Which of the following is true regarding the flow of heat?

The correct statement about the flow of heat is that it can only flow from hot to cold substances without external work. This principle is grounded in the second law of thermodynamics, which asserts that heat energy naturally moves from regions of higher temperature to regions of lower temperature in an isolated system.

This natural flow occurs because the energy distribution is more favorable in cooler areas when heat is introduced, leading to a more uniform energy level within the system. The concept emphasizes that under normal circumstances, heat does not flow in the opposite direction—that is, from colder regions to warmer regions—unless external energy, such as work performed by a heat pump, is applied to cause such a movement.

The correct statement is foundational in understanding thermal processes and energy transfer in various systems, whether in heating applications, refrigeration, or even in understanding natural phenomena. In contrast, options that suggest heat can spontaneously flow from cold to hot or in both directions without any influence of external energy are contrary to established thermodynamic principles.