SECOND LAW

The Second Law of Thermodynamics, also known as the Law of Increased Entropy, says that over time the state of disorganization or entropy in a system will always increase. What do we mean by this? Take this example – why does your desk always get messier as the week progresses? Or more importantly, why doesn’t your office go from messy to clean without you having to do work on it? This is the arrow of time in thermodynamics. As time increases, so too does disorganization.

This phenomenon happens in any system. Over time, usable energy will eventually give way to unusable energy. While energy cannot be created or destroyed according to the First Law, it can change form a useful state to a less-useful state, like thermal energy (heat).

Over time, every system moves from a state of low to high entropImage source)

In our light bulb example again, the longer we leave our light bulb on, converting electrical energy into radiant energy, the more usable energy we continue to convert into unusable energy in the form of heat. As usable energy within a system, decreases and unusable energy increases, then we say that the entropy of a system has increased. Stated mathematically:

Here, the total entropy ΔSuniverse within the universe equals the total entropy within a system ΔSsys plus the total energy within all surroundings ΔSsurr, all of which cannot be less than 0. Why? Because at all times, at all hours of the day, all energy is being transformed from one form into another, and one of those forms is unusable energy. Driving in your car uses mechanical energy to produce the kinetic energy of motion, but in the process, you also transform a ton of energy into heat. It’s an inevitable byproduct.

Another way to think about entropy is with probabilities. Take a box filled with puzzle pieces as an example. What’s the probability that you dump all of the puzzle pieces out of the box, and one of the pieces randomly lands where it connects perfectly with another piece? It’s a low probability. In that same box, what’s the probability of a piece landing randomly where it doesn’t fit with another piece? It’s a high probability.

Total chaos! Entropy gets the upper hand with probability. (Image source)

In this puzzle example, the randomly placed puzzle piece represents a higher form of disorder or entropy. This is why tires release air when punctured, or why ice cubs left out at room temperature eventually melts, or why the electrons in a circuit flow from negative to positive. Sure, it could be possible for all of these actions to occur in reverse, but the probability of them occurring is so low, and the cards of increasing probability are stacked so high, that they simply never occur.

In electronics, we see the Second Law of Thermodynamics at work with the Seebeck Effect. This phenomenon occurs when heat is applied to one of two conductors, which causes heated electrons to flow toward the cooler conductor. If you connect this pair of heated conductors together in a circuit, then the heating effect will cause a direct current (DC) to flow through the circuit. In this situation, we have electrons in a lower state of entropy in a cold conductor reaching a higher state of entropy through the application of heat, and so disorder increases.

The Seebeck Effect using heat to generate a direct current. (Image source)

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