Thermodynamics

Introduction:

What is Thermodynamics?

· Thermodynamics is the branch of physics that deals with the concepts of heat and temperature and the inter conversion of heat and other forms of energy.

· Thermodynamics is a macroscopic science. It deals with bulk systems and does not go into the molecular constitution of matter.

· Thermodynamic description involves relatively few macroscopic variables of the system, which are suggested by common sense and can be usually measured directly.

· Example: Would involve specifying the co-ordinates and velocities of the huge number of molecules constituting the gas.

Thermodynamics vs Mechanics:

· In Thermodynamics only the state of the object which means only consider macroscopic variables like pressure, volume and temperature.

· In mechanics consider the motion, velocity and acceleration of the object.

· Example: In mechanics if a bullet is fired from a gun we will consider the motion of bullet and its velocity, acceleration etc.

Bullet getting fired from gun

Consider a second scenario where bullet is fired on a wall. When it hits the wall its kinetic energy gets converted to heat. Due to heat generation there will be a change in temperature and the internal energy of the bullet. These are the macroscopic variables in Thermodynamics.

Bullet fired from the gun and hitting a wall

Thermal Equilibrium:

What is Thermal Equilibrium?

Two systems are said to be in thermal equilibrium with each other if they have the same temperature.

Thermal Equilibrium

How it is Attained?

Consider a gas inside a closed rigid container, completely insulated from its surroundings, with fixed values of pressure, volume, temperature, mass and composition that do not change with time, is in a state of thermodynamic equilibrium.

Consider two gases A and B occupying two different containers. Pressure and volume of a given mass of gas can be chosen to be its two independent variables.

Let the pressure and volume of the gases be (PA, VA) and (PB, VB) respectively. Suppose first that the two systems are put in proximity but are separated by an adiabatic wall – an insulating wall (can be movable) that does not allow flow of energy (heat) from one to another. The systems are insulated from the rest of the surroundings also by similar adiabatic walls.

Systems A and B (two gases) separated by an adiabatic wall – an insulating wall that does not allow flow of heat.

In figure A:

It is found that any possible pair of values (PA, VA) will be in equilibrium with any possible pair of values (PB, VB).

The same systems A and B separated by a diathermic wall – a conducting wall that allows heat to flow from one to another. In this case, thermal equilibrium is attained in due course.

In figure B:

Suppose that the adiabatic wall is replaced by a diathermic wall – a conducting wall that allows energy flow (heat) from one to another. It is then found that the macroscopic variables of the systems A and B change spontaneously until both the systems attain equilibrium states. After that there is no change in their states.

Conclusion:

The pressure and volume variables of the two gases change to (PB , VB ) and (PA , VA ) such that the new states of A and B are in equilibrium with each other .There is no more energy flow from one to another .We then say that the system A is in thermal equilibrium with the system B.

Definition of Temperature (Zeroth Law of Thermodynamics):

What is Zeroth Law of Thermodynamics?

This law identifies thermal equilibrium and introduces temperature as a tool for identifying equilibrium.

“Zeroth law of thermodynamics states that when two systems are in thermal equilibrium through a third system separately then they are in thermal equilibrium with each other also”.

Example for Zeroth Law of Thermodynamics:

· Consider two systems A and B which are separated by an adiabatic wall. Heat flow happens between systems A and C, and between B and C, due to which all 3 systems attain thermal equilibrium.

· Systems A and B are in thermal equilibrium with C. Then they will be in equilibrium with each other also.

· Zeroth Law of Thermodynamics suggested that there should be some physical quantity which should have same value for the system to be in thermal equilibrium.

· This physical quantity which determines whether system is in equilibrium or not is Temperature.

· Temperature is the quantity which determines whether the system is in thermal equilibrium with the neighboring system.

· When the temperature becomes equal then the flow of heat stops.

How does Zeroth Law of Thermodynamics leads to the Concept of Temperature?

Concept of Temperature: Temperature is a physical quantity which has the same value for all systems which are in thermal equilibrium with each other. Temperature of a system determines whether it is in thermal equilibrium or not with another system.

· There exists a scalar quantity called "temperature" which is a property of all thermodynamic system such that temperature equality is a necessary and sufficient condition for thermal equilibrium.

· Heat may be defined as energy in transit.

· Word heat is used only if there is a transfer of energy from one thermodynamic system to another.

· When two systems at different temperatures are kept in contact with each other then, after some time temperatures of both the systems become equal and this phenomenon can be described by saying that energy has flown from one system to another.

· This flow of energy from one system to another on account of temperature difference is called heat transfer.

· Flow of heat is a non-mechanical mode of energy transfer.

· Heat flow depends not only on initial and find states but also on path it's.