First Law Of Thermodynamics
Chapter 2
Friday, 19 June 2015
Tuesday, 16 June 2015
THERMODYNAMICS THEORY
Basic Concept
Chapter 1
Intensive and Extensive Properties
Intensive property: Whose value is independent of the
size or extent i.e. mass of the system. These are, e.g., pressure p
and temperature T.
Extensive property: Whose value depends on the size or
extent i.e. mass of the system (upper case letters as the symbols).
e.g., Volume, Mass (V, M). If mass is increased, the value of extensive
property also increases. e.g., volume V, internal energy U,
enthalpy H, entropy S, etc.
Specific property: It is a special case of an intensive
property. It is the value of an extensive property per unit mass of
system. (Lower case letters as symbols) eg: specific volume, density (v,
ρ).
Thermodynamic System and Control Volume
- In
our study of thermodynamics, we will choose a small part of the universe
to which we will apply the laws of thermodynamics.We call this subset a SYSTEM.
- The
thermodynamic system is analogous to the free body diagram to which we apply
the laws of mechanics, (i.e. Newton’s Laws of Motion).
- The system is a macroscopically identifiable collection of matter on which we focus our attention(e.g., the water kettle or the aircraft engine).
System
Definition
- System: A quantity of matter in space which is analyzed during a problem.
- Surroundings: Everything external to the system.
- System Boundary: A separation present between system and surrounding. Classification of the system boundary:-
- Control Mass System.
- Control Volume System.
The choice of boundary depends on
the problem being analyzed.
Types of System
Closed System (Control Mass System)
 |
| Fig. A Closed System |
- It’s a system of fixed mass with fixed identity.
- This type of system is usually referred to as “closed system”.
- There is no mass transfer across the system boundary.
- Energy transfer may take place into or out of the system.
Open System (Control Volume System)
 |
| Fig. A Open System |
- Its a system of fixed volume.
- This type of system is usually referred to as "open system” or a "control volume".
- Mass transfer can take place across a control volume.
- Energy transfer may also occur into or out of the system.
- A control volume can be seen as a fixed region across which mass and energy transfers are studied.
- Control Surface – Its the boundary of a control volume across which the transfer of both mass and energy takes place.
- Th e mass of a control volume (open system) may or may not be fixed.
- When the net influx of mass across the control surface equals zero then the mass of the system is fixed and vice versa.
- The identity of mass in a control volume always changes unlike the case for a control mass system (closed system ).
- Most of the engineering devices, in general, represent an open system or control volume.
Example:
- Heat exchanger - Fluid enters an d leaves the system continuously with the transfer of heat across the system boundary.
- Pump - A continuous flow of fluid takes place through the system with a transfer of mechanical energy from the surroundings to the system.
Isolated System
 |
| Fig. A An Isolated System |
- It is a system of fixed mass with same identity and fixed energy.
- No interaction of mass or energy takes place between the system and the surroundings.
- In more informal words an isolated system is like a closed shop amidst a busy market.
Quasi-Static Process
The
processes can be restrained or unrestrained
W e need restrained processes in practice.
 |
| Fig. A quasi – static process |
A quasi – static process is one in which:-
- The deviation from thermodynamic equilibrium is infinitesimal.
- All states of the system passes through are equilibrium states.
- If we remove the weights slowly one by one the pressure of the gas will displace the piston gradually. It is quasistatic.
- On
the other hand if we remove a ll the weights at once the piston
will be kicked up by the gas
pressure. (This is unrestrained
expansion) but we don’t consider that the work is done – because it is not
in a sustained manner.
- In both
cases the systems have undergone a change of state.
- Another e.g., if a person climbs down a ladder from roof to ground, it is a quasistatic process. On the other hand if he jumps then its not a quasistatic process.
Laws of Thermodynamics
• The
Zeroth Law deals with thermal equilibrium and provides a means for measuring temperatures.
• The
First Law deals with the conservation of energy and introduces the concept
of internal energy.
• The
Second Law of thermodynamics provides with the guidelines on the conversion
heat energy of matter into work. It also introduces the concept of entropy.
• The
Third Law of thermodynamics defines the absolute zero of entropy. The entropy
of a pure crystalline substance at absolute zero temperature is zero.
Summation of 3 Laws
• Firstly, there isn’t a meaningful
temperature of the source from which we can get the full conversion of heat to work. Only at
infinite temperature one can dream of getting the full 1 kW work output.
• Secondly, more interestingly, there isn’t enough work
available to produce 0 K. In other words, 0K is unattainable. This is precisely
the Third law.
• Because,
we
don’t know what 0 K looks like, we haven’t got a starting point for the
temperature scale!! That is why all temperature scales are at best
empirical.
You can’t get something for nothing :
To
get work output you must give some thermal energy.
You can’t get something for very little:
To get some work output there is a minimum amount of thermal energy t hat needs to be given.
You can't get everything:
However much work yo u are willing to give 0 K can’t be reached.
Violation of all 3 laws:
Try to get everything for nothing.
