# First Law of Thermodynamics

Mechanics

Oct 27, 2013 (4 years and 6 months ago)

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First Law of Thermodynamics

The first law of thermodynamics is the application of the
conservation of
energy

principle to heat and thermodynamic processes:

The first law makes use of the key concepts of
internal energy
,
heat
, and
system work
. It is used extensively in the discussion of
heat engines
.

It is typical for chemistry texts to write the first law as ΔU=Q+W. It is the
same law, of course
-

the thermodynamic expression of the conservation of
energy principle. It is just that W is defined as the work done
on

the system
by

th
e system. In the context of physics, the common
scenario is one of adding heat to a volume of gas and using the expansion of
that gas to do work, as in the pushing down of a piston in an internal
combustion engine. In the context of chemical reactions and
process, it may
be more common to deal with situations where work is done on the system
rather than by it.

Internal Energy in the Thermodynamic Identity

Index

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engine
concepts

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Second Law of Thermodynamics

The second law of thermodynamics is a general pr
inciple which
places constraints upon the direction of
heat transfer
and the
attainable efficiencies of
heat engines
. In so doing, it goes
beyond the limitations imposed by the
first law of
thermodynamics
. It's implications may be visualized in terms of
the waterfall analogy.

Second
Law

statements

Heat engine

Refrigerator

Entropy

Heat
transfer

The maximum efficiency which can be achieved is the
Carnot efficiency
.

Qualitative statements of the Second Law of Thermodynamics

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concepts

Heat
engine
concepts

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Second Law: Heat Engines

Second Law of Thermodynamics: It is impossible to extract an amount of heat
Q
H

from
a hot reservoir and use it all to do work W . Some amount of heat Q
C

must be exhausted to a cold reservoir. This precludes a perfect
heat engine
.

This is sometimes calle
d the "first form" of the second law, and is referred to
as the Kelvin
-
Planck statement of the second law.

Alternative statements: Second Law of Thermodynamics

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law
concepts

Heat
engine
concepts

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Second Law: Refrigerator

Second Law of Thermodynamics: It is not possible for
heat

to flow from a
colder body to a warmer body without any
work

having been done to
accomplish this flow. Energy w
ill not flow spontaneously from a low
temperature

object to a higher temperature object. This precludes a perfect
refrigerator
. The statements about refrigerators apply to air conditioners and
heat pumps
, which embody the same principles.

This is the "
second form" or Clausius statement of the second law.

Alte
rnative statements: Second Law of Thermodynamics

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concepts

Heat
engine
concepts

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Second Law: Entropy

Second Law of Thermodynamics: In any cyclic process the entropy will either
increase or remain the same.

Entropy
:

a state variable whose change is defined for a
reversible process at T where Q is the heat
absorbed.

Entropy:

a measure of the amount of energy which is
unavailable to do work.

Entropy
:

a measure of the disorder of a system.

Entropy
:

a measure of the multiplicity of a system.

Since entropy gives information about the evolution of an isolated system
with time, it is said to give us the direction of "
time's arrow
" . If snapshots of
a system at two different times shows one state which is more disordered,
then it could be implied that this state came later in time. For an isolated
system
, the natural course of events takes the system to a more disordered
(higher entropy) state.

Alternative statements: Second Law of Thermodynamics

Biological systems are highly ordered; how does that square with entropy?

Index

Second
law
concepts

Heat
engine
concepts

Entropy
concepts

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Qualitative Statements: Second Law
of Thermodynamics

The
second law of thermodynamics

is a profound principle of nature which
affects

the way energy can be used. There are several approaches to stating
this principle qualitatively. Here are some approaches to giving the basic
sense of the principle.

1. Heat will not flow spontaneously from a cold object to a hot
object.

Further

discussion

2. Any system which is free of external influences becomes
more disordered with time. This disorder can be expressed in
terms of the quantity called entropy.

Further

discussion

3. You cannot create a heat engine which extracts heat and
converts it all to useful work.

Further

discussion

4. There is a thermal bottleneck which contrains devices which
convert stored energy to heat and then use the heat to
accomplish work. For a given mechanical efficiency of the
devices, a machine which includes the
conversion to heat as one
of the steps will be inherently less efficient than one which is
purely mechanical.

Further

discussion

Alternative statements: Second Law of Thermodynamics

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Second
law
concepts

Heat
engine
concepts

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Internal Energy

Internal energy is defined as the
energy

associated with the random,
disordered motion of molecules. It is separated in scale from the macroscopic
ordered energy associated with moving objects; it refers to the invisible
microscopic energy

on the atomic and molecular scale. For example, a room
temperature glass of water sitting on a table has no apparent energy, either
potential

or
kinetic
. But on the microscopic scale it is a seething mass of high
speed molecules traveling at hundreds of meters per second. If the water were
tossed
across the room, this microscopic energy would not necessarily be
changed when we superimpose an ordered large scale motion on the water as
a whole.

U

is the most common symbol used for internal energy.

Related energy quantities which are particularly useful in chemical
thermodynamics are
enthalpy
,
Helmholtz free energy
, and
Gibbs free energy
.

Temperature and kinetic energy

Equipartition of energy

Thermal energy

Can a molecule's trajectory be predicted like that of a baseball?

In
dex

Internal
energy
concepts

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