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What is Thermodynamics?

Updated on January 28, 2020

A basic easy learning and revision guide to Thermodynamics. I’ve tried to make this article easy to follow by using my own learning techniques i.e. very simple and without repetition.

The Basics

● Thermodynamics in its simplest definition is; the study of HEAT (thermo) and MOVEMENT (dynamics).

● Thermodynamics is a branch of physics associated with the flow of HEAT and ENERGY into or out of a SYSTEM as it undergoes physical or chemical change.

● Any “SYSTEM” can be classed as a SOLID, a LIQUID or a GAS.

● The Laws of Thermodynamics describe accepted physical quantities that define thermodynamic systems.

● These physical quantities are; ENERGY, ENTROPY and TEMPERATURE.

● The Laws of Thermodynamics govern the DIRECTION of a spontaneous process.

● If enough separate interactions are involved then the DIRECTION will ALWAYS be towards increased ENTROPY.

● If you increase the HEAT in any system what you are actually doing is increasing the ENERGY in the system. i.e. HEAT = ENERGY.

The Four Laws of Thermodynamics


● It is called the ZEROTH law because it was developed after the first and second laws had already been established and named.

● It was considered more important so it was given a lower number.

● When two systems are in thermal equilibrium with a third system then those two systems are in thermal equilibrium with each other.


● The First Law is also known as the LAW OF CONSERVATION OF ENERGY.

● This specifies that energy CANNOT be created or destroyed in an isolated system.

● It states that energy can only be transferred or changed from one form to another.

● There are two types of processes: HEAT and MOVEMENT (WORK) that can lead to an alteration in the internal energy of a system.


● The Second Law represents the amount of energy in a system’s thermal energy per unit of temperature that is no longer available to convert into useful work.

● Usually defined as the degree of DISORDER or RANDOMNESS in the system.

● The more disordered a substance is then the GREATER the Entropy.

● This specifies that the Entropy of any isolated system ALWAYS increases with time OR remains constant.

● The Entropy of a system increases when there are more available MICROSTATES.

● The Entropy of a substance at 1 atmospheric (atm) pressure determines if a reaction will take place spontaneously.

● Entropies typically INCREASE with molecular weight.

● The Entropy of a solid is LESS than that of a liquid which is LESS than a gas.

● It is also a measure of the here and now RANDOM activity in a system. How long it took and how many changes were involved to get to the present is not important.


● The Third Law states that the ENTROPY of a system will approach a constant value as the temperature nears ABSOLUTE ZERO.

● The Entropy of a perfect crystal at ABSOLUTE ZERO is exactly EQUAL to ZERO.

Spontaneous and Non-Spontaneous (ENDERGONIC) Processes

● A SPONTANEOUS process is capable of moving in a given direction WITHOUT needing to be influenced by an outside source of energy.

● Spontaneous processes are also called NATURAL processes and depending on the process can happen very quickly or take millions of years.

● A NON-SPONTANEOUS (Endergonic) reaction is a chemical reaction in which the normal change in free energy is positive and energy is absorbed.

Thermodynamic Systems

● A thermodynamic system is one that INTERACTS and EXCHANGES energy with the area around it.

ADIABATIC: describes a system that changes with no transfer of heat in or out.

ISOBARIC: describes a system that changes but the pressure stays constant.

ISOTHERMAL: describes a system that changes in every way but the temperature stays constant.

ISOVOLUMIC: describes a system that changes but the volume stays constant.

Some Helpful Facts

● The definition of ABSOLUTE ZERO is: The COLDEST temperature that is theoretically possible.

● The definition of ENDERGONIC is: A reaction that absorbs energy FROM its surroundings.

● The definition of EXERGONIC is: A reaction that releases energy INTO its surroundings.

● The definition of a MICROSTATE is: The description of a system that depends on the states of each element of that system.

● Energy can be shared between MICROSTATES of a system.

● All parts of the Universe that are NOT within the THERMODYNAMIC SYSTEM of interest are known as SURROUNDINGS.

● The term REVERSIBILITY is used to describe systems that are in equilibrium (i.e. balance) with themselves and the environment around them. A change in one direction will be balanced by an equal change in the opposite direction.

● Zones of HIGH temperatures give off energy to zones with LOWER temperatures.

● Heat will always move from a high to a lower temperature. A HEAT SOURCE is the area where you will find a HIGHER temperature. A HEAT SINK is the area you will find a LOWER temperature.

● ENTHALPY is a measure of the heat content of a CHEMICAL or PHYSICAL system. Therefore a change in heat will be an ENTHALPY CHANGE.

A Final Thought

Trying to put even the basics of Thermodynamics and its Laws into easy to understand (by me) bite-sizes tested my rudimentary writing skills to the limit.

If you are studying and learning all about the Thermodynamics then good on you and I wish you well.

If there are any mistakes please let me know.

© 2018 Brian OldWolf


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