Atmospheric Thermodynamics Thermodynamic Systems This section examines the atmosphere as a system, and several atmospheric phenomena that have become critical in the understanding and guardianship of our environment. The current state of the atmosphere is the result of a multitude of facts. The energy from the sun produces the movements or currents in the atmosphere. This energy, the Earth's movement relative to the sun and the components of the atmosphere and of the Earth’s surface maintains the long-term climate, the short-term weather, and the temperature conditions. These provide conditions fit for the forms of life found on Earth. The condition of the physical world affects and is affected by the life present. The entire system is therefore called the biogeochemical system. In the last century especially, this system, which evolved over billions of years, has been subject to rapid changes due to industrial activities increasing at unprecedented rates. This section discusses some of the basic science and details of the interactions involving the atmosphere. We begin by examining the nature of the sun's energy, and the actions and reactions it
44
Embed
ugphysag.files.wordpress.com · Web viewAtmospheric ThermodynamicsThermodynamic SystemsThis section examines the atmosphere as a system, and several atmospheric phenomena that have
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Atmospheric Thermodynamics
Thermodynamic Systems
This section examines the atmosphere as a system, and several atmospheric phenomena that have
become critical in the understanding and guardianship of our environment. The current state of
the atmosphere is the result of a multitude of facts. The energy from the sun produces the
movements or currents in the atmosphere. This energy, the Earth's movement relative to the sun
and the components of the atmosphere and of the Earth’s surface maintains the long-term
climate, the short-term weather, and the temperature conditions. These provide conditions fit for
the forms of life found on Earth. The condition of the physical world affects and is affected by
the life present. The entire system is therefore called the biogeochemical system. In the last
century especially, this system, which evolved over billions of years, has been subject to rapid
changes due to industrial activities increasing at unprecedented rates.
This section discusses some of the basic science and details of the interactions involving the
atmosphere. We begin by examining the nature of the sun's energy, and the actions and reactions
it produces in the atmosphere. We then discuss how industrial activity has perturbed atmospheric
conditions, and what policy actions are being taken to reduce our impact. Due the complexity of
the atmospheric system, there are still large amounts of scientific uncertainty in predicting
changes precisely, but we do know enough to describe and project qualitative features of the
system that lead to an understanding of the impacts of large scale human activity.
A thermodynamic system is a quantity of matter or any region of space to which one directs
attention for the purposes of analysis. The quantity of matter or region of space must be within
prescribed boundary which may be deformable or rigid. It may even be imaginary.
Surrounding
everything outside a system boundary is referred to as the surrounding. Usually, term
surrounding is restricted to those things outside the system that in some way influence the
system.
Types of Systems
The First Law of Therodynamics
the first law of thermodynamics is often applied to process as the system changes from state to
another. Heat is a form of energy that cannot be destroyed. Because of this, there is an amount of
energy associated with every thermodynamics system called its internal energy, U which can be
changed by adding or subtracting energy of any form, and that the algebraic sum of the added or
subtracted amount is equal to the change in of internal energy, dU of the system. Assume that a
system transform from state (1) to another state (2), then there would be a change in internal
energy considering the difference in energy, Q supplied to the system and the work, W done by
the system. This can be expressed as:
dQ−dW=dU
dQ=dU+dW - - - - - - - - - - - - - - (1)
Equation (1) can every thermodynamic process such as (i) isobaric, (ii) isochoric, (iii) isothermal
and (iv) adiabatic processes.
Enthalpy
In trying to offer solution to the problems involving systems, certain products or sum of
properties occur with regularity. One of such a combination can be demonstrated by going
through a simple experiment below. Just add heat to a constant-pressure cylinder containing a
gas trapped a piston.
Heat is added slowly to the system (the gas in the cylinder) maintained at a constant pressure
under a condition where there is no friction between the cylinder and the piston. Also if the
kinetic and potential changes are neglected, and all other work mode are absent, then the first law
of thermodynamics requires that:
Q−W=U2−U1
- - - - - - - - - - - - - ()
where Q is the energy put into the system, W the work done in raising the piston up and U2 –U1 is
the change in internal energy.
