Laws of Thermodynamics

We, like other living beings, are open systems: we exchange matter and energy with our surroundings. Food supplies chemical energy, and our bodies transfer energy through work and heat. These exchanges obey the same physical laws that describe engines, gases, and hot and cold objects.

Thermodynamics has four laws, conventionally called the zeroth, first, second, and third laws. Each applies under specified conditions.

Therefore, the deep impression which classical thermodynamics made on me. It is the only physical theory of universal content, which I am convinced, that within the framework of applicability of its basic concepts will never be overthrown. —Albert Einstein

First law: energy is conserved

Energy cannot be created or destroyed. It can be transferred between a system and its surroundings as heat or work, or carried by matter entering or leaving the system. An isolated system exchanges neither energy nor matter with its surroundings, so its total energy remains constant.

A perpetual motion machine of the first kind would produce work indefinitely without an energy supply or a corresponding reduction in stored energy. The first law rules it out.

Second law: total entropy cannot decrease

The entropy of an isolated system cannot decrease. It increases in an irreversible process and remains constant in the ideal reversible limit. An open system can lose entropy by exporting enough entropy to its surroundings; living organisms do this. Local order therefore does not violate the second law.

Entropy is the price of structure. —Ilya Prigogine

Entropy is often described as disorder, but that shorthand can mislead. In statistical thermodynamics, entropy relates to the number of microscopic arrangements compatible with a macroscopic state. It also helps determine how much energy is available to do work under specified conditions.

Heat engines can convert some heat into work. What the second law forbids is a cyclic engine whose sole effect is to draw heat from a single reservoir and convert all of it into work. A working engine must have another effect, such as transferring heat to a colder reservoir.

Nothing in life is certain except death, taxes, and the second law of thermodynamics. —Seth Lloyd

Equilibrium principles depend on the constraints imposed on the system:

• At fixed energy, volume, and composition, an isolated system reaches equilibrium at maximum entropy.
• At fixed entropy, volume, and composition, stable equilibrium minimizes internal energy.

These principles do not mean that every system always moves toward lower internal or potential energy. A closed system exchanges no matter, but it can exchange energy; its entropy need not remain constant. The appropriate equilibrium criterion depends on what is held fixed.

A ball rolling downhill loses gravitational potential energy, but that energy becomes motion, heat, sound, or deformation. It settles only if energy is dissipated or transferred away. Likewise, a compressed spring can release stored energy into motion and oscillate before damping brings it to rest.

If your theory is found to be against the Second Law of Thermodynamics, I give you no hope; there is nothing for it but to collapse in deepest humiliation. —Arthur Eddington

It is a remarkable fact that the second law of thermodynamics has played in the history of science a fundamental role far beyond its original scope. Suffice it to mention Boltzmann’s work on kinetic theory, Planck’s discovery of quantum theory or Einstein’s theory of spontaneous emission, which were all based on the second law of thermodynamics. —Ilya Prigogine

The increase of disorder or entropy is what distinguishes the past from the future, giving a direction to time. —Stephen Hawking

For a chemical reaction, consider the reacting system together with its surroundings. An exothermic reaction releases heat, increasing the surroundings’ entropy. An endothermic reaction absorbs heat, but it can still be thermodynamically favorable if other entropy changes more than compensate. Heat release alone does not establish whether a reaction is favorable, and thermodynamic favorability does not establish how quickly it proceeds.

I think you should always bear in mind that entropy is not on your side. —Elon Musk

Real processes often dissipate energy that could otherwise have done useful work. Engineering can reduce these losses. The attainable efficiency depends on the process and its operating conditions; the first law by itself does not tell us how much of an energy supply can be converted into useful work.

Third law: entropy near absolute zero

For a perfect crystal with a unique ground state, entropy approaches zero as temperature approaches absolute zero. Some materials retain residual entropy because their microscopic states are not uniquely ordered. A crystal’s low entropy does not mean that all atomic motion disappears.

The third law concerns low-temperature limits. It does not establish a universal rate at which entropy grows, nor does it show that every system becomes more disordered at every moment.

Zeroth law: thermal equilibrium

If two systems are each in thermal equilibrium with a third system, they are in thermal equilibrium with each other. This relation underlies temperature measurement: a thermometer can indicate whether other systems share a temperature.

A mnemonic

The laws are sometimes summarized as a game:

• 0. There is a game. (zeroth law)
• 1. You can’t win. (first law)
• 2. You can’t break even. (second law)
• 3. You can’t even get out of the game. (third law)

This is a memorable analogy, rather than a theorem or a substitute for the laws’ conditions. Business convenience, organizational disorder, and physical entropy require different explanations.