State the second law of thermodynamics. How does the second law explain why heat travels from a substance at higher temperature to one at lower temperature?

Entropy is one of the cornerstones of thermodynamics and a pivotal part of understanding physical processes. In essence, entropy is a measure of the molecular disorder within a system, often interpreted as a measure of randomness or chaos. The second law of thermodynamics highlights entropy’s inevitability by stating that the total entropy of an isolated system can never decrease over time. It either remains constant for a reversible process or increases for an irreversible one.

An increase in entropy corresponds to a loss of energy available to do work, thereby rendering systems naturally inclined to move towards a more disordered state. This concept is critical because it sets the fundamental direction of energy flow and the extent to which energy transformations can occur within any given system.

Heat transfer is a phenomenon that occurs when there is a temperature difference within a system or between different systems in thermal contact. The second law of thermodynamics describes the directionality of this transfer, stating that heat will spontaneously transfer from a warmer object to a cooler one until thermal equilibrium is reached.

Heat can be transferred in three main ways: conduction, in solid materials; convection, in fluids; and radiation, through empty space. Each of these mechanisms abides by the principle that heat moves from high to low temperature regions, increasing the randomness—hence the entropy—of the involved substances. In doing so, the transfer of heat can be seen as nature’s way of dispersing energy to achieve a balance and maximize disorder.

Thermodynamic processes are the paths taken by a system from one state to another. They include isothermal (constant temperature), adiabatic (no heat exchange), isobaric (constant pressure), and isochoric (constant volume) processes. Each of these processes follows the laws of thermodynamics, with the second law providing the direction in which these processes will naturally proceed.

The second law introduces the concept of irreversibility in natural processes and explains why certain processes occur spontaneously while others do not. For example, it tells us that without the input of extra energy, heat will not flow from a cold object to a hot object because such a process would decrease the overall entropy of the system, which is contrary to the second law.

The concept of molecular disorder relates to the arrangement and energy distribution of particles within a physical system. A system with high molecular disorder has particles that are spread out and have a wide range of energies, such as in a gas. A system with low molecular disorder, like a solid, has particles that are tightly packed and have relatively uniform energies.

The second law of thermodynamics is often visualized at this microscopic level; it indicates that systems will shift towards the state with greater molecular disorder. The distribution of heat, as part of energy exchange, is fundamentally a quest to increase disorder. This concept explains why cooling a hot substance or heating a cool one increases disorder as the particle energies become more varied, hence driving the direction of natural processes.