a piston, thus converting heat energy into
kinetic energy. This is the basic principle behind heat engines.
Entropy
All thermodynamic
systems generate waste heat. This waste results in an increase in entropy,
which for a closed system is "a quantitative measure of the amount of
thermal energy not available to do work," according to the American
Heritage Dictionary. Entropy in any closed system always increases;
it neverdecreases. Additionally, moving parts produce waste heat
due to friction, and radiative heat inevitably leaks from the system.
Entropy
is also defined as "a measure of the disorder or randomness in a closed system,"
which also inexorably increases. You can mix hot and cold water, but because a
large cup of warm water is more disordered than two smaller cups containing hot
and cold water, you can never separate it back into hot and cold without adding
energy to the system. Put another way, you can’t unscramble an egg or remove
cream from your coffee. While some processes appear to be completely
reversible, in practice, none actually are. Entropy, therefore, provides us
with an arrow of time: forward is the direction of increasing entropy.
The four laws of thermodynamics
The fundamental principles of
thermodynamics were originally expressed in three laws. Later, it was
determined that a more fundamental law had been neglected, apparently because
it had seemed so obvious that it did not need to be stated explicitly. To form
a complete set of rules, scientists decided this most fundamental law needed to
be included. The problem, though, was that the first three laws had already
been established and were well known by their assigned numbers. When faced with
the prospect of renumbering the existing laws, which would cause considerable
confusion, or placing the pre-eminent law at the end of the list, which would
make no logical sense, a British physicist, Ralph H. Fowler, came up with
an alternative that solved the dilemma: he called the new law the “Zeroth Law.”
In brief, these laws are:
The Zeroth Law states that if two bodies are in thermal equilibrium with
some third body, then they are also in equilibrium with each other. This
establishes temperature as a fundamental and measurable property of matter.
The First Law states that the total increase in the energy of a system is
equal to the increase in thermal energy plus the work done on the system. This
states that heat is a form of energy and is therefore subject to the principle
of conservation.
The Second Law states that heat energy cannot be transferred from a body at
a lower temperature to a body at a higher temperature without the addition of
energy. This is why it costs money to run an air conditioner.
The Third Law states that the entropy of a pure crystal at absolute zero is
zero. As explained above, entropy is sometimes called "waste energy,"
i.e., energy that is unable to do work, and since there is no heat energy
whatsoever at absolute zero, there can be no waste energy. Entropy is also a
measure of the disorder in a system, and while a perfect crystal is by
definition perfectly ordered, any positive value of temperature means there is
motion within the crystal, which causes disorder. For these reasons, there can
be no physical system with lower entropy, so entropy always has a positive
value.
The
science of thermodynamics has been developed over centuries, and its principles
apply to nearly every device ever invented. Its importance in modern technology
cannot be overstated.
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