# What Is Entropy?

How entropy counts the ways particles and energy can spread out, and why hot drinks cool, inks mix and heat flows one way\. Covers coin counting and a heat engine\.

By Alvaro Tolosa

Topic: Physics & Chemistry

[Open the interactive Jig](https://ask.jig.com/j/yrcWisKC7JTa)

## Transcript

### 1. What is entropy?

A hot drink cools down\. A drop of ink spreads through water\. Entropy is a way of measuring how spread out energy and things are, and it explains why these changes only go one way\.

Drag anywhere to turn the view and look around the workshop\.

### 2. Energy spreads out

Tiny particles are always jiggling\. Hot ones jiggle fast, and when they bump into slower ones they share their energy\. So heat squeezed into one corner soon spreads out until every part of the box is equally warm\.

### 3. Neat or spread out?

There are only a few ways to pack particles neatly into one corner\. There are very many ways to scatter them all over the box, and most of those ways look alike\. That is why spread out is what we almost always see\.

Scientists say an arrangement with more possible ways has higher entropy\. "Disordered" really means "can happen in many more ways"\.

### 4. Counting the ways

Throw 10 coins\. Only one way gives all heads, but 252 different ways give five heads and five tails\. Entropy is a count of ways like this\. With trillions of particles instead of 10 coins, the most mixed result wins almost every time\.

With just 100 coins, the chance of all heads is about 1 in a million million million million million\.

### 5. Mixing never unmixes by itself

Lift the wall between green and orange particles and they wander into each other\. They mix, and they stay mixed\. Unmixing is not forbidden by any force; it is just so unlikely that you would never see it happen\.

The same happens when milk swirls into coffee or ink spreads in water\.

### 6. Heat flows from hot to cold

Move the slider to let heat flow\. Fast, hot particles pass energy to slow, cold ones until both sides are equally warm\. On the way, the heat can turn the engine wheel\.

Sliding back plays the film in reverse\. Heat never flows from cold to hot by itself\.

### 7. An engine rides the flow

Steam engines and car engines let heat run from hot to cold and catch part of it as motion\. They can never catch all of it\. Some heat always spreads out into the cooler air around them, and that spreading raises entropy\.

Even a perfect engine running between boiling water \(100 °C\) and melting ice \(0 °C\) could turn only about 27% of the heat into motion\.

### 8. The arrow of time

Things spread, mix and cool, but they never do the reverse by themselves\. So the total entropy of everything keeps growing\. That is how we can tell the past from the future: a film of an egg unbreaking looks wrong at once\.

### 9. Order still grows in places

Entropy does not stop order from forming\. A snowflake, a growing plant or a fridge makes neat order in one place by sending even more heat out around it\. Counted all together, entropy still grows\.