What is Thermodynamics?: Thermodynamics is the field of physics that deals with the relationship between heat and other properties such as pressure, density, temperature, etc. in a substance. In other words, thermodynamics is the branch of science that deals with the concepts of heat and temperature and the inter-conversion of heat and other forms of energy.
Specifically, thermodynamics focused mainly on how heat transfer is related to various energy changes within a physical system undergoing a thermodynamic process. These processes usually result in work being done by the system and are guided by the laws of thermodynamics.
Thermodynamic System
An assembly of a very large number of particles having a certain value of pressure, volume, and temperature is called a thermodynamic system.
Surroundings: Everything outside the system which can have a direct effect on the system is called its surroundings.
Equation of State: The mathematical relation between the pressure, volume, and temperature of a thermodynamic system is called its equation of state.
Thermodynamic Variables
The quantities like pressure (P), volume (V), and temperature (T) help us to study the behavior of a thermodynamic system called thermodynamic variables.
Thermal Equilibrium
Two systems are said to be in equilibrium with each other if they have the same temperature.
Thermodynamic Equilibrium
A system is said to be in the state of thermodynamic equilibrium if the microscopic variables describing the thermodynamic state of the system do not change with time.
Explain the laws of thermodynamics:
There are four thermodynamic laws:-
Laws of thermodynamics
Thermodynamics is principally based on a set of four laws that are universally valid when applied to the system that falls within the constraints implied by each.
Zeroth law of thermodynamics
It states that if two systems are each in thermal equilibrium with a third, they are also in thermal equilibrium with one another. It implies that thermal equilibrium is an equivalence relation in the set of thermodynamic systems under consideration.
The system is said to be in equilibrium if the small, random exchanges between them (Ex: Brownian motion) do not lead to a net change in energy. The first law provides an empirical definition of temperature and justification for the construction of practical thermometers.
Internal Energy: It is the sum of molecular kinetic and potential energies in the frame of reference relative to which the center of mass of the system is at rest.
- The intermolecular potential energy of a real gas is a function of its volume.
- The internal energy of a gas is a function of the temperature.
- The internal energy of a system is a thermodynamic state variable.
- The internal energy of an ideal gas is kinetic in nature.
| Heat | Work |
| 1. Heat is a mode of energy transfer that produces random motion. 2. Heat is a mode of energy transfer due to temperature differences in the system and the surroundings. | 1. Work is the mode of energy transfer bought about by means that do not involve temperature differences. 2. Work may be regarded as the mode of energy transfer that produces organized motion. |
The sign convention used:
- Heat observed by a system is positive. Heat given out by a system is -ve.
- Work done by a system is positive. Work done on a system is -ve.
- The increase in the internal energy of a system is positive. The decrease in the internal energy of a system is negative.
The first law of thermodynamics
The thermodynamic first law states that the internal energy of an isolated system is constant. The first law is an expression of the principle of the conservation of energy which is usually formulated by assuming that the change in the internal energy of a closed thermodynamic system is equal to the difference between the heat supplied to the system and the amount of work done by the system on its surroundings.
It is important to note that internal energy is a state of the system. In other words, the specific internal energy of the system may be achieved by any combination of heat and work; the manner by which a system achieves specific internal energy is path-independent.
Limitations of the first law of thermodynamics
- Thermodynamic first law does not indicate the direction of transfer of
- Thermodynamic first law does not tell anything about the conditions under which heat can be converted into mechanical
- The first law of thermodynamics does not indicate the extent to which heat energy can be converted into mechanical work
The second law of thermodynamics
The thermodynamic second law states that heat cannot spontaneously flow from a colder location to a hotter location. It being more an expression of the universal principle of decay observable in nature explains the phenomenon of irreversibility in nature.
Third law of Thermodynamics
The thermodynamic third law states that as a system approaches absolute zero, all processes cease and the entropy of the system approaches a minimum value. The third law provides an absolute reference point for the determination of entropy.
The entropy determined relative to this point is the absolute entropy. The entropy of all systems and of all states of a system is the smallest at absolute zero, or equivalently however it is impossible to reach the absolute zero of temperature by any finite number of processes.
Absolute zero refers to the temperature at which all activity would stop if it were possible to happen. It is -273.16 degrees Celsius or -459.67 degrees Fahrenheit or 0 K.
Cause of the thermodynamic system
A thermodynamic system can be classified and distinguished into three classes of systems as listed in the table given below:
| Types of system | Mass flow | Work | heat |
| Open | yes | Yes | Yes |
| Closed | No | Yes | Yes |
| Thermally isolated | No | Yes | No |
| Mechanically isolated | No | No | Yes |
| Isolated | No | No | No |
As time passes in an isolated system, internal differences in the system tend to even out and pressures and temperatures tend to equalize, as do density differences. A system in which all equalizing properties and processes have gone to completion is considered to be a state of thermodynamic equilibrium.
The thermodynamic processes which develop so slowly as to allow each intermediate step to be an equilibrium state are said to be reversible processes.
