Second law of thermodynamics and entropy

What does the Second Law of Thermodynamics state?

The entropy of the universe only increases.

What is entropy?

Entropy is the disorder of a system, which can be thought of as the number of states a system could take on.

What is a closed system?

A closed system is a system that is isolated from its surroundings and does not exchange matter or energy with the outside environment.

What is an open system?

An open system interacts thermodynamically with its surroundings, exchanging heat, matter, or energy.

Give an example of an open system.

A campfire, because it releases heat and light into the surroundings and interacts with the environment.

Give an example of an approximate closed system.

An ice cooler, which is thermodynamically insulated from its surroundings to slow heat transfer, but is not perfect.

What is considered the ultimate closed system?

The universe, because it has nothing outside of it to interact with thermodynamically.

Why is an ice cooler not a perfect closed system?

Because heat from the environment will eventually transfer to the ice inside, causing it to melt.

What happens to molecules in diffusion over time in a closed container?

Molecules spread out to occupy the entire volume of the container, increasing the number of possible states and thus increasing entropy.

How does diffusion illustrate the Second Law of Thermodynamics?

Diffusion shows that a system moves from fewer possible states (lower entropy) to more possible states (higher entropy), which is an irreversible process.

What makes a process irreversible?

A process is irreversible if it moves from a state of lower entropy to higher entropy with extremely low probability of spontaneously returning to the original state.

Why are processes like diffusion considered irreversible in practice?

Because the number of molecules is so large (20–30 zeros) that the probability of all molecules randomly returning to the initial state is essentially zero.

What is an example of an everyday reversible-seeming process?

A billiard ball colliding with another, which seems reversible at a macro level but still generates some heat microscopically.

Why are macro-level reversible processes actually only approximately reversible?

Because microscopic interactions, friction, and heat generation increase the entropy slightly, so the process is never truly reversible.

How does heat contribute to increasing entropy?

Heat dissipated from bodies, friction, electronics, and other activities increases the number of possible molecular states, raising entropy.

Give examples of sources of heat increasing the universe’s entropy.

Body heat while moving, friction from using tools, running computers, and electrons traveling in wires all release heat into the universe.

Why is the probability of molecules spontaneously returning to lower-entropy states so low?

Because the number of molecules in real systems is extremely large, making the odds of all molecules randomly moving to their original positions astronomically small.

How is disorder in a system related to the number of possible states?

As disorder increases, the number of possible states increases, which corresponds to higher entropy.

How does the Second Law of Thermodynamics relate to molecular-level interactions?

Every molecular interaction, collision, and energy transfer increases the number of possible states, contributing to the overall increase of entropy in the universe.

Why do people often observe reversible processes at the macroscopic level?

Because changes in entropy are very small and not noticeable at the scale of everyday objects, even though entropy increases microscopically