## What is Thermodynamics?

The branch of physics in which observer deals with temperature related properties is called thermodynamics.

## Laws of Thermodynamics

##### Zeroth Law

If we have two systems, A and B are separated by an adiabatic wall and both are separated using a conducting wall from the third system, C, then, A and B are said to be in thermal equilibrium with each other, if both are separately in equilibrium with C.

##### First Law of Thermodynamics

This law is related to conservation of energy. According to this law, energy can neither be created nor be destroyed but can be transformed from one form to another.

dQ=dU+dW

where,

• dQ is the small change in heat.
• dU is the small change in internal energy of the system
• dW is the small amount of work done by the system on the surroundings.
##### Second Law of Thermodynamics

According to this law, an isolated system's entropy will never decrease over time i.e. randomness will never decrease. Also entropy of irreversible processes is always greater than that of reversible processes.

#### System

The part of the universe in which we are interested.

#### Surroundings

All the universe except the system comes under the surroundings.

## Three Types of Systems

Isolated system : A system which does not allow exchange of energy as well as matter is known as an isolated system.

Closed System : A system which allows exchange of energy but does not allow the exchange of matter is known as a closed system.

Open System : A system which allows exchange of energy as well as matter is known as an open system.

## Phase Transition

The conversion of a sample of a material from one phase to another is known as phase transition. The temperature at the transition point remains constant until all the material changes from one phase to another. It is isothermal and isobaric eg. water.

The three phases of water are totally dependent upon temperature.

• At low temperature, it behaves as solid
• At high temperature, it behaves as liquids and gases.

## Triple Point of Water

The point at which three phases coexist is known as triple point in equilibrium or three curves (vaporization, melting and sublimation) intersect is known as triple point. For water, the triple point is 273.16 Kelvin or (0.01ºC) and triple point pressure is 0.46 cm of Hg column. Whereas the SI unit of pressure is pascal.

## Vaporization of Water

The process in which liquid state turns into gaseous state is known as vaporization of water. This is done by increasing the temperature of the substance. In this, water changes into vapors by increasing temperature.

The vaporization line is also known as the steam line. At any point on the vaporization line the substance co-exists in liquid and gaseous state but at temperature and pressure above the vaporization line the substance will exist only in liquid state as at higher pressure the vapors get condensed into liquid, but if the pressure gets decreased, the water gets evaporated into vapors and thus all the points below vaporization line will lie in vapor state.

## Sublimation of Water

The process in which solid directly gets converted into a gaseous state is known as sublimation, in case of water, ice directly gets converted into vapors. The most common example of sublimation process is naphthalene balls, solid carbon dioxide and iodine.

In the case of water, the sublimation line is also known as the hoar-frost line. All points above the sublimation line will lie in solid state whereas all points below the sublimation line lie in the gaseous state.

## Fusion Line

At every point on this line the substance can co-exist in equilibrium in solid and liquid states. When pressure is increased to a value greater than pressure corresponding to a point on the fusion line then solid gets melted into liquid. At all points above the fusion line the substance exists in the liquid phase whereas at all the points below the fusion line, the substance will exist only in the solid phase. This line is also known as the Ice line.

## Melting of Water

The process in which solid state gets converted into liquid state is known as melting. In case of water, ice gets converted into water.

## Melting Point

The temperature at which a substance has its solid and liquid phases at thermal equilibrium with each other is known as the melting point of that substance. Its value is different for different substances and varies with pressure.

## Boiling Point

The temperature at which a substance has its liquid and vapor phases at thermal equilibrium with each other is known as the boiling point of that substance. Boiling point increases with increasing pressure. Cooking is faster in pressure cookers and slower at high altitudes.

In the case of water, the point where ice, vapors and water co-exist is known as the triple point of water. We can measure the triple point by measuring temperature but pressure has to be specific throughout the process, say 1 atmosphere.

## Critical Point

The end point or the terminal point of the vaporization curve or liquid-vapor equilibrium line is known as the critical point. Away from the critical point means temperature and pressure less than the critical point. For temperature and pressure greater than the critical point, there is no change in the phase of the substance but the properties of substance will change.

## First Order Phase Transition

If Gibbs free energy is continuous but its first order derivatives are discontinuous at transition point, then it is called the first order phase transition.

• Amount of discontinuity is finite ⇒ discontinuous nature
• Amount of discontinuity is infinite ⇒ diverging nature

Examples:

Water-vapor transition away from the critical point.

• ice-water transition
• ice-vapor transition
1. Volume changes discontinuously.
2. Number density changes discontinuously at transition point.
3. Energy, magnetization changes discontinuously at transition point
4. Density changes discontinuously at transition point.

## Second Order Phase Transition

The phase transition in which Gibbs free energy and its first order derivatives are continuous but its second order derivatives are discontinuous at the transition point is called second order phase transition.

• Entropy, volume and magnetization change continuously at transition point.
• Specific heat, compressibility, magnetic susceptibility change discontinuously at transition point.
• Example: Transition from Helium 1-Helium 2, transition from normal conducting state to superconducting state and vice versa, Transition from paramagnetic material to ferromagnetic material, liquid transition at critical point.
• Only properties of the substance will change while the physical state remains the same.

## Latent Heat

The amount of heat energy required to change the phase of 1g of a material at constant temperature is called latent heat.

Calorimetry principle: Heat is transferred from higher temperature to lower temperature until the temperature of two bodies remain same.

## Extensive and Intensive Variables

If the total system is divided into two equal parts, then properties that are independent of this division are known as intensive properties and those properties that become exactly halved are known as extensive properties.

For example : Number of particles, Volume, Internal energy, entropy comes under the category of extensive properties.

Pressure, Temperature, chemical potential, particle density comes under the category of intensive properties.

## Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for

• Bachelors in Science in Physics
• Masters in Science in Physics

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