This is the first lecture on thermodynamics and in this lecture, we will grow some conceptual knowledge about system, surroundings, boundary, state variables and processes. You need to understand these terms since, you find these terms most of the definitions, derivations or experiments involving thermodynamics.
What is ‘System’ in Thermodynamics?
A system can be considered as a region of space separated from surrounding by a real or imaginary boundary. In simple word, a quantity of matter under investigation around which we can draw a boundary. Generally, a system can be of three types:
Closed System, Open System and Isolated System
What is a Closed System?
In a closed system, no matter can be transferred from system to surroundings or vice versa. For example, an airtight container of gas can be considered as a closed system. Since the container is airtight no gas molecule can escape from the container.
What is an Open System?
In case of an open system, matter can be removed from or added to the system. A bowl of hot water can be said as an open system because water molecules can vaporize from the bowl and spread to the surroundings.
What is an Isolated System?
Isolated systems have no interaction surrounding, i.e. neither matter nor energy can be transferred between the system and surroundings. There are very few examples of this kind of systems. An adiabatic airtight container falls into this category. No heat energy or matter can escape from the system.
N.B. These definitions of systems are valid for thermodynamics only. For broader sense, the definitions can be complicated.
What is Surroundings in Thermodynamics?
Surroundings are the other remaining portion of the universe other than the system of investigation. More precisely, all other matter that can interact with the system.
What is Boundary in Thermodynamics?
A real or imaginary wall which separates the system from its surroundings. A boundary can be rigid or non-rigid. It can be diathermic (conductor of heat) or adiabatic (non-conductor of heat).
What are State Variables in Thermodynamics?
A state of a system can be described by some particular values of sufficient numbers of states variables. States variables are macroscopic in nature, for example, temperature, pressure, volume, composition, surface area, mass etc. Generally, all values of state variables are not necessary for describing the states of a system.
For example, let us take a system of n mole of ideal gas having volume V and pressure P at a temperature T. Here, n, V, P, and T can be considered as the state variables for the system but all the four variables are not necessary for defining the system at all. Now, see the reason why it is said so.
Since the system is describing an ideal gas, it must obey the ideal gas equation:
PV = nRT
From this equation, we can easily work out the 4th state variable if three variables are known already. For example, V can be easily calculated if n, P, T are known.
Thus minimum numbers of states variables to describe a system = (Number of states variables – numbers of equations relating them)
For the ideal gas system, the minimum number of state variables necessary = 4 – 1 = 3.
Thus, any three state variables among the four variables (n, V, P, and T) are necessary to describe the full system.
Variables are classified into two categories, Extensive and Intensive Variables.
What are Extensive and Intensive Variables in Thermodynamics?
Let us take a system and it is divided into two or more parts without altering the state of the system. For example, an airtight container of ideal gas is divided into two parts by an imaginary wall. Now, Extensive Variables are those which changes its values upon division from the original system whereas Intensive Variables do not change its values upon division. For the ideal gas container, the state variables are, n, P, V and T. Carefully notice that n and V change its original values upon division, so these are Extensive Variables, on the other hand, P and T remain same so these are Intensive Variables.
In, simple word, Extensive variables are mass dependent but Intensive Variables are independent of the mass of the matter of the system.
Examples of Extensive and Intensive Variables
Volume, Heat capacity, Enthalpy, Entropy, Energy, Free energy, Length, mass etc.
Pressure, Temperature, Concentration, Density, Dipole moment, Viscosity, Surface Tension, Vapour Pressure, Specific Gravity, Molar Volume, Chemical Potential, EMF of a cell, refractive index etc.
What is a Thermodynamic Process?
A thermodynamic process is a path by which a system changes its states. Many of texts use the following processes as a convention.
Isothermal Process: State of a system changes its state under constant temperature.
Isobaric Process: State of a system changes its state under constant pressure.
Isochoric Process: State of a system changes its state under constant volume.
Adiabatic Process: State of a system changes its state such a way that no heat enters or leaves the system.
Quasi-Static or Reversible process: In this process, a system maintains an equilibrium condition at every moment during the process of change of state occurs. This type of process involves the change of the state variables of a system in an infinitesimal manner at every instance during the process such that the system remains at equilibrium with the surroundings.