But the work done on the piston is given as:
dW=pdV
W=p [V 2−V 1 ] - - - - - - - - - - - - - - ()
Putting equation () in equation () gives:
Q−p [V 2−V 1 ]=U2−U1
Q=p [V 2−V 1 ]+U 2−U 1
The quantity in the parenthesis is a combination of thermodynamic properties (pressure, volume
and internal energy) of the system. It is called the enthalpy, H of the system.
H=U+ pV
Equation () is energy equation of the system can be expressed as:
Q=H 2−H 1
Assignment
A frictionless piston is used to provide a constant pressure of 400 kPa in a cylinder containing
steam originally on 200ºC with a volume of 2m3. If 3500 kJ of heat is added, determine (i) the
initial enthalpy, (ii) the final enthalpy, (iii) the final volume and (iv) the final temperature of the
system.
A Cyclic Process
let us consider a cyclic process. It is a transformation which brings the system back to its initial
state. It can be represented a closed curve. The total work done in the cyclic process is given
geometrical by the area enclosed by curve which represents the curve. There is amount energy,
QH absorbed in the cycle while another amount of energy, QL also rejected. The net work done
by the system is given as:
QH=QL=W NET
-- - -- - - - - - - - - - - - ()
The change in internal energy over the cycle is zero.
Examples of cycle process are Carnot, Otto and others. Their principles form the basis of
engines. The efficiency of the engines can be determined thus:
Efficiency of an engine, η=Net work done, WNET
the total energyput into the system, QH
η=W NET
QH
By substitution, the efficiency becomes:
η=QH -Q L
QH=1−
QL
QH
- - - - - - - - - - - - - - - - - ()
Entropy
It can also be proved that efficiency can be defined in terms of temperatures TH and TL at which
energy QH are QL are absorbed and rejected respectively.
η=1−T L
TH=1−
QL
QH
- - - - - - - - - - - - - - - - - ()
Hence,
T L
TH=QL
QH
- - - - - - - - - - - - - - - - -()
Rearranging equation () gives:
QH
T H=QL
T L
- - - - - - - - - - - - - - - - -()
the quantity
QT
is known as entropy with a symbol, S. It is preferably expressed in differential
form as:
dS=dQT
- - - - - - - - - - - - - - - - -()
The energy Q, can be express as:
dQ=TdS - - - - - - - - - - - - - - - - -()
Helmholtz Functions
Helmholtz Functions also known as Helmholtz free energy is defined as: F=U−TS
Gibbs Function
Gibbs Functions also known as Gibbs free energy is defined as: G=H−TS
Atmospheric Systems and Maxwell Relation
Atmospheric systems are given masses and composition of the atmosphere under study. Once a
system has been identified the rest of the atmosphere is the surrounding of the identified system.
The following can be the system of the atmosphere: (i) the composition of air with a humidity
that ranges from 0% to saturation and clouds of saturated air and either water drops or ice
crystals. Unsaturated air is a mixture of gases in constant composition with molecular weight
being 28.964 and water vapour whose content can be expressed by the vapour pressure and by its
molar fraction.
Molar Fraction
molar fraction can be defined as the ratio of the number of moles of water vapour to the total
number of moles. Since the number of moles of water vapour is always much smaller than that of
dry air, molar fraction can be given another definition. It is by approximation. It is given as the
ratio of the moles of water vapour to the number of moles of dry air.
The atmospheric systems can be regarded as ideal gases. These systems; dry air, water vapour
and moist air; all obey ideal gas equation given as
pV=mRTM
=nRT
- - - - - - - - - - - - - - - - - ()
where p is the pressure, V the volume, m mass R the universal gas constant (8.3 Jmol–1K–1) M the
average molecular weight (28.9) and n the number of m. the presence of any of the following (i)
water vapour, (ii) cloud and (iii) water droplets changes the the atmospheric parameters.
First Law of Thermodynamics
From our previous studies, the first law can be stated thus: