THERMODYNAMICS

 

 

Thermodynamics

Thermodynamics, science of the relationship between heat, work, temperature, and energy. In broad terms, thermodynamics deals with the transfer of energy from one place to another and from one form to another. The key concept is that heat is a form of energy corresponding to a definite amount of mechanical work.

The most important laws of thermodynamics are:

• The zeroth law of thermodynamics. When two systems are each in thermal equilibrium with a third system, the first two systems are in thermal equilibrium with each other. This property makes it meaningful to use thermometers as the “third system” and to define a temperature scale.

• The first law of thermodynamics, or the law of conservation of energy. The change in a system’s internal energy is equal to the difference between heat added to the system from its surroundings and work done by the system on its surroundings.

• The second law of thermodynamics. Heat does not flow spontaneously from a colder region to a hotter region, or, equivalently, heat at a given temperature cannot be converted entirely into work. Consequently, the entropy of a closed system, or heat energy per unit temperature, increases over time toward some maximum value. Thus, all closed systems tend toward an equilibrium state in which entropy is at a maximum and no energy is available to do useful work.

• The third law of thermodynamics. The entropy of a perfect crystal of an element in its most stable form tends to zero as the temperature approaches absolute zero. This allows an absolute scale for entropy to be established that, from a statistical point of view, determines the degree of randomness or disorder in a system.

Basic Concepts of Thermodynamics – Thermodynamic Terms

Thermodynamics has its own unique vocabulary associated with it. Good understanding of the basic concepts forms a sound understanding of various topics discussed in thermodynamics preventing possible misunderstandings.

Thermodynamic Systems

A thermodynamic system is a specific portion of matter with a definite boundary on which our attention is focussed. The system boundary may be real or imaginary, fixed or deformable.
There are three types of systems:

• Isolated System – An isolated system cannot exchange both energy and mass with its surroundings. The universe is considered an isolated system.
• Closed System – Across the boundary of the closed system, the transfer of energy takes place but the transfer of mass doesn’t take place. Refrigerator, compression of gas in the piston-cylinder assembly are examples of closed systems.
• Open System – In an open system, the mass and energy both may be transferred between the system and surroundings. A steam turbine is an example of an open system.

Surrounding

Everything outside the system that has a direct influence on the behaviour of the system is known as a surrounding.

Thermodynamic Process

A system undergoes a thermodynamic process when there is some energetic change within the system that is associated with changes in pressure, volume and internal energy.

There are four types of thermodynamic process that have their unique properties, and they are:

• Adiabatic Process – A process where no heat transfer into or out of the system occurs.
• Isochoric Process – A process where no change in volume occurs and the system does no work.
• Isobaric Process – A process in which no change in pressure occurs.
• Isothermal Process – A process in which no change in temperature occurs.

A thermodynamic cycle is a process, or a combination of processes conducted such that the initial and final states of the system are the same. A thermodynamic cycle is also known as cyclic operation or cyclic processes.

Thermodynamic Equilibrium

At a given state, all properties of a system have fixed values. Thus, if the value of even one property changes, the system’s state changes to a different one. In a system that is in equilibrium, no changes in the value of properties occur when it is isolated from its surroundings.

• When the temperature is the same throughout the entire system, we consider the system to be in thermal equilibrium.
• When there is no change in pressure at any point of the system, we consider the system to be in mechanical equilibrium.
• When the chemical composition of a system does not vary with time, we consider the system to be in chemical equilibrium.
• Phase equilibrium in a two-phase system is when the mass of each phase reaches an equilibrium level.

A thermodynamic system is said to be in thermodynamic equilibrium if it is in chemical equilibrium, mechanical equilibrium and thermal equilibrium and the relevant parameters cease to vary with time.