Thermodynamic Systems:
If we want to analyze movement of energy over space, then we must define the space that would be used for the observation, we would call it as a SYSTEM, separated from the adjoining space that is known as "Surroundings", by a boundary that may be real or may be virtual depending upon the nature of the observation. The boundary is called as SYSTEM BOUNDARY. So, we shall define a system properly. A thermodynamics system refers to a three dimensional space occupied by a certain amount of matter known as ''Working Substance'', and it is the space under consideration. It must be bounded by an arbitrary surface which may be real or imaginary, may be at rest or in motion as well as it may change its size and shape. All thermodynamic systems contain three basic elements:
Classification of Thermodynamic Systems:
Systems can be classified as being (i) closed, (ii) open, or (iii) isolated.
(i) Closed System:
Mass cannot cross the boundaries, but energy can.
(ii) Open System or Control Volume:
Both mass and energy can cross the boundaries.
(iii) Isolated System:
Neither mass nor energy can cross its boundaries.
Property, Equilibrium and State:
A property is any measurable characteristic of a system. The common properties include:
Properties can be intensive or extensive. Intensive properties are those whose values are independent of the mass possessed by the system, such as pressure, temperature, and velocity. Extensive properties are those whose values are dependent of the mass possessed by the system, such as volume, enthalpy, and entropy (enthalpy and entropy will be introduced in following sections).
Extensive properties are denoted by uppercase letters, such as volume (V), enthalpy (H) and entropy (S). Per unit mass of extensive properties are called specific properties and denoted by lowercase letters. For example, specific volume v = V/m, specific enthalpy h = H/m and specific entropy s = S/m (enthalpy and entropy will be introduced in following sections).
Note that work and heat are not properties. They are dependent of the process from one state to another state.
When the properties of a system are assumed constant from point to point and there is no change over time, the system is in a thermodynamic equilibrium.
The state of a system is its condition as described by giving values to its properties at a particular instant. For example, gas is in a tank. At state 1, its mass is 2 kg, temperature is 20oC, and volume is 1.5 m3. At state 2, its mass is 2 kg, temperature is 25oC, and volume is 2.5 m3.
A system is said to be at steady state if none of its properties changes with time.
Process, Path and Cycle:
The changes that a system undergoes from one equilibrium state to another are called a process. The series of states through which a system passes during a process is called path.
In thermodynamics the concept of quasi-equilibrium processes is used. It is a sufficiently slow process that allows the system to adjust itself internally so that its properties in one part of the system do not change any faster than those at other parts.
When a system in a given initial state experiences a series of quasi-equilibrium processes and returns to the initial state, the system undergoes a cycle. For example, the piston of car engine undergoes Intake stroke, Compression stroke, Combustion stroke, Exhaust stroke and goes back to Intake again. It is a cycle.
- System boundary: The imaginary surface that bounds the system.
- System volume: The volume within the imaginary surface.
- The surroundings: The surroundings is everything external to the system.
So we get a space of certain volume where ENERGY TRANSFER (movement of energy) is going on, what may or may not be real, and distinct, it may be virtual (in case of flow system ), again if real boundary exists, then it may be fixed (rigid boundary like constant volume system) or may be flexible (like cylinder-piston assembly). For a certain experiment the system and surroundings together is called UNIVERSE.
The interface between the system & surroundings is called as "SYSTEM BOUNDARIES", which may be real & distinct in some cases where as some of them are virtual, but it may be real, solid & distinct.
The interface between the system & surroundings is called as "SYSTEM BOUNDARIES", which may be real & distinct in some cases where as some of them are virtual, but it may be real, solid & distinct.
Systems can be classified as being (i) closed, (ii) open, or (iii) isolated.
(i) Closed System:Mass cannot cross the boundaries, but energy can.
Both mass and energy can cross the boundaries.
Neither mass nor energy can cross its boundaries.
Property, Equilibrium and State:
- pressure (P)
- temperature (T)
- volume (V)
- velocity (v)
- mass (m)
- enthalpy (H)
- entropy (S)
Properties can be intensive or extensive. Intensive properties are those whose values are independent of the mass possessed by the system, such as pressure, temperature, and velocity. Extensive properties are those whose values are dependent of the mass possessed by the system, such as volume, enthalpy, and entropy (enthalpy and entropy will be introduced in following sections).
Extensive properties are denoted by uppercase letters, such as volume (V), enthalpy (H) and entropy (S). Per unit mass of extensive properties are called specific properties and denoted by lowercase letters. For example, specific volume v = V/m, specific enthalpy h = H/m and specific entropy s = S/m (enthalpy and entropy will be introduced in following sections).
Note that work and heat are not properties. They are dependent of the process from one state to another state.
When the properties of a system are assumed constant from point to point and there is no change over time, the system is in a thermodynamic equilibrium.
The state of a system is its condition as described by giving values to its properties at a particular instant. For example, gas is in a tank. At state 1, its mass is 2 kg, temperature is 20oC, and volume is 1.5 m3. At state 2, its mass is 2 kg, temperature is 25oC, and volume is 2.5 m3.
A system is said to be at steady state if none of its properties changes with time.
Process, Path and Cycle:
The changes that a system undergoes from one equilibrium state to another are called a process. The series of states through which a system passes during a process is called path.
In thermodynamics the concept of quasi-equilibrium processes is used. It is a sufficiently slow process that allows the system to adjust itself internally so that its properties in one part of the system do not change any faster than those at other parts.
When a system in a given initial state experiences a series of quasi-equilibrium processes and returns to the initial state, the system undergoes a cycle. For example, the piston of car engine undergoes Intake stroke, Compression stroke, Combustion stroke, Exhaust stroke and goes back to Intake again. It is a cycle.
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