Laws of Thermodynamics

State and Understand the Four Laws of Thermodynamics

The laws of thermodynamics describe how energy, entropy, and temperature function in physical systems.
The laws of thermodynamics describe how energy, entropy, and temperature function in physical systems. Mmdi, Getty Images

Although many people are only aware of three laws of thermodynamics, there are actually four laws. These are some of the most important scientific laws, particularly in physics, physical chemistry, and engineering. Here are simple statements of the laws of thermodynamics and an explanation of the key implications of these laws.

Zeroeth Law of Thermodynamics

The zeroeth law of thermodynamics states that two thermodynamic systems in equilibrium with a third system are in thermal equilibrium with each other.

Essentially, if A=B and B=C, then A=C. The zeroth law was actually described after the first three laws had been around awhile. Rather than reordering the numbers, the law was ranked zero.

  • The law seems obvious, yet it is still important. The zeroeth law of thermodynamics makes it possible to define temperature and thus to build thermometers that measure it.

First Law of Thermodynamics

The first law is also known as the Law of Conservation of Energy. The first law of thermodynamics states that energy cannot be created nor destroyed, but can only change forms. Another way of stating this law is to say the total energy of a system and its surroundings is constant. Yet another way to state this law is to say the change in the energy of a system equals heat flow from the surroundings minus work done by the system on its surroundings.

Key implications of this law are:

  • The total energy of the universe is a constant.
  • In a thermodynamic cycle, net heat put into the system equals net work that is done by the system.
  • This laws makes perpetual motion machines of the first kind impossible. This type of perpetual motion machine produces work without needing an input of energy.

Second Law of Thermodynamics

The second law of thermodynamics states that the entropy of an isolated system that is not in equilibrium will increase over time.

Entropy (the disorder of a system) will approach a maximum value when the system is at equilibrium.

  • The take-home point here is that the entropy of the universe will never decrease. It can stay the same or increase.
  • The second law makes perpetual motion machines of the second kind impossible. These are machines that spontaneously change thermal energy (heat) into mechanical work. While the machine works when the work done equals the input of thermal energy, it violates the entropy part of the second law. There is only one heat reservoir in a perpetual motion machine of the second kind, which is somehow spontaneously cooled without transferring heat.

Third Law of Thermodynamics

The third law of thermodynamics states that as temperature approaches absolute zero, the entropy of a system approaches a constant minimum. The entropy of a perfect crystal approaches zero as the temperature nears absolute zero.

  • The third law establishes an absolute reference point by which we can measure entropy.
  • Note the law says the value of entropy approaches a minimum, not that it equals zero. This is important because while entropy is 0 at 0K for most things, there is a tiny amount of residual energy for certain systems (e.g., non-crystalline solids or glasses) at 0K.

    Sadi Carnot formulated the first thermodynamic principle (now the second law of thermodynamics) in 1824. Since that time, the four laws have been described and renumbered multiple times and in different ways depending on the scientific discipline. The present number system and description of the laws has remained fairly constant since the latter part of the 20th century. New laws have been proposed, but they aren't generally included in textbooks.