Q: Who cares about the second law of thermodynamics?
A: Anyone who wonders how the material world -- our world of energy and matter --works.
Q: Big deal?
A: The biggest, most powerful, most general idea in all of science. Why paper, trees, coal, gas and all things like them burn (and why people "should" spontaneously catch fire in air), why sand and dry ice even in pure oxygen can't ever burn, why the sun will eventually cool down, why iron rusts (but not why it rusts faster nearer the ocean), why there are hurricanes or any weather at all on earth, what makes things break, why houses get torn apart in tornadoes or explosions, why everything living tends to die.
That's for starters.
Q: Just STARTERS? OK, OK, I'm impressed. So, what IS the second law of thermodynamics?
Well, wait a minute, what's the first law?
A: The first law is very simple and important but pretty dull: You can't create or destroy energy.
You can just change it from one form to another, for example, electricity to heat, heat that will boil water and make steam, hot steam to push a piston (mechanical energy) or rotate a turbine that makes electricity that in turn can be changed to light in a light bulb or to sound in an audio speaker system, and so forth. That's it. Important but dull.
The second law of thermodynamics looks mathematically simple
but it has so many subtle and complex implications that it makes most chem majors
sweat a lot before (and after) they graduate. Fortunately its practical, down-to-earth
applications are easy and crystal clear. These are what we'll talk about. From
them we'll get to very sophisticated conclusions about how material substances
and objects affect our lives.
Q: Sounds fair. I'm listening.
A: Look at the direction that energy flows in any happening or process or event. That is the first step to understanding what the second law of thermodynamics is and what it applies to.
Energy spontaneously tends to flow only from being concentrated in one place
to becoming diffused or dispersed and spread out.
(Later we'll come back to those two tricky words "spontaneously" and "tends".) That's it, the big idea. The perfect illustration is: A hot frying pan cools down when it is taken off the kitchen stove. Its thermal energy ("heat") flows out to the cooler room air. The opposite never happens.
Q: Come on. All this build up for that dumb example?
A: Don't put me down. I could have snowed you with differential equations and diagrams instead of what you see everyday. We're being practical and visual rather than going the math route, essential as that is in chemistry.
The big deal is that all types of energy spread out like the energy in that hot pan does (unless somehow they're hindered from doing so) They don't tend to stay concentrated in a small space; they flow toward becoming dispersed if they can -- like electricity in a battery or a power line or lightning, wind from a high pressure weather system or air compressed in a tire, all heated objects, loud sounds, water or boulders that are high up on a mountain, your car's kinetic energy when you take your foot off the gas. All these different kinds of energy spread out if there's a way they can do so.
Get the picture? The second law of thermodynamics summarizes that totally different events involving all kinds of energy have a common cause. A blowout in a tire or a car battery shorting out or slowly running down -- what could seem to be more unlike than those! Yet the reason for their occurring is the same, the tendency for concentrated energy not to stay localized, to disperse if it has a chance and isn't hindered somehow.
A major goal in life is to find true BIG ideas that describe how the world works, to understand why and how things happen around us in terms of a small number of basic principles. Principles that can be tested and checked. You can't do better than the second law of thermodynamics. And the direction of energy flow is just a tip of the iceberg of that law.
Q : Iceberg? Titanic iceberg?
A: Come on now. You know that's just a figure of speech to give a feeling for the size of this principle. But... OK, let's get literal: Run that Titanic movie as the ship hits the iceberg. See those steel plates ripped open and the ship begin to sink. Realistic, right? Can you imagine a real happening in which the reverse occurs? A sinking ship whose steel side heals up as it comes up out of the water and floats? Ridiculous. Too stupid to think about. But why is it stupid? Because it is so improbable from your and my experience. Only a movie run backward would show that kind of unrealistic fantasy. The second law isn't some weird scientific idea. It fits with everything common happening that we know.
Our psychological sense of time is based on the second law.
It summarizes what we have seen, what we have experienced, what we think will happen.
Sinking ships are like rocks rolling down a mountain -- as they sink, their potential energy due to being high above sea-bottom is diffused, spread out to the water that they push aside (or, in the case of mountain rocks, diffused as they roll down to the valley and hit other rocks, give those a bit of kinetic energy, and warm them slightly by friction.)
In a video that is run backward, you may have laughed at some diver who zooms up from the water to a ten-meter diving board, but you're never fooled that the video is going forward, i.e., that you are seeing an event as it actually happened in real time. Unconsciously, you are mentally comparing what you see now with your past practical experience -- and that has all followed the second law. Even though you may never have heard of the law before, in the years of your everyday experience you have seen thousands and thousands of examples of energy flowing from being concentrated to becoming diffused.
