Download First law of thermodynamics

SYSTEM, SURROUNDINGS, BOUNDARY
First law of thermodynamics
The first law of thermodynamics is a version of the law of conservation of energy, adapted for
thermodynamic systems. The law of conservation of energy states that the total energy of an isolated
system is constant; energy can be transformed from one form to another, but cannot be created or
destroyed.
For a closed system, in any process, the change in the internal energy is considered due to a
combination of heat added to the system and work done by the system. Taking
as a change
in internal energy, one writes
where and
are quantities of heat supplied to the system by its surroundings and of work
done by the system on its surroundings, respectively. This sign convention is implicit in
Clausius' statement of the law given above, and is consistent with the use of thermodynamics to
study heat engines, which provide useful work that is regarded as positive.
In modern style of teaching science, however, it is conventional to use the IUPAC convention by
which the first law is formulated in terms of the work done on the system. With this alternate
sign convention for work, the first law for a closed system may be written:
[20]
This convention follows physicists such as Max Planck,[21] and considers all net energy transfers
to the system as positive and all net energy transfers from the system as negative, irrespective of
any use for the system as an engine or other device.
When a system expands in a fictive quasistatic process, the work done by the system on the
environment is the product, P dV, of pressure, P, and volume change, dV, whereas the work
done on the system is -P dV. Using either sign convention for work, the change in internal
energy of the system is:
where δQ denotes the infinitesimal increment of heat supplied to the system from its
surroundings.
Work and heat are expressions of actual physical processes of supply or removal of energy,
while the internal energy U is a mathematical abstraction that keeps account of the exchanges of
energy that befall the system. Thus the term heat for Q means "that amount of energy added or
removed by conduction of heat or by thermal radiation", rather than referring to a form of energy
within the system. Likewise, the term work energy for W means "that amount of energy gained
or lost as the result of work". Internal energy is a property of the system whereas work done and
heat supplied are not. A significant result of this distinction is that a given internal energy change
ΔU can be achieved by, in principle, many combinations of heat and work.
Quick
The first law of thermodynamics states that during any cycle a system (control mass) undergoes,
the cyclic integral of the heat is proportional to the cyclic integral of the work.
Equation
(Eq2)
∮δQ =∮δW
basic statement of the first law of thermodynamics
Nomenclature
∮δQ the cyclic integral of the heat transfer, represents the net heat transfer during the cycle
∮δW the cyclic integral of the work, represents the net work during the cycle
J
proportionality factor that depends on the units used for work and heat
Consider as a control mass the gas in the container as shown:
Let this system go through a cycle that is made up of two processes. In the first process work is
done on the system by the paddle that turns as the weight is lowered. Let the system then return
to its initial state by transferring heat from the system until the cycle has been completed.
Originally, work was measured in mechanical units of force times distance, such as foot pounds
force or joules, and heat was measured in thermal units, such as the British thermal unit (Btu) or
the calorie. Measurements of work and heat were made during a cycle for a wide variety of
systems and for various amounts of work and heat. When the amounts of work and heat were
compared, it was found that they were always proportional. Such observations led to the
formulation of the first law of thermodynamics, which in equation form is written:
(Eq1)
J
∮δQ =∮δW
The basis of every law of nature is experimental evidence, and this is true also of the first law of
thermodynamics. Many different experiments have been conducted on the first law, and every
one thus far has verified it either directly or indirectly. The first law has never been disproved.
The units for work and heat or for any other form of energy either are the same or are directly
proportional. In SI units, the joule is used as the unit for both work and heat ad for any other
energy unit. In English units, the basic unit for work is the foot pound force, and the basic unit
for heat is the British thermal unit (Btu). The Btu is defined in terms of the basic SI metric units:
1 Btu = 778.17 ft lbf
This unit is termed the International British thermal unit. For much engineering work, the
accuracy of other data does not warrant more accuracy than the relation 1 Btu = 778 ft lbf.
Because these units are equivalent, it is not necessary to include the factor J explicitly in Eq1,
but simply to recognize that for any system of units, each equation must have consistent units
throughout. Therefore, Eq1 may be written as:
(Eq2)
∮δQ =∮δW
which can be considered the basic statement of the first law of thermodynamics.
Zeroth law of thermodynamics
Statement of Zeroth law of thermodynamics : "If two systems are each in thermal equilibrium (uniform
temperature at all points in system)with third system, they are also in thermal equilibrium with each
other".
The statement is the basis of the concept of temperature as well as it is basis of all temperature
measurement. Why/Reason? If we want to measure the temperature of our body (system A), we put
thermometer (system B) in our mouth and waif for some time so that both system A and B comes to
equal temperature i.e thermal equilibrium.
First law of thermodynamics
Statement of first law of thermodynamics : For a non-flow processes for system,
(1) "If a system undergoes a cycle ( the initial and final state are same after completion of process), then
the algebraic sum of all work delivered to the surrounding is proportional to the algebraic sum of heat
taken from the surrounding". or
(2) "Heat and work are mutually convertible one into other.
But according to the law of conversational of energy, energy can neither be created nor created, it just
change the form of it".
(3) "It is impossible to construct a perpetual motion machine of the first kind, PMM-I. Where PMM-I is
capable of producing energy without corresponding use of input energy".
Ultimately first law deals with energy conservation. In automobile, heat of combustion is converted to
work/rotation of wheel. The figure shows dropping of weight stirs the water and thus temperature of
water increases, so, work in form of potential energy is converted to heat energy or rise of temperature.
Second law of thermodynamics
Before look at the second law of thermodynamics, let us discuss where first law of thermodynamics
fails. According to the first law of thermodynamics, whenever any process occurs, there may be either
heat interaction or work interaction. But we cannot exactly tell by first law that in which direction
interaction takes place. i.e. if in case of heat transfer, we cannot tell by first law, from where to where
heat will be transferred. Also first law does not tell the extent of energy conversion in form of heat and
work. So, there must be a law which tells the unsolved answer for first law. The law which gives all
answers of limitation of first law is 'Second law of thermodynamics.
Statement of Second law of thermodynamics :
Second law can be stated in two ways according to the
(i) Direction of heat flow and
(ii) Extent of energy conversion
Direction of heat flow / Clausius statement
"It is impossible to construct a heat pump (heat pump or refrigerator) which operates in a cycle will
remove the heat continuously from heat sink (low temperature reservoir) and transfer it to heat source
(higher temperature reservoir) without any amount of external work being done on it". or In other
words,
"Heat cannot itself flow from a colder body to a hotter body".
Thus, Clausius statement of second law gives direction of heat and work interaction. The statement is
mostly suitable for heat pump. The statement is given by Scientist named Clausius
Extent of energy conversion / Kelvin-Plank statement
"It is impossible to construct an engine which will work in a complete cycle that produce equal amount
of work that is equivalent to heat absorption" or In other words,
"It is impossible to construct an engine which gives 100% efficiency, thus this statement gives extent of
energy conversion. This statement is mostly suitable for heat engine. The statement is given by two
Scientist named Kelvin and Plank".
Third law of thermodynamics
Statement of Third law of thermodynamics : The entropy of a pure crystal body or crystal structure is
almost zero/constant at absolute zero temperature. Absolute zero means Zero Kelvin or -273.15 degree
centigrade. Actually entropy is measure of disorder of the system and this disorder increases as the
temperature increases and decrease as temperature of the system decreases.