THERMODYNAMIC PROCESS
A thermodynamic process is said to occur if the thermodynamic variables of a system undergo a change with time.
Types of thermodynamic processes
- Isobaric process: Occurs at constant pressure.
- Isochoric process: Occurs at constant volume. It is also called Isovolumetric or Isometric.
- Isothermal process: Occurs at a constant temperature.
- Adiabatic process: Occurs without the loss or gain of energy by heat.
- Isentropic process: Reversible adiabatic process that occurs at constant entropy.
- Isenthalpic process: Constant enthalpy.
- Steady-state process: Occurs without a change in internal energy.
Conditions for an isothermal process
- The walls of the vessel must be perfectly conducting to allow a free exchange of heat between the system and the surroundings
- The process of compression or expansion should be very slow to provide sufficient time for the exchange of heat.
Conditions of the Adiabatic Process
- The walls of the vessel must be perfectly insulated so that there cannot be any exchange of heat between the gas and the
- The process of compression & expansion should be sudden so that heat does not get time to get exchanged with the surroundings.
Thermodynamic Potential
These are different quantitative measures of the stored energy in a system. Potentials are used for measuring energy changes in systems as they evolve from an initial state to a final state. The potential used depends on the constraints of the system, such as constant temperature or pressure. The five most well-known potentials are:
- Helmholtz free: It is denoted by F.
- Enthalpy: The symbol of Enthalpy is H.
- Internal energy: It is denoted by U.
- Landau potential: Landau potential is also called grand potential. It is denoted by Ω.
- Gibbs free energy: The symbol of Gibbs free energy is G.
Indicator diagram
A graphical representation of the state of the system with the help of two thermodynamic variables is called an indicator diagram.
Differences between Cyclic and Non-Cyclic process
Cyclic process: Any process in which the system returns to its initial state after undergoing a series of changes is known as a cyclic process.
Non-Cyclic process: A non-cyclic process is one in which the system does not return to its initial state.
Specific Heats of a Gas
If gas is heated, its volume and pressure change with the increase in temperature. So the amount of heat required to raise one gram of gas temperature through 1 degree Celsius is not fixed. That is a gas does not possess a unique or single specific heat. A gas can have any value of specific heat depending on the conditions under which it is heated.
Limited specific heat of a gas
Suppose the mass (m) of a gas enclosed in a cylinder fitted with an air-tight and frictionless piston.
- Suppose the gas is suddenly compressed. No heat is supplied to the gas i.e. ∆Q = 0. But the temperature of the gas rises due to compression. The specific heat of gas is zero.
C = ∆Q/m∆T = 0/m∆T = 0
- The gas is heated & allowed to expand such that the rise in temperature of the gas due to the heat supplied is equal to the fall in temperature of the gas due to the expansion of the gas itself. Hence, the net rise in temperature is zero i.e. ∆T = 0. The specific heat of the gas is infinite.
c = ∆Q/m∆T = ∆Q/mx0 = ∞
- The gas is heated & allowed to expand at such a rate that the fall in temperature due to expansion is less than the rise in temperature due to heat supplied. The temperature of the gas will rise i.e. ∆T is positive. The specific heat of the gas is positive.
C = ∆Q/m∆T = positive value
- The gas is heated & allowed to expand at such a rate that the fall in temperature due to expansion is more than the rise in temperature due to heat supplied. The temperature of the gas will decrease i.e. ∆T is negative. The specific value of gas is negative.
C = ∆Q/m∆T = negative value.
Molar Specific Heat at Constant Volume
It is defined as the amount of heat required to raise the temperature of 1 mole of gas through 1 degree Celsius at constant volume. It is denoted by CV.
Molar-specific heat at constant pressure
It is defined as the amount of heat required to raise the temperature of 1 mole of gas through 1 degree Celsius at constant pressure. It is denoted by Cp.
Heat Engine
It is a device that converts continuously heat energy into mechanical energy in a cyclic process.
- Source: It is a heat reservoir at a higher temperature.
- Sink: It is a heat reservoir at a lower temperature.
- Working substance: A working substance is any material that performs mechanical work when heat is supplied to it.
The efficiency of a heat engine
The efficiency of a heat engine is defined as the ratio of the net work done by the engine in one cycle to the amount of heat absorbed by the working substance from the source.
External combustion engine
In such a heat engine, the heat needed for the working substance is produced by burning the fuel outside the cylinder and piston arrangement of the engine. A steam engine is an external combustion engine.
Internal combustion engine
In such a heat engine, the heat needed for the engine is produced by burning the fuel inside the main cylinder. Petrol and diesel engines are internal combustion engines.
Differences between Reversible and Irreversible process
Reversible Process
Any process which can be made to process in the reverse direction by variation in its conditions such that any change occurring in any part of the direct process is exactly reversed in the corresponding part of the reverse process.
Irreversible Process
Any process that cannot be retraced in the reverse direction is called an irreversible process.
Carnot Theorem
It states that the engine does not work between two given temperatures can have an efficiency greater than that of the Carnot engine working between the same two temperatures and the efficiency of the Carnot engine is not dependent on the nature of the working substance.
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