Thermodynamics

Introduction:

We know that matter is made up of atoms or molecules and it may be in any one of its three States: solid, liquid and gas. whatever be the state of the matter, its properties are determine by the force acting between its constituent particles.

In kinetics theory we discuss the properties of the gases on the basis of interparticle force. In this method we consider the position and velocity of each particle of the system and the force acting between them. Thus the method is known as “microscopic method of approach”.

There is another method, where we do not think about the atom or molecules, but we consider a matter in bulk. To study these properties of the system it is sufficient to know only a few properties of the system, such as pressure, temperature, Volume ete. This method is known as macroscopic method of approach. All properties of matter related to temperature can be studied in this way. This forms the subject matter of thermodynamics.

System and its Surroundings:

• Any portion of matter which is considered as separated from its surroundings is called a ‘system’.

• All those things which are outside that system and influence its behaviour are known as the ‘surroundings’ of the system.

Example:

Let a gas be filled in a cylinder fitted with a piston and heated by a burner. Here the ‘gas is the system’; while the ‘piston and burner are the surroundings’.

Thermodynamic system may be of three types:

₪ Open system: It can exchange both energy and matter with the surrounding.

₪ Closed system: It can exchange only energy (not matter) with the surroundings.

₪ Isolated system: It can exchange neither energy nor matter with the surrounding.

Zeroth Law of Thermodynamics:

This fundamental law states that if two systems A and B are separately in thermal equilibrium with a third system C, then A and B are in thermal equilibrium with each other.

Suppose there are three systems A, B and C. Systems A and B are isolated from each other but are in thermal contact with C. Experiments show that both A and B individually attain thermal equilibrium with C. If now A and B are put in thermal contact of each other, no further change takes place. That is, A and B are found to be in thermal equilibrium with each other.

Mechanical Equivalent of Heat:

Whenever mechanical work is transformed into heat, or heat into mechani-cal work, there is a constant ratio between the work and the amount of heat. This ratio is called “mechanical equivalent of heat and is denoted by J. Thus, if W be the amount of work done and the amount of heat produced, we have

W/Q = J

W = JQ

The value of J = 4.186 joule/calorie

It is clear from the definition of J that it is not a physical quantity. It is only a conversion factor used to convert energy from heat units (kilo-calorie or calorie) to mechanical unit and vice-versa.

Thermodynamic Equilibrium:

Mechanical Equilibrium: When there are no unbalanced forces between the system and its surroundings, the system is said to be in mechanical equilibrium.

 Chemical Equilibrium: If the system has no tendency to undergo a change in internal structure and also has no tendency to transfer matter from one part of the system to another, it is said to be in chemical equilibrium.

Thermal Equilibrium: If all parts of the system are at the same temperature which is equal to the temperature of the surroundings, the system is said to be in thermal equilibrium’.

֍ When the system is under all the three types of equilibrium, it is said to be in ‘thermodynamic equilibrium’. In this condition any change in the state of the system or of the surroundings would not occur.

֍ If he conditions for any one of the three types of equilibrium are not satisfied, the system is said to be in a ‘non-equilibrium state’.

Thermodynamic coordinates:

The state of a thermodynamic system in equilibrium can be completely specified in terms of certain measurable (macroscopic) quantities. In the case of a gas, for example, these quantities are pressure P , volume V and temperature T. They are functions of the state of the system alone and return to the same values whenever the system returns to the same equilibrium state. They are known as ‘state functions’ or ‘state variables’ or ‘thermodynamic coordinates’ of the system.

Equation of State:

The equation of state of a thermodynamic system in at equilibrium is a functional relationship among the state variables of that system. Any two of the three variables p, V and T are enough to specify the state of a gas and to fix the value of the third variable.

֍  If the system is in a non-equilibrium state then this state cannot be described in terms of thermodynamic coordinates.

Work:

When a system undergoes a displacement under the action of a force, ‘work’ is said to be done. Its magnitude being equal to the product of the force and the component of the displacement parallel to the force.

Explanation:

suppose a hydrostatic system contained in a cylinder fitted with a movable piston on which both system & surrounding act. Let P be the instantaneous pressure of the gas while expanding against the piston. The force on the piston is PA, where A is the cross section of the piston(fig.1).The work done by the gas in displacing out the piston through an infinitesimal distance ds is

                            dw= pAds = pdv ; Where dv = Ads is the infinitesimal increase in the volume of the gas.

The total work done by the gas in expanding from initial Volume v_i to the final Volume V_f would be

 

 

The value of this integral can be obtained graphically by plotting the p-V curve of the gas. The nature of the curve depends upon the conditions under which the gas has expanded. One type of the curve is shown in figure-2.

It is clear that the integral is the area (shown shaded) under the p-V curve between the initial and final states. This area is, therefore, the measure of the work done by the gas.

Internal energy:

Internal energy of a system is defined as the total energy possessed by the system due to molecular motion and molecular configuration. It is represented by U.

Internal energy, U= U_K + U_p    where  U_k = Internal kinetic energy and U_p =   Internal potential energy .

Heat:

It is the energy that exchanged between a system and its surrounding because of the temperature difference between them. SI unit of heat is joule.

The First Law of Thermodynamics:

The first law of thermodynamics is simply the principle of conservation of energy applied to a thermodynamic system.

 

 

 

 

Mathematically, the change in the internal energy of a system in passing from an initial state i to a final state f is

 

 

where Q is the amount of heat absorbed by the system and W the external work done by the system. This equation is taken as  mathematical form of the first law of thermodynamics.

Differential Form of The First Law of Thermodynamics:

If the system undergoes only an infinitesimal change in state absorbing an infinitesimal amount of heat dQ and doing an infinitesimal    amount of work dW, then the infinitesimal change in internal energy dU would be given by

dU=dQ-dW

or dQ=du+dw

This is the differential form of the first law of thermodynamics. (Q, U and W must all be expressed in the same unit).

Real-World Applications of Thermodynamics:

1.Refrigerators and Air Conditioners

• Work on the refrigeration cycle (reverse heat engine).

• Use evaporation and condensation to absorb and release heat.

2.Engines (Cars, Bikes, Trucks)

• Internal combustion engines follow the First Law (energy conversion) and Second Law (efficiency limits).

• Converts chemical energy→ mechanical work.

 3.Heat Pumps (Winter Heating Systems)

Transfer heat from cold environment to warm space – opposite of refrigerators.

4.Pressure Cookers

• Uses thermodynamics of pressure-temperature relation to cook food faster.

• Higher pressure increases boiling point of water.

Conclusion:

Thermodynamics is one of the most powerful branches of physics because it helps us understand how heat, energy, and work interact in nature and technology. From engines and refrigerators to stars and chemical reactions, every system follows the laws of thermodynamics. By learning the basic laws, concepts, and applications, students can build a strong foundation for advanced physics, engineering, and daily-life problem solving. A clear understanding of these principles not only boosts academic performance but also develops scientific thinking and reasoning skills.

FAQs on Thermodynamics (With Answers)

Q1.What is thermodynamics?

A1.Thermodynamics is a branch of physics that studies the relationship between heat, energy, and work. It explains how energy is transferred and transformed in physical systems.

Q2.What is an example of thermodynamics in real life?

A2.Running AC or refrigerator, cooking in a pressure cooker, car engines, and steam power plants are everyday examples of thermodynamics.

Q3.What is absolute zero?

A3.Absolute zero (0 K or -273.15°C) is the lowest possible temperature where molecular motion theoretically stops.

Q4.Why is thermodynamics important?

A4.It helps us understand engines, power plants, refrigeration, chemical reactions, biological systems, and almost every energy-related technology in modern life.