The laws of thermodynamics primarily govern systems that transfer thermal energy into at least another form of energy, or into work. They represent the absolute fundamental rules which all thermodynamic systems follow when undergoing any energy change.
The zeroth law
Thermodynamic systems obey similar fundamental rules to those of basic arithmetic. The transitive law of mathematics states that if X = Y and Y = Z, then it must be true that X = Z. In thermodynamics, this transitive law also applies. If two systems are in thermal equilibrium with a third system, then they must also be in thermal equilibrium with each other. The zeroth law was officially noted at a later date than the other three laws of thermodynamics. Its name was selected on the basis that this law is even more fundamental than the others.
The first law of thermodynamics
The increase in internal energy of a system is equal to the heat supplied to the system plus the work done. This law explains how an increase of heat can only ever result in two outcomes. Either the internal energy is changed, or the work is done. In realistic terms, the most likely outcome is a combination of the two. The first law of thermodynamics distinguishes between energy transfer in the form of heat, and energy transfer in the form of work done. It also acts as a fundamental principle of the law of conservation of energy. Energy cannot be created or destroyed. It can only ever change form.
The second law of thermodynamics
The second law of thermodynamics explains how a transfer of energy can never be truly
100% efficient, even in theoretical terms. To transfer heat energy from a body of lower temperature to a body of higher temperature will inevitably require work to be done and energy to be lost.
The third law of thermodynamics
It is realistically impossible to reduce any thermodynamic system to a temperature of absolute zero.
Absolute zero is the state at which the internal energy of a solid is exactly 0. The third law of thermodynamics explains how this stage can never be reached without an infinite number of operations. Instead, it will reduce to a constant close to absolute zero, but quantum restraints will prevent further reduction.