The zeroth law of thermodynamics discusses thermal equilibrium, and in so doing, gives meaning to the term temperature. The first law describes the conservation of energy, and dictates that energy cannot be created nor destroyed. The second law says, in a variety ways, that no system can be 100% efficient, and as such, every system has losses.
If you are curious, the reason that there is a zeroth law is that it was developed decades after the first two, but precedes them in terms of scientific principle, in that it lays a foundation for further study. Rather than shift laws one and two up to two and three, it was decided that they be left alone, and that the new law be referred to as law zero. This was a great choice, because it gave scientists the opportunity to use the term "zeroth", and as a result, sound very intelligent.
In summary, law zero says there will be lunch, law one says that it will not be free, and law two says that actually, you have to tip.
While the zeroth and first laws of thermodynamics set up a structure for studying energy and temperature, the second law gets the wheels turning, by offering something somewhat unusual for science: direction.
The simplest statement of the second law is that heat travels spontaneously from hot bodies to cold ones. That is, the heat from a steaming cup of coffee leaves the liquid and travels to the cooler surrounding air in the room. This simple statement, so obvious in nature, gives rise to some of the most profound concepts in science.
No one would think for a moment that additional heat from the room would spontaneously transfer to an already scolding cup of coffee. That would be like an already wealthy person taking additional funds from someone who is financially constrained. Though such an occurence is not unsual within society, it is a road seldom taken by our equitable mother nature. The second law states that such a clear disregard for equilibrium is so improbable, that it is illegal. In nature, everything searches for equilibrium.
Let us take the air in a classroom for example. What are the chances that all of the particles that make up this volume of air will spontaneously shift to one side of the room, leaving a vacuum on the other side of the room (and causing terrible consequences for those students left in a vacuum of space)? The probability of this occuring is so low, that we may simply conclude that it is nil.
Let us now reverse the problem. If one were to manually displace all of the air particles in the classroom to one side, what would happen if they were set free? If the air were left to its own devices, it would immediately try to balance itself in the given space. The air would naturally disperse to fill the volume with air of equal pressure and temperature (assuming that the boundaries encompassing the space are uniform).
One might refer to the original state, where all of the particles are grouped together at one side, as 'organized' or orderly. The second situation, where the air is evenly dispersed, could then be deemed disorganized or disorderly. We could even call this dispersed situation 'random', or even better, 'chaotic'.
It turns out that scientists like to measure everything, including randomness. In thermodynamics, the quantity of chaos is called entropy. A very chaotic, disorganized state, is one with high entropy, whereas a very organized system has low entropy. Entropy is a measure of energy per unit temperature, or Joules per degree Kelvin (J/K).
Returning to the dispersing air within a room, we could conclude that the original, organized state had little entropy, while the random state, that it naturally tended towards, is one of high entropy. So, the notion that heat moves from hot bodies to cold ones leads directly to the idea that the entropy of a system increases with time. A kindergarten teacher can leave a room full of children calmly sitting at tables colouring, and return to complete pandemonium in just a few minutes. A closed system tends towards chaos: it is the second law.
So, how does this relate in any particular way to the Autumn season? I'm glad you asked.
Last week, I was raking leaves with my two-year-old daughter. After organizing the leaves into distinct piles, it was time for her to jump in them. After a half an hour of fun, I wanted to place the leaves in bags - my daughter, a lovely but at times possessive little girl, objected to the removal of her piles. I conceded, and left the piles of leaves as they lay, with the intention to return to bag them at a later time, when one particular little girl was napping.
As I raked the leaves into piles, again, and finally disposed of them, I thought of the second law of thermodynamics. The only way to bring about an increase in the organization of a system is to interfere with it. However, we must realize that when we do so, we become part of the system. The total system, consisting of the leaves, the rake, and my body, experiences an overall increase in entropy. I may, philosophically speaking, decrease the entropy of the leaves, but my concerted, non-spontaneous effort to do so requires an input of energy on my part, which causes my body temperature to increase. Overall, there is a net increase in entropy; there always is.
When I originally introduced the second law of thermodynamics, I stated it another way: no system can be 100% efficient. It turns out that a system that gains entropy has an inherent inefficiency associated with it. And since every system gains entropy with time, no system is perfectly efficient. This gives rise to yet another statement of the second law: there is no such thing as a perpetual motion device.
Every system has at least some loss associated with it. No contact is perfectly frictionless. A pendulum swaying back and forth will, after enough time, come to rest. Its mechanical energy will eventually all be converted into heat. The notion that no body can undergo perpetual motion is easy to relate to - we all need to rest. One spinoff that is not so rosy, but is equally true, is that nobody can live forever.
But you know, the second law can work for us under certain conditions... The next time someone calls you lazy, just remind them that there is no such thing as a perpetual motion device.
The final statement of the second law is easily the most compelling, as it imposes a constraint on time itself: time can only flow in one direction.
Imagine you are shown two pictures of a billiards table. The first picture shows fifteen coloured balls organized into a triangular shape with the white ball on the opposite side of the table, while the second has all of the balls placed randomly around the table. You could confidently ascertain that the second picture was taken after the first without even watching the game. How did you know? Since an increase in randomness occurs naturally as time moves forward, the second law goes so far as to stipulate the direction of time's arrow.
Most principles in science have a certain symmetry to them, that is, they look the same no matter how you view them. The second law of thermodynamics is special, because it implies that things move in one direction only. The notion of permanent change is rather unsettling, but we must accept the fact that our universe will have more entropy tomorrow than it did today. The second law proves that in life, there is no undo button.
The harsh reality is that the decisions we make in life cannot be unmade. And, if left unchecked, things naturally tend towards a big mess. It may take months to paint a masterpiece, carefully placing each grain of colour in just the right spot. It takes only minutes for the painting to be ruined by rain if left outside.
Thankfully, we can restore order to chaotic situations if we are willing to put in the work. Such actions raise our body temperature, which is welcomed now, but is embraced even more during the season that lurks around the corner.