Thermodynamics is the science of the relations between heat, work and the properties of systems.
All of the underlined words need to be precisely undefined! In the study of any special branch of physics we usually start with a separation of a restricted region of space or a finite portion of matter from its surroundings. The portion that is set aside and on which attention is focused is called the system.
System is a fixed and identifiable collection of matter enclosed by a real or imaginary surface which is impermeable to matter but which may change its shape or volume. The surface is called the boundary.
Everything outside the system which has a direct bearing on the system's behaviour.
Macroscopic Vs Microscopic
When a system has been chosen, the next step is to describe it in terms of quantities related to the behaviour of the system or its interactions with the surrounding or both. Two points of view may be adopted: macroscopic or microscopic,
Microscopic approach considers the behaviour of every molecule by using statistical methods. In Macroscopic approach we are concerned with the gross or average effects of many molecules' infractions. These effects, such as pressure and temperature, can be perceived by our senses and can be measured with instruments. This approach greatly reduces the complexity of the problem and we use this approach in this course. This is known as "Classical Thermodynamics".
The conditions of the system, and the substance within it, is defined by the properties of a system. A Property is any observable characteristic of a system. Properties can be defined only when they are uniform throughout a system eg impossible to define the pressure of an exploding system.
Examples: Length, volume, pressure, density, refractive index, etc., systems have lots of properties.
A property which is quite important in thermodynamics is the pressure.
A fluid exerts forces on its boundaries, due to the change in momentum of molecules when they collide with the boundary. These forces are not concentrated at one particular point, due to random motion of molecules, but are distributed. Therefore, we can define pressure as the normal force exerted on a surface, divided by the are of the surface.
The unit of pressure is the force 1 Newton acting on a square metre area, which is called a Pascal.
1 Pa = 1 N/m2 and 1 bar = 105 Pa
In most thermodynamics investigations we are concerned with the absolute pressure. However, most pressure gauges measure the difference between the absolute pressure and the atmospheric pressure which is known as gauge pressure.
Extensive & Intensive Properties
In general one can make distinction between two types of properties.
(i ) Extensive:
Extensive properties are those whose value is the sum of the values for each subdivision of the system, eg mass, volume.
Properties are those which have a finite value as the size of the system approaches zero, eg pressure, temperature, etc.
Extensive properties may be made intensive by dividing them by the system mass. For example:
system volume = 12 m3, mass = 4 kg
system specific volume = 12/4 = m3/kg State
The state of a system is fixed by defining all its properties or sufficient properties so that all others may be described. The state of a system changes if any property changes. In most of the simple systems that we shall consider a small number of properties will be enough to completely define the state of a system.
A system is in thermodynamic equilibrium if no tendency towards spontaneous change exists within the system. Energy transfers across the system disturb the equilibrium state of the system but may not shift the system significantly from its equilibrium state if carried out at low rates of change.
I mentioned earlier that to define the properties of a system, they have to be uniform throughout the system. Therefore to define the state of a system, the system must be in equilibrium. (Inequilibrium of course implies non-uniformity of one or more properties).
A process is the description of what happens when a system changes its state by going through a succession of equilibrium states.
Property Diagram and Path Consider a system which we are monitoring and assume that properties X (pressure) and Y (eg volume), which are being measured, are enough to define the state of the system. Then if we plot X versus Y, we get a Property diagram.
A point, such as 1, on the diagram represents the properties of the system at a particular instant and is known as a state point. Three different hypothetical processes have been drawn:
Process 1 - 2: Is relatively undefined. We cannot guess what happens between the two equilibrium states 1 and 2.
Process 3 - 4 In this case the properties have been measured at points a, b, c, d, ..., etc, and so we can draw a dotted line through the points.
Process 5 - 6: In this case the properties have been measured continuously and we have obtained an infinity of equilibrium states between 5 and 6. We are now justified in drawing a full line. This line is called the path of the process. Note: to fully define the process we need to monitor the system - surrounding interactions as well.
A cyclic process is one for which the initial and final states of the system are identical.
INTERACTIONS BETWEEN SYSTEMS (WORK and HEAT)
What happens when we bring two systems into contact?
The configuration or states of (A) and (B) may be altered until after a certain time when the systems reach equilibrium, and there is no change. Systems (A) and (B) can interact even at a distance, eg earth and moon (tide). But in Thermodynamic we are concerned with only two kinds of interactions, work and heat. Thermodynamics Definition of Heat and Work
In mechanics work is defined as a force acting through a displacement x, the displacement being in the direction of the force. That is:
W = F*x
or in the case in which F is varying
Unit of work - 1 Nm = 1 Joule
But this definition of work is very restrictive and not adequate in Thermodynamic. (Example a system consisting of a battery!) Work - Interactions Between Systems I
"Positive work is done by a system, during a process, when he ONLY effect external to the system could be reduced to the rise of a weight".
Definition seems arbitrary! But partly is forced on us since we have to make distinctions between work and heat as a result of the Second Law of Thermodynamics.
A simple way to visualise this:
The system boundary moves in such a way as to push the lever system to a new dotted position. The work done as in mechanics is (WxL) Nm. This does not show the width of the application of Thermodynamic definition. So consider a battery which we take as our system. The battery terminals are connected to a resistor through a switch:
When the switch is shut for a time, current flows through the resistor and it becomes "Warmer". Is this a work interaction? Of course from mechanics we would say NO! (since no force has moved its point of application). But now let us image that:
That is we have imagined a practical arrangement which includes a pulley and an electric motor.
If we again close the switch, the electric current will drive the electric motor which winds up a string attached to a weight. That is the sole effect external to the system could be the rise of a weight. This interaction is work! Some Notes
(i ) The "ONLY effect", is necessary since, an interaction in the form of heat transfer could result in the rise of a weight as a part of its effect.
(ii) External to the system - Work is defined with respect to a system boundary. If you choose a different system and hence boundary, then work may be changed.
(iii) "Could be reduced to" - This means that a weight does not actually have to be raised, but we must be able to visualise a real physical method of raising the weight by hypothetical changes in the surroundings!
(iv) "Positive Work"; also implies negative work!
"If a system does positive work, then obviously the surrounding do an equal amount of negative work and vice versa. In symbols:
W system+ Wsurroundings = 0
Important to know:
Work is a Transient. It is present during the interaction but does not exist either before or after the interaction. It is something which happens to a system but it is not a characteristic of a system ie not a property!