Zeroth Law of Thermodynamics
- If two systems (say A and B) are in thermal equilibrium with a third system (say C) separately (that is A and C are in thermal equilibrium; B an d C are in thermal equilibrium) then they are in thermal equilibrium themselves (that is A and B will be in thermal equilibrium).
- All temperature measurements are based on Zeroth law of thermodynamics.
International Temperature Scale
To
provide
a standard for temperature measurement taking into account both
theoretical and practical considerations, the International Temperature
Scale (I
TS) was adopted in 1927 . This scale has been refined and extended in
several
revisions, most recently in 1990. The International Temperature
Scale of 1990 (ITS-90) is defined in such a way t hat the temperature measured on it conforms with the thermodynamic temperature, the unit
of which is the kelvin, to within the limits of
accuracy of measurement obtainable in 1990. The
ITS–90 is based on the assigned values of temperature of a number of reproducible fixed points (Table).Interpolation between the fixed-point temperatures is
accomplished by formulas that give the relation between readings of standard
instruments and values of the ITS. In the range from 0.65 to 5.0 K, ITS-90
is defined by equations giving the temperature as functions of the vapor
pressures of p articular helium isotopes . The range from 3.0 to 24.5561 K is
based on measurements using a helium constant-volume gas thermometer. In
the range fr om 13.8033 to 1234.93 K, ITS-90 is defined by means of certain
platinum resistance thermometers. Above 1234.9 K the temperature is
defined using Planck’s equation for blackbody radiation and
measurements of the intensity of visible-spectrum radiation the absolute
temperature
d W=
F d l = pad l = pd V
The magnitude of the work done is given by the area under the path 1-2, as shown in Fig. since p is at all times a thermodynamic co-ordinate, all the states passed through by the system as the volume changes from V1 to V2 must be equilibrium states, and the path 1-2 must be quasi-static. The piston moves infinitely slowly so that every state passed through is an equilibrium state.
scale. The absolute temperature
scale is also known as Kelvin temperature scale. In defining the Kelvin
temperature scale also, the triple point of water is taken as the standard
reference point. For a Carnot engine operating between reservoirs at temperature θand
θtp, θtp
being the triple point of water arbitrarily assigned the value of 273.16 K.
Time Constants: The time constant is the amount of time required for a thermocouple to indicate 63.2% of step change in temperature o f a surrounding media. Some of the factors influencing the measured time constant are sheath wall thickness, degree of insulation compaction, and distance of junction from the welded can on an ungrounded thermocouple. In addition, the velocity of a gas past the thermocouple probe greatly influences the time constant measurement.
In general, time constants for
measurement of gas can be estimated to be ten times as long as those for
measurement of liquid. The time constant also varies inversely proportional
to the square root of the velocity of the media.
Work a path function
Work is one of the basic modes of
energy transfer. The work done by a system is a path function, and not a
point function. Therefore, work is not a property of the system, and
it cannot be said that the work is possessed by the system. It
is an interaction across the boundary. What is stored in the system is
energy, but not work. A decrease in energy of the system appears as workdone.
Therefore, work is energy in transit and it can be identified
only when the system undergoes a process.
Free Expansion with Zero Work Transfer
Free Expansion Let us consider an insulated
container (Figure) which is divided into two compartments A an d B by a thin diaphragm.
Compartment A contains a mass of gas, while compartment B is
completely evacuated. If the diaphragm is punctured, the gas in A will
expand into B until the pressures in A
and B become equal. This is known as free or unrestrained
expansion. The
process of free expansion is
irreversible Also work done is zero during free
expansion.
 |
| Free Expansion |
pdV-work or
Displacement Work
Let the gas in the cylinder (Figure
shown in below) be a system having initially the pressure p1 and volume V1. The system is in thermodynamic equilibrium, the state
of which is described by the coordinates p1
, V1.
The piston is the only boundary which moves due to gas pressure. Let the
piston move out to a new final position 2, which is also a thermodynamic
equilibrium state specified by pressure p2
an d volume V2 . At any intermediate point in the
travel of the piston, let the pressure be p and the volume V. This must also be an equilibrium state, since macroscopic properties p and V
for
equilibrium states. When the piston moves an infinitesimal distance dl, and if ‘a' be the area of the piston, the force F acting on the piston F = p.a. and the infinitesimal amount of work done by the gas
on the piston.
where dV = a dl
= infinitesimal displacement volume. The differential sign in dW with the line drawn at the to p of it will be explained
later.
V2
W1 2= pdV
V1
 |
| Fig. Quasi-static pdV Work |
The integration ∫pdV can be performed only on a quasi-static path.
Heat Transfer- A Path Function
Heat transfer is a path function, that is, the amount o heat transffered when a system changes from 1 state to 2 depends on the intermediate states through which the system passes, i.e. its path. Therefore dQ is an inexact differential, and we write
2
1dQ=Q1-2 or 1Q2 ≠ Q2 - Q1
The displacement work is given by:-
2 2
W1-2 = ∫1 dW = ∫1 pdV ≠ W2 -W1
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