P is the pressure in n/m 2. The volume of v is m 3. Multiplied by PV is n * m, which means it can be converted into J Joule. Needless to say, it is indeed a measure of energy. For example, the most common is a gasoline engine. The high temperature and high pressure gas in the cylinder can output torque after expansion, that is, it can be converted into mechanical energy. It can be understood that the container expands under the action of pressure in the container, such as a hydrogen balloon. In the process of expansion, gas does work, and the measurement of work done by gas can be expressed by PV. But quantifying this situation is very complicated and requires calculus. When they expand, both p and v will change. P will generally decrease and v will increase. Usually, the energy of PV is like this, which is the work done by the infinite expansion of this gas volume to absolute zero vacuum. But in practice, the changes of P and V are limited, so δ V or δ P will be introduced.