T Nation

Engineering Question

I’m having a hard time understanding formal definitions of and putting into my own words the concepts of entropy and enthalpy. Entropy is giving me the most trouble. The best I can understand it is that entropy is the measure of irreversibility of a heat transfer.

In other words, it is the amount of heat that cannot be recovered from a steam cycle. Is this close to correct?

[quote]HardcoreHorn wrote:
I’m having a hard time understanding formal definitions of and putting into my own words the concepts of entropy and enthalpy. Entropy is giving me the most trouble. The best I can understand it is that entropy is the measure of irreversibility of a heat transfer.

In other words, it is the amount of heat that cannot be recovered from a steam cycle. Is this close to correct?[/quote]

Entropy has to do with disorder, or “lost” energy, so I’d say your analogy works. Heat transfer is one of the most unorganized energy transfer methods.

Well, no…entropy isn’t a measure of energy. The simplest way to put it is the relation dS=dQ/T, meaning the change in entropy is equivalent to the heat absorbed in a reversible process divided by the absolute temperature.

Entropy is one of those things that no one really understands, I’m convinced. Its fundamental definition is rooted in statistical thermodynamics, where it is a measure of the total number of possible microstates of the system. I really can’t go into any more detail than that without hurting myself.

If entropy isn’t a measure of energy, why is its unit of measurement, at least in the context of a theoretical steam cycle, BTU/lbm F?

In more laymans terms, Entropy is the tendency for the universe to disintegrate into disorder without the application of energy, ie your house becomes increasingly disordered unless you apply energy to placing it back in order by cleaning and putting things away… it is also the reason no “perpetual motion” machine can exist, as energy is lost in every conversion, therefore whatever is fueling the motion will eventually dissipate as lost energy and no longer be available within your closed system to keep the motion going.

[quote]HardcoreHorn wrote:
If entropy isn’t a measure of energy, why is its unit of measurement, at least in the context of a theoretical steam cycle, BTU/lbm F?[/quote]

Its unit is J/K for most of us.

BTU/lbm is a measure of specific energy (energy per mass), so BTU/lbm F is specific entropy. How is it, in your mind, that a unit of energy divided by some other unit is still a measure of energy?

[quote]HardcoreHorn wrote:
If entropy isn’t a measure of energy, why is its unit of measurement, at least in the context of a theoretical steam cycle, BTU/lbm F?[/quote]

check the post above (by wfifer). That’s the basis… dQ/T.

if you have lbm in the denominator, you’re talking specific entropy, that is, entropy per lb of the system

Also, you may need to take thermo 2 or 3 classes before you really begin to “get” it. Accept the definition and move on; it’s hard to understand - esp. in layman’s terms.

[quote]wfifer wrote:
Well, no…entropy isn’t a measure of energy. The simplest way to put it is the relation dS=dQ/T, meaning the change in entropy is equivalent to the heat absorbed in a reversible process divided by the absolute temperature.

Entropy is one of those things that no one really understands, I’m convinced. Its fundamental definition is rooted in statistical thermodynamics, where it is a measure of the total number of possible microstates of the system. I really can’t go into any more detail than that without hurting myself. [/quote]

You’re crazy man. Entropy is a completely reversible hydrostatic state brought about through the agitation of particles at a subatomic level. The coorelation of possible microstates would be similar to the coorelation between light traveling by waves or by particles. Fundamentally it’s an EZ-Bake Oven. Sheesh.

[quote]dswithers wrote:
In more laymans terms, Entropy is the tendency for the universe to disintegrate into disorder without the application of energy, ie your house becomes increasingly disordered unless you apply energy to placing it back in order by cleaning and putting things away… it is also the reason no “perpetual motion” machine can exist, as energy is lost in every conversion, therefore whatever is fueling the motion will eventually dissipate as lost energy and no longer be available within your closed system to keep the motion going.[/quote]

Yes, but what happens when we die?

I’m pretty sure you’re going to hell. =)

[quote]wfifer wrote:
I’m pretty sure you’re going to hell. =)[/quote]

You’re right. I never should have gotten involved with you and that goat. She was asking for it, you gotta admit. Wearing that bell and chewing that hay so coquettishly.

Hey, your mom’s not the hottest piece of ass in town, but she’s no goat.

there are some unnecessarily complicated definitions of entropy up in here. don’t sweat it man, no one gets entropy at first. as you begin to understand the subject matter and do the calculations, it’ll start to make sense.

here’s something to think about: if the universe keeps expanding, eventually there will be the “entropy death of the universe” where the universe attains a maximum entropy. As everything must happen in a way that the universe’s entropy increases, basically there is no energy for anything to happen, thermodynamically speaking.

on second thought, that probably wasn’t any better, but I’m just putting it out there.

[quote]wfifer wrote:
Hey, your mom’s not the hottest piece of ass in town, but she’s no goat.[/quote]

We’ll save that distinction for your wife.

[quote]wfifer wrote:
HardcoreHorn wrote:
If entropy isn’t a measure of energy, why is its unit of measurement, at least in the context of a theoretical steam cycle, BTU/lbm F?

Its unit is J/K for most of us.

BTU/lbm is a measure of specific energy (energy per mass), so BTU/lbm F is specific entropy. How is it, in your mind, that a unit of energy divided by some other unit is still a measure of energy?[/quote]

Yes, you’re right; I made a mistake. Thanks for the help. I get it for the most part now. I’ll just have to wait until Thermo next semester to get a real hold on it.

[quote]veruvius wrote:
there are some unnecessarily complicated definitions of entropy up in here. don’t sweat it man, no one gets entropy at first. as you begin to understand the subject matter and do the calculations, it’ll start to make sense.

here’s something to think about: if the universe keeps expanding, eventually there will be the “entropy death of the universe” where the universe attains a maximum entropy. As everything must happen in a way that the universe’s entropy increases, basically there is no energy for anything to happen, thermodynamically speaking.

on second thought, that probably wasn’t any better, but I’m just putting it out there.[/quote]

The class I’m taking involving entropy is Boilers, so I’m only trying to define it in that context. But your definition is helpful as well. It reinforces the idea that entropy is not recoverable, which is important for my study of boilers.

[quote]HugeMutha wrote:
wfifer wrote:
Hey, your mom’s not the hottest piece of ass in town, but she’s no goat.

We’ll save that distinction for your wife.[/quote]
So I fucked your sister a few times–I never said I’d marry her!

Just consider enthalpy to be order, and entropy to be disorder. Consider this analogy…

It takes an energy expenditure, that is, real thought, to keep a forum discussion orderly. Without input energy, the discussion turns disorderly, entropy sets in, and the discussion turns to screwing goats.

[quote]wfifer wrote:
HugeMutha wrote:
wfifer wrote:
Hey, your mom’s not the hottest piece of ass in town, but she’s no goat.

We’ll save that distinction for your wife.
So I fucked your sister a few times–I never said I’d marry her![/quote]

I don’t have a sister. That was my goat.