A Simple Explanation of Entropy

In this post, I’ll explain the concept of entropy using a real-life example.

First, some definitions

Thermodynamics is the study of how energy is transferred and transformed in physical systems.

A system is a specific portion of the physical world that is being studied using the laws of thermodynamics.

Equilibrium is the state a system is in when there is no transfer of energy or matter.

Now, the explanation

It’s the end of the day and you stop at a coffee shop on your way home from work. You get a hot cup of coffee. You get home and place the cup on a table in your living room. The coffee is too hot to drink just yet, so you decide to take a quick shower and drink it when you get out.

Naturally, you get sidetracked. You planned on taking a five-minute shower, but you start wondering about those disgraceful people who pour milk before cereal. Obviously, you know this is an immoral act. But why? Why is it so atrocious? And what goes on in the minds of those poor lunatics who know not which food item to pour first?

You wonder about the psychology behind milk-first-pourers. You wonder, is this the result of nature, or of nurture? Is there any hope for these people to turn a new leaf?

Before you know it, you’ve considered dozens of aspects of this relatively unimportant phenomenon in life. It’s been forty-five minutes. Oh no, you realize what this means. You get out of the shower and rush to your coffee. It’s too late. Your coffee has gone cold.

Why has your coffee gone cold? To answer that, we need to analyze the thermodynamic process that took place — and learn a bit about entropy in the process.

The system we’re analyzing here is the cup of coffee. The system’s environment is the living room.

Here’s your cup when you first left it. The coffee is piping hot.

The second law of thermodynamics tells us that heat always moves from hotter areas to colder areas. This system is not in equilibrium since the coffee is hotter than everything around it. As you waste your time thinking about cereal, the heat from the coffee moves into the cooler air.

The coffee gets colder and the air gets hotter. This happens until the coffee and the living room are the same temperature.

By now you’re probably thinking, “Cool story, but how does entropy factor into this?” Well, entropy can be thought of as the measure of how decentralized energy is in a given system. The more decentralized, the higher the entropy.

When you first put the cup of coffee down on the table, the system had low entropy. This is because energy (heat) was highly centralized in one area (the coffee cup). After forty-five minutes, the heat decentralized, the system reached equilibrium, and entropy increased.

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