Heat naturally flows from a hotter source to a colder source due to the fundamental principle of thermodynamics known as the Second Law of Thermodynamics. This principle is based on the concept of entropy, which is a measure of the disorder or randomness of a system.
The Second Law of Thermodynamics states that in an isolated system, the total entropy tends to increase or remain constant over time. Entropy can be thought of as a measure of the distribution of energy within a system. When energy is distributed evenly, the system is in a state of higher entropy.
When there is a temperature difference between two objects or systems, heat transfer occurs spontaneously from the hotter object to the colder object. This is because the hotter object has higher internal energy, and its molecules are more energetically active. The random thermal motion of the molecules causes them to collide with neighboring molecules and transfer energy in the form of heat.
As heat is transferred from the hotter object to the colder object, the energy distribution becomes more balanced, resulting in an increase in the entropy of the system. This transfer of heat continues until the temperatures of both objects equalize, reaching a state of thermal equilibrium where there is no net heat flow.
In summary, heat moves from a hotter source to a colder source because it is a natural process driven by the tendency of systems to increase their entropy and achieve a more balanced energy distribution.