Thermodynamics: A Beginners Guide
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.
● 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
THE ZEROTH LAW
● 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 (ENERGY > E)
● 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 (ENTROPY > S)
● 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 (TEMPERATURE > T)
● 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.
● 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