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Thermodynamics: First, Second and Zeroth Laws Made Easy

Updated on June 9, 2014

The Laws of Thermodynamics

Thermodynamics is a branch of physics that has been formally studied since the mid seventeenth century and became particularly important with the advent of the steam engine. But what is Thermodynamics and what are the three main laws of Thermodynamics?

Thermodynamics is the study of heat and its relation to other forms of energy. Familiar concepts such as temperature and pressure can be accurately defined as can the more obscure concept of entropy.

The First Law of Thermodynamics

The First Law of Thermodynamics - Conservation of Energy


The first law of thermodynamics expresses the concept of conservation of energy in the context of heat being passed to a system or body and can be written as:

(Heat given to a body) = (increase of internal energy) + (work done by the body)


ΔQ = ΔU + ΔW

e.g. where heat ΔQ is supplied to a gas which raises the internal energy from U to (U + ΔU) and allows the gas to do external work of ΔW

Second Law of Thermodynamics

What is the Second Law of Thermodynamics? - Direction of Heat-Flow


The first law of thermodynamics would not be violated if:

i) A heat engine converted internal heat entirely into mechanical energy

i.e. ΔW = ΔQ and ΔU = 0

ii) Heat gets transferred from a cold body to a hot one without any loss of energy

Through experimentation we know that neither of these processes can occur, so this observation is expressed as The Second Law of Thermodynamics: Here are two versions of the law from the mid-nineteenth century:

Clausius (1850):

No heat pump, reversible of irreversible, can transfer internal energy from a low temperature reservoir to a high temperature reservoir without work being done on it by some external agent.

Kelvin (1851):

No heat engine, reversible of irreversible, operating in a cycle, can take in heat from its surroundings and totally convert it into work.

The second law determines the direction of heat-flow, but as you can see this is also important information about the maximum efficiency of engines that were being developed at this time.

Zeroth Law of Thermodynamics

What is the Zeroth Law of Thermodynamics? - Thermal Equilibrium


The second Law of Thermodynamics is probably the most often quoted and the First Law is a statement of the conservation of energy, which is itself a fundamental principle on which the second law was based, but even more fundamental (and hence given, at a later stage, the rather unusual title of Zeroth Law) is the concept of thermal equilibrium on which the other laws depend:

The Zeroth Law of Thermodynamics states that if two bodies A and B are separately in thermal equilibrium with a third body C, then A and B are in thermal equilibrium with each other.

The History of Thermodynamics

The History of Thermodynamics

Great Physicists

Man has been fascinated by thermodynamics ever since (s)he first discovered fire, but it wasn't until the mid seventeenth century that it became a real scientific discipline. Otto von Guericke made the first vacuum pump in 1650 in order to disprove Aristotle's supposition that 'nature abhors a vacuum' and using a similar apparatus, in 1656, physicist and chemist Robert Boyle and Robert Hooke noticed, in experimentation, a relationship between temperature, pressure and volume (Boyle's Law)

In 1697 Thomas Savery built the first steam engine, followed by Thomas Newcomen in 1712, but these engines were very inefficient and attracted the attention of scientists, who applied the laws of thermodynamics to the engine designs to improve efficiency and that is when thermodynamics really became an important discipline.

Professor Joseph Black (University of Glasgow) developed the concepts of heat capacity and latent heat, fundamental concepts of thermodynamics. James Watt worked for Black as an instrument maker helping to experiment on his steam engine and dramatically increased the steam engine's efficiency. The first and second laws of thermodynamics were developed in the 1850s and statistical thermodynamics by physicists James Clerk Maxwell, Ludwig Boltzmann, Max Planck, Rudolf Clausius and J. Willard Gibbs.

What is Entropy?

Statistical Thermodynamics

Statistical Thermodynamics
Statistical Thermodynamics

What is Entropy?

Statistical Thermodynamics

The First and Second Laws of Thermodynamics resulted from observation and experimentation, the Zeroth Law was a prerequisite for the other two (hence the strange name) This is Classical Thermodynamics, considering whole systems in thermodynamic equilibrium, rather than the actions of molecules and atoms that make up those systems. But to explain the causes of these laws, statistical thermodynamics (also known as statistical mechanics) is required, which gives rise to the concept of Entropy.

Statistical thermodynamics is the study of the probabilities of the constituent parts (i.e. atoms and molecules) of the whole complex system, being in certain locations or states, or of certain events occurring (e.g. similar to Brownian Motion, the random movements of particles in a fluid, which was studied with similar stochastic or probabilistic theory, by Albert Einstein and later by Black, Scholes and Merton to model the movements of stock markets!)

A simple explanation of what Entropy is, is disorder. The Third Law of Thermodynamics (see next module) implies that disorder will always increase (e.g. a child's bedroom never gets less messy - this can be proved statistically)

Statistical Thermodynamics is a complex subject, but the concept can be simplified by considering just two atoms and the distribution of quanta of energy (i.e. individual identically sized lumps of energy) If we have no quanta of energy there is only one possible distribution (two atoms each with no energy), with one quantum we have two (either atom can have the quantum), with two quanta we have three possible arrangements and the number of possible arrangement of the atoms increases as the number of quanta increases. These quata of energy are heat and the more heat the higher the temperature.

If we have two solid bodies A and B with temperatures Ta and Tb (Ta > Tb) and the number of ways of distributing quanta of energy in the particles of each each is Wa and Wb (Wa > Wb because A has more energy) the total number of ways of arranging the energy quanta is Wa x Wb. If A and B are put in thermal contact the quanta can move between the two bodies resulting in a new number of ways of arranging the quanta in each of wa and wb where wa < Wa and wb > Wb, because (wa x wb) > (Wa x Wb) i.e. because there are more ways of arrange the quanta if the bodies are both the same temperature the probability of heat flowing from hot to cold is higher than the other way. So this is a statistical explanation of temperature and heat-flow.

Change of Entropy is defined as:

δS = δQ /T

Which implies that =>

S = k ln W

(where k is the Boltzmann Constant)

For a reversible process ΔS = 0 but for a system initially not in equilibrium ΔS > 0

i.e. entropy rises

The Third Law of Thermodynamics

Statistical Thermodynamics
Statistical Thermodynamics

The Third Law of Thermodynamics

Statistical Thermodynamics

The Third law of thermodynamics: As a system approaches absolute zero, all processes cease and the entropy of the system approaches a minimum value.

The third law of thermodynamics results from statistical analysis of the complex system and its entropy (see previous module) and the impossibility of reaching a temperature of absolute zero (-273.15 Celsius or -459.67 °F i.e. 0 kelvin) or below. Another version of the law is: "the entropy of all systems and of all states of a system is smallest at absolute zero,"

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    • Redneck Lady Luck profile image

      Lorelei Cohen 5 years ago from Canada

      I think that I had better hop in my coffee pot and swim a few laps in there so I can take a second look at thermodynamics. Yep, you are well up and beyond my comprehension of such matters. I look at a pot on the stove, and my brain alike a cave man says, "Hot. Don't touch." Your brain looks at that same pot and does some pretty amazing calculations of why.

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      fenellashorty 5 years ago

      Well done Andy. Nicely explained