hutchphd said:
I think you need to rethink that.....what you have described is a dehumidifier.
Yes! That's exactly what I'm thinking of. Why isn't this the most efficient heating device using electricity? I do realize that as relative humidity decreases, water is better able to evaporate from our skin, causing cooling directly and more quickly. Maybe this is why a dehumidifier is a bad idea for cold dry areas.
russ_watters said:
You seem to be describing the heat transfer backwards. If the water vapor condenses on the coils it heats the coils. The now liquid water will then run off the coils and drain. What happens to it after that doesn't matter. It doesn't condense on the coils and then evaporate again. You're not losing anything you are just gaining.
If the heat from condensation is generated on the outside, surely a smaller amount of that heat is transferred inside, than if you were to generate that heat inside in the first place. Likewise for the electricity spent in running the system.
russ_watters said:
The downside of this for heating has already been largely explained: it only works if your outdoor temperature and dew point are well above freezing, and at that time you don't need much heating anyway. If it's below freezing the water vapor will frost onto the coils and then you have to spend extra energy re-heating it to get it off the coils. Most of the available energy gain is lost to re-heating it.
If both units were inside, as with a dehumidifier, this wouldn't happen.
russ_watters said:
Also noted was that there is a lot less water in the air when it is cold than when it is hot. How much? Air with a dew point of 30F has less than half as much water as at 50F. at 10F it's 1/6th. A humid summer day at 70F dewpoint has twice as much as at 50F.
I do realize this, yes. But there's some.
russ_watters said:
For gases, pretty much yes.
Very interesting. And for liquids? And solids?
russ_watters said:
Loses. Potential energy is negative. What happens in evaporation (at 100C) is you input heat and instead of increasing the temperature of the water you just break a chemical bond.
You mean the liquid water loses the potential energy, and the vapor gains it, correct?
When you say it breaks the chemical bond, you're not talking about a molecular bond, are you? Because when plants create carbohydrate during photosynthesis, by combining carbon, hydrogen and oxygen, energy is stored there, and is released when either plants or animals break up the molecular bond in the carbohydrate molecules. During photosynthesis plants also break the