Explain the First and Second Law of Thermodynamics?



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    The first law: energy cannot be created or destroyed, though it can change form and be converted. Think of it like this: When you flip a light switch, the electrical energy there is not destroyed, it is converted, by means of wires and filaments, into light energy. Conversely, you must have the electrical energy to create the light energy, because you cannot simply create the light energy from nothing.

    The second law: potential energy (that is, energy that is remaining in an object) will always be less than the energy in the original state. Imagine that our light bulb is now on a battery-run circuit. Once the battery is used, the potential energy lowers, leaving it at less than the initial state. That battery cannot ever regain the energy it has given out, and cannot ever has as much energy as it initially had. This is known as entropy, the idea that everything is slowly breaking down over time. 

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    Thermodynamics (Thermo + dynamics) means the motion of heat.  The laws of thermodynamics describe how heat moves.  They’re actually pretty straightforward and their existence is common sense.  Only the why gets a little complicated.

    The first law states that (1) energy can neither be created or destroyed, only changed; (2) flow of heat is a form of energy transfer; and (3) energy can be transferred to a closed system by work input and from a closed system by heat output.  For example, if you have two trains steaming toward each other, work is being put into the train system by the engines, which are converting the potential energy in fuel into kinetic energy.  When the trains collide, they eventually come to a stop, thus losing all the energy put into them by the engines.  But since that energy cannot be destroyed by the collision, it must go somewhere.  It is lost as heat, according to the first law.

    The second law states that heat flows from a higher temperature to a lower temperature until thermal equilibrium has been reached.  Put a handful of ice cubes in a pot of boiling water, and the second law says that the ice will never get colder while the water gets hotter, instead, the ice will always melt, and the water will get colder, until there is no difference in temperature between water and melted ice.  This is all common sense, but a common mistake in understanding the second law is when the word “entropy” is thrown around. 

    (Entropy change is not the process of things growing more and more chaotic or disorganized over time.  Entropy is directly related to how much energy molecules have (i.e. how hot they are) and entropy change tends toward thermal equilibrium.  Once that equilibrium is reached, entropy stays constant.  Melting ice has a positive change in entropy, where as cooling water has a negative change in entropy.  They reach a middle ground in temperature, and the amount of entropy holds there until another system is introduced.  Natural entropic processes can be reversed by work input (e.g. water can be refrozen in a freezer; it can be boiled again on a stove), as stated in the first law.)

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      Ever since highschool physics I thought that entropy was the reason why my room always gets messy (as I think that might have been the example given in the textbook), but I guess I can’t blame the laws of physics for that anymore. Does entropy apply to other systems besides the flow and trasfer of energy/heat? In terms of earth system as a whole, it is a lack of equilibrium that makes this earth inhabitable for life and life actually seems to be preventing an equilibrium state from being achieved. How, if at all, is this related to entropy? I found the following abstract (http://rsta.royalsocietypublishing.org/content/368/1910/181.abstract) which basically describes what I am refering to, but I dont really understand the reference to “maximum entropy production” as it seems to be in conflict with what you mentioned about entropy change tending towards equilibrium and then remaining constant.

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      This is a great question and is exactly why it’s important not to define entropy as a measure of disorder increasing naturally over time. Clearly, ordered processes happen on Earth (evolution — or just “the coming into existence of higher forms of life”, so as not to get too controversial — being one of the most beautiful). But those processes are entirely counter-intuitive to the “increasing disorder” definition of entropy.

      I like to picture entropy as a refrigerator on a hot day, because refrigeration is such an unnatural process. The second law says that the fridge should heat up, yet it doesn’t. The hot day outside transfers a whole lot of heat into the fridge; it “produces entropy” in the fridge. Yet the fridge is plugged in, and electricity surges into the system to keep it cool.

      The confusion with entropy comes from the fact that it requires a naming (isolation) of a “system” in which entropy is measured. A system is anything, an ice cube, a bedroom, or a star, and it is only a system because it is being analyzed. If we say the system is the earth, disregarding everything else, then the second law breaks down — or seems to. But the first law picks up its slack: the Sun is a source of constant work-input in the earth-system, exactly like the plug of a fridge. If the sun were to burn out, all life here would die, and the earth would gradually become a cold rock in space. (Which ironically would be a decrease in entropy to equalize with the rest of the universe.)

      But it doesn’t. In order to hold its thermal equilibrium (that is, remaining at a temperature that can support life), entropy is produced on Earth, its maximum entropy production (as in the abstract) equals the amount of work the sun does on the earth system.

      Fundamental Thermodynamic Relation (first law of Thermodynamics), restated:

      Earth’s Change in Entropy = Maximum Entropy Production – Sun’s Work-input = 0

      I believe that’s what the abstract was getting at, but I’d have to read the whole article to be sure.

      On a side note, though purists might disagree, you could say that the tendency of your room to get messier is an increase in entropy. Laziness is withholding work in the form of picking up from being put into the bedroom-system, and it tends toward messiness. Its entropy will maximize (i.e. the mess will reach equilibrium) when absolutely everything has been strewn about, and nothing new is added to the mess.

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      Thanks for the thorough response. I cant say that I fully grok the concept yet, but the fridge example certainly helped, and I’ll just let this one settle a bit and hopefully time will work its magic.

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