In the simplest form, thermodynamics is the study of energy. In particular, it is the study of how heat energy behaves and how it can be converted into other forms of energy. The laws of thermodynamics are essential for understanding many everyday phenomena, such as why a hot cup of coffee will eventually cool down to room temperature.
The first law of thermodynamics, also known as the law of conservation of energy, states that energy cannot be created or destroyed. This means that the total amount of energy in the universe is always constant. The second law of thermodynamics says that entropy always increases over time. Entropy is a measure of disorderliness or randomness in a system. Thus, the second law states that systems tend to become more disordered over time.
The third law of thermodynamics says that absolute zero cannot be reached. Absolute zero is the lowest possible temperature where all thermal motion ceases. This means that it is impossible to achieve perfect order in a system (i.e., entropy equals zero).
Thermodynamic processes are classified into two types: reversible and irreversible processes. A reversible process is one in which there is no net change in entropy; an irreversible process is one in which there is a net increase in entropy. Examples of reversible processes include adiabatic expansion (such as when a gas expands without exchanging heat with its surroundings) and mixing two perfect gases at constant temperature (such as when nitrogen and oxygen mix to form air). Examples of irreversible processes include mixing two imperfect gases (such as when nitrogen and oxygen mix to form smog) or allowing heat to flow from a hotter object to a cooler object (which increases entropy).