Why energy transfer and conversion are one-way. Because of the principle of entropy increase, the transfer or conversion of energy must be in the direction of entropy increase of the system, just as heat can only be spontaneously transferred from a high-temperature object to a low-temperature object, and water will always flow downwards (gravitational potential energy is converted into kinetic energy).

The conversion and transfer of energy is directional. Energy will neither be produced out of thin air nor disappear out of thin air. It can only be transferred from one object to another, and the forms of energy can also be converted to each other. In the process of transfer and transformation, the total amount of energy remains unchanged.

Energy conversion and transfer actually refers to heat conduction. According to the second law of thermodynamics, the time course of heat conduction is directional. This process can be carried out spontaneously in one direction, but not in the opposite direction, so the transformation and transfer of energy is directional.