Question

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?

Short answer

The second law of thermodynamics states that the entropy of any isolated system always increases. It explains that heat travels from a substance at higher temperature to one at lower temperature because such a process results in an increase in the total entropy of the system.

Step-by-step solution

Understanding the Second Law of Thermodynamics

The second law of thermodynamics states that in any cyclic process, the entropy of the system and its surroundings will either increase or remain constant. Entropy is a measure of disorder, and this law implies that systems tend to move towards a state of higher disorder or randomness.

Heat Flow and the Second Law

Heat naturally flows from a substance at a higher temperature to one at a lower temperature because doing so increases the overall entropy of the combined system. As heat leaves the hotter body, its molecules become less energetic and more orderly, while the molecules of the colder body gain energy and become more disordered. The overall effect is an increase in the total entropy.

Conclusion

The tendency of heat to flow from higher to lower temperature substances is an example of the second law of thermodynamics. The movement of heat increases the total entropy of the system, which is consistent with the natural direction of processes prescribed by this law.

Key concepts

Entropy

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

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

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.

Molecular Disorder

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.