The first law of thermodynamics
The first law of
thermodynamics distinguishes between two kinds of transfer of energy, transfer
of energy as heat and as thermodynamics work. The first law of thermodynamics
also relates work and heat to the system state called internal energy.
Before going deep into the concepts we need to explain some
important terms of energy
Internal energy:
Energy is associated with the molecules of the system which
includes the kinetic and potential energy of molecules.
What is the first law of thermodynamics?
The first law of thermodynamics is a version law of conservation of energy:
The law of conservation states that
“Energy can neither be created nor be destroyed; can be transferred
from one form to another but the total energy of an isolated system remains
constant.
The first law of thermodynamics definition:
The first law of thermodynamics states that:
“Energy
can neither be created nor be destroyed but it transformed from one form to
another.”
Equations of the first law of thermodynamics:
In the case in which matter is not transferred:
For the closed system, the change in internal energy is
equal to the difference between the energy given to the system and the work
done by the system.
U
= Q-W
U= denotes the internal energy of the system
Q=denotes the amount of energy supplied to a system
W= denotes the amount of work done by the system
In the case in which matter is transferred:
For an open system in which matter can be transferred another statement is needed. When the two systems with internal energies U1 AND U2 are separated by an impermeable wall when
the wall is made permeable or even removed the internal of the combined system
or a new system is equal to the sum of internal energies U1 AND U2.
U0 = U1
AND U2
U0= denotes the inter energy of the combined system
U1 and u2 = denotes the internal energies of a separated system.
What are examples of the first law of thermodynamics?
Examples of the first law of thermodynamics:
·
A heat engine is an example of the first law of
thermodynamics in which thermal energy is converted to mechanical energy.
·
Human metabolism
History :
Many statements encountered as the consequences to the first law of thermodynamics the few main
statements are explained below:
Hess statement: In 1840, Germain Hess stated a law for the
heat of reaction in a chemical reaction that is called conservation law.
Julius Robert statement: In 1842, Julius Robert von Mayer made a
statement, which is also considered a consequence of the first law of thermodynamics.
In the process, the heat used to produce expansion is
universally interconvertible into work at constant pressure.
This statement is rendered by Truesdell in 1980.
Rudolf Clausius (Original statement of the first law of
thermodynamics:
In 1850, Rudolf
Clausius gave the first statement of the first law of thermodynamics, which referred
to cyclic thermodynamics processes.
“In all cases in which work is produced by the agency of
heat, an amount of work produced is proportional to the amount of heat utilized;
and conversely, the amount of heat utilized proportion to the amount of work.”
Clausius also stated another form, which is referred to the
internal energy of the system.
In the thermodynamics process in a closed system, the
increment in internal energy of the system is equal to the difference between
heat accumulated from the system and work done by the system.
U = H - W
The first law of thermodynamics for the closed system:
A close system is a system in which there is no transfer of
matter. Work done for a closed system is the product of the pressure and change
in volume of the system.
W= -pv
P = constant external pressure on the system
V= change in the volume of the system
Explanation:
The internal energy of the system changes when energy leaves
and enters the system. The internal energy of the system increases or decreases
depending upon the interaction of the work with the boundaries of the system.
The internal energy of the system increases if work is done on the system and
decreases if work is done by the system. This also happens by adding energy or
heat to the system. The internal energy of the system increases if heat is
added to the system and decreases when the heat is removed from the system.
Relation between system and surroundings:
The total energy of the system remains constant throughout
the process. if energy is absorbed by the system then energy is released by the
surrounding. If the energy is lost then the energy is absorbed by the system.
U
system= -u surrounding
U= energy of the system
U= energy of the surrounding
Sign convections:
Some sign convections are commonly used for the heat and
work when heat add and release from the system, and work is done on and by the
system.
|
process |
Sign
convention for work |
Sign
convention for heat |
|
Work done on
the system |
+ |
|
|
Work done by
the system |
- |
|
|
Heat added to
the system |
|
+ |
|
The heat
extracted from the system |
|
- |
Sign conventions for the process:
There is some sign convention for the process which takes
place like adiabatic, isothermal in a closed system.
|
The
process |
Internal
energy change |
Heat
transfer of thermal energy |
Work( w=-PV) |
example |
|
q=o Adiabatic |
+ |
0 |
+ |
Isolated
system in which no heat enter can leave the system. |
|
V=0
constant volume |
+ |
+ |
0 |
A hard,
pressure isolated system |
|
Constant
pressure |
+ or - |
Enthalpy
(H) |
-PV |
Most
processes that occur are constant external pressure.
|
|
T=0
isothermal |
0 |
+ |
|
There is
no change in the temperature like in the temperature bath. |
What are the limitations of the first law of thermodynamics?
Limitations of the first law of thermodynamics:
There are some limitations in the first law of
thermodynamics, therefore there is a need for the second law of thermodynamics.
The few limitations of the first law of thermodynamics are as follows
The first law of thermodynamics fails to explain the state
change of the system, which a system undergoes during the process.
The first law of thermodynamics explains us energy transfer from
one form to another but don does not explain how energy is transformed from another
or the flow of energy.
Perpetual motion machine: perpetual motion machine is
a physical example that violates the first law of thermodynamics. The main
principle of this is “ It is impossible
to construct a machine which converts all the energy into mechanical work
without wasting or consuming it.
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