Thermodynamics Overview

An iron bar, tapered at the end, glowing from the heat.
A heated metal bar. Dave King/Getty Images

Thermodynamics is the field of physics that deals with the relationship between heat and other properties (such as pressure, density, temperature, etc.) in a substance.

Specifically, thermodynamics focuses largely on how a heat transfer is related to various energy changes within a physical system undergoing a thermodynamic process. Such processes usually result in work being done by the system and are guided by the laws of thermodynamics.

Basic Concepts of Heat Transfer

Broadly speaking, the heat of a material is understood as a representation of the energy contained within the particles of that material. This is known as the kinetic theory of gases, though the concept applies in varying degrees to solids and liquids as well. The heat from the motion of these particles can transfer into nearby particles, and therefore into other parts of the material or other materials, through a variety of means:

  • Thermal Contact is when two substances can affect each other's temperature.
  • Thermal Equilibrium is when two substances in thermal contact no longer transfer heat.
  • Thermal Expansion takes place when a substance expands in volume as it gains heat. Thermal contraction also exists.
  • Conduction is when heat flows through a heated solid.
  • Convection is when heated particles transfer heat to another substance, such as cooking something in boiling water.
  • Radiation is when heat is transferred through electromagnetic waves, such as from the sun.
  • Insulation is when a low-conducting material is used to prevent heat transfer.

Thermodynamic Processes

A system undergoes a thermodynamic process when there is some sort of energetic change within the system, generally associated with changes in pressure, volume, internal energy (i.e. temperature), or any sort of heat transfer.

There are several specific types of thermodynamic processes that have special properties:

States of Matter

A state of matter is a description of the type of physical structure that a material substance manifests, with properties that describe how the material holds together (or doesn't). There are five states of matter, though only the first three of them are usually included in the way we think about states of matter:

Many substances can transition between the gas, liquid, and solid phases of matter, while only a few rare substances are known to be able to enter a superfluid state. Plasma is a distinct state of matter, such as lightning 

Heat Capacity

The heat capacity, C, of an object is the ratio of change in heat (energy change, ΔQ, where the Greek symbol Delta, Δ, denotes a change in the quantity) to change in temperature (ΔT).

C = ΔQ / ΔT

The heat capacity of a substance indicates the ease with which a substance heats up. A good thermal conductor would have a low heat capacity, indicating that a small amount of energy causes a large temperature change. A good thermal insulator would have a large heat capacity, indicating that much energy transfer is needed for a temperature change.

Ideal Gas Equations

There are various ideal gas equations which relate temperature (T1), pressure (P1), and volume (V1). These values after a thermodynamic change is indicated by (T2), (P2), and (V2). For a given amount of a substance, n (measured in moles), the following relationships hold:

Boyle's Law (T is constant):
P1V1 = P2V2

Charles/Gay-Lussac Law (P is constant):
V1/T1 = V2/T2

Ideal Gas Law:
P1V1/T1 = P2V2/T2 = nR

R is the ideal gas constant, R = 8.3145 J/mol*K.

For a given amount of matter, therefore, nR is constant, which gives the Ideal Gas Law.

Laws of Thermodynamics

  • Zeroeth Law of Thermodynamics - Two systems each in thermal equilibrium with a third system are in thermal equilibrium to each other.
  • First Law of Thermodynamics - The change in the energy of a system is the amount of energy added to the system minus the energy spent doing work.
  • Second Law of Thermodynamics - It is impossible for a process to have as its sole result the transfer of heat from a cooler body to a hotter one.
  • Third Law of Thermodynamics - It is impossible to reduce any system to absolute zero in a finite series of operations. This means that a perfectly efficient heat engine cannot be created.

The Second Law & Entropy

The Second Law of Thermodynamics can be restated to talk about entropy, which is a quantitative measurement of the disorder in a system. The change in heat divided by the absolute temperature is the entropy change of the process. Defined this way, the Second Law can be restated as:

In any closed system, the entropy of the system will either remain constant or increase.

By "closed system" it means that every part of the process is included when calculating the entropy of the system.

More About Thermodynamics

In some ways, treating thermodynamics as a distinct discipline of physics is misleading. Thermodynamics touches on virtually every field of physics, from astrophysics to biophysics, because they all deal in some fashion with the change of energy in a system. Without the ability of a system to use energy within the system to do work — the heart of thermodynamics — there would be nothing for physicists to study.

That having been said, there are some fields use thermodynamics in passing as they go about studying other phenomena, while there are a wide range of fields which focus heavily on the thermodynamics situations involved. Here are some of the sub-fields of thermodynamics:

  • Cryophysics / Cryogenics / Low Temperature Physics - the study of physical properties in low temperature situations, far below temperatures experienced on even the coldest regions of the Earth. An example of this is the study of superfluids.
  • Fluid Dynamics / Fluid Mechanics - the study of the physical properties of "fluids," specifically defined in this case to be liquids and gases.
  • High Pressure Physics - the study of physics in extremely high pressure systems, generally related to fluid dynamics.
  • Meteorology / Weather Physics - the physics of the weather, pressure systems in the atmosphere, etc.
  • Plasma Physics - the study of matter in the plasma state.