Heat transfer occurs due to a temperature difference between two bodies or systems. Heat naturally flows from a region of higher temperature to a region of lower temperature until thermal equilibrium is reached. This behavior is governed by the second law of thermodynamics, specifically the concept of entropy.
Entropy is a measure of the disorder or randomness in a system. The second law of thermodynamics states that the total entropy of an isolated system tends to increase over time. In simpler terms, natural processes tend to move towards a state of greater disorder.
When two bodies or systems are in thermal contact and at different temperatures, the molecules or particles in the hotter body have higher kinetic energy and are moving more rapidly. As a result, they collide with the molecules or particles in the cooler body, transferring some of their energy.
The transfer of energy occurs until both bodies reach the same temperature, which corresponds to thermal equilibrium. At this point, there is no temperature difference, and the average kinetic energy and molecular motion of the particles are the same in both bodies. In this state, the system has reached maximum entropy because the energy is evenly distributed.
If there were no temperature difference between two bodies, it means that their particles have the same average kinetic energy. As a result, there is no net transfer of energy between them, and no heat flows. This is consistent with the second law of thermodynamics, which dictates that heat flows from hot to cold bodies in order to increase the overall entropy of the system.
In summary, heat transfer occurs due to temperature differences because it is a natural process driven by the tendency to increase entropy. Heat flows from hot to cold bodies until thermal equilibrium is reached, equalizing the temperatures and maximizing the entropy of the system. When there is no temperature difference, there is no net flow of heat because the energy is already evenly distributed between the bodies.