A swimmer doesn't come shooting up out of the water to the diving board, rocks in a valley don't suddenly roll up a mountain, outside air doesn't rush into a flat tire, batteries don't get charged by sitting around. Those events all would have energy spontaneously become more concentrated, the opposite of energy spreading out. We sense that videos or movies are shown in the right direction of showing time going forward only if the events in them agree with our lifetime experience about the direction of energy flow: concentrated to dispersed and spread out. The second law points the direction of how we feel time goes.
Q: So that's why people call the second law "time's arrow"?
A: You got it. Now let's clear up those tricky words, "spontaneously" and "tends" in the statement of the second law, "Energy spontaneously tends to flow only from being concentrated in one place to becoming diffused and spread out."
Q: Why bother? "Spontaneously" means fast, quick, ad lib .......
A: Hold it! That may apply to people, but we're talking about how things and chemicals behave. In the second law "spontaneously" means only that any energy which is available in the object or substance for diffusing will spread out from it -- if given a chance. It doesn't have anything to do with how fast or slow that occurs after the dispersal of energy starts, or even when it might start. That's why "tends" is so important to understand as part of the second law.
The energy available in a hot frying pan or in a loud BOOM
from a drum immediately and rapidly begins to spread out to their environments.
Nothing hinders that from happening. Lots of ordinary and also unhappy events
are like that. But there are an enormous number of "energy diffusing"
second-law happenings that are hindered so they don't occur right away.
Here's a simple illustration: If I hold a half-pound rock in my fingers so it
is ready to fall, it has potential energy concentrated in it because it is up
above the ground. If the second law is so great and powerful, why doesn't
the energy that has been concentrated in the rock spread out? Obviously, it
can't do that because my fingers are "bonding" to it, keeping it from
falling. The second law isn't violated. That rock tends to fall and diffuse
its energy to the air and to the ground as it hits -- and it will do
so spontaneously by itself, without any help -- the second I open my
fingers and "unbond" the rock.
Q: Is that really so important?
A: Yes, it is. Many philosophers and novelists learned about the second law only from physicists.
Unfortunately, physics emphasizes what happens in a closed or isolated system of tiny particles rather than in our real open-flow world of trees, shiny steel, sunshine, rocks and people, the world of sun energy and things made out of chemicals. Thus, many readers of popular philosophy articles and recent novels have been misled and frightened by talk about the second law as a fast-approaching doomsday. The writers pass too quickly over the fact that it is a tendency rather than a prediction of what will happen right away.
In many real-world chemicals and things the second law can be obstructed or hindered for millions of years. Certainly, the mountains of the world haven't all slid down to sea level in the last several hundred centuries! Similar to my fingers holding the small rock (but millions of times more tightly), even overhanging stone in cliffs or mountains is bonded, chemically bonded, to adjacent atoms in the stone and so the stone can't obey the second law tendency for it to fall to a lower level. Here, as in countless other examples, the second law is blocked by the strength of chemical bonds. It takes a huge number of repetitions of outside energy input like freezing and thawing and earthquakes and windy rainstorms to break the bonds along even a weak bond-line, make a crack, and free particles or pebbles or rocks so they can follow the second law by falling to a lower level. (But even then, they may just fall into a mile-high valley and be kept from dropping any closer to sea level; so here in a different way the second law is further hindered.)
Blockage of the second law is absolutely necessary for us to be alive and happy. Not one of the complex chemical substances in our body and few in the things we enjoy would exist for a microsecond if the second law wasn't obstructed. Its tendency is never eliminated but, fortunately for us, there are a huge number of compounds in which it is blocked for our lifetimes and even far longer.
Q: About time we got to something human. Are we through with rocks?
A: Yes, but don't forget what they have illustrated.
I think it is helpful to see some of the ways the second law works in the ordinary dusty world of actual objects before looking at its relation to pure substances, the chemicals that make up those objects. Chem profs approach the second law the other way around, starting with atoms and molecules first. And that's certainly OK. Professors rightfully avoid much talk about the behavior of big visible things at all. In the limited time of a chemistry course they can only develop the nature of atoms and molecules and of chemical substances. Objects made from chemicals like a gear or a bridge or a wooden house or a book or a bone just have to be assumed to behave like their constituent substances. What they do because they are big things composed of tiny molecules just has to be left for other courses or our future experience.
We'll go further to find how the second law affects common objects and how it is really the mother of all serious Murphy's Laws that apply to things. (We'll omit the zillions of humorous or stupid variations about the way fallible humans behave, as well as all the problems surrounding computers and programs!)
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