All energy flows linearly; that is, in one direction. There are several basic forms of energy, including kinetic, potential, heat, chemical, electrical, electrochemical, electromagnetic, sound and nuclear.
For example, heat energy travels between molecules from high heat energy to low heat energy or from hot to cold, in a linear fashion. Hot molecules have high heat energy that is absorbed from a heat source, which causes the molecules to vibrate vigorously. This vibration causes collisions between molecules, transferring heat energy.
Another example is electrical energy, which is the energy of moving electrons within an electrical conductor. The linear flow of electrons is from negative to positive because electrons are negatively charged particles that are repulsed by the negative end of an electrical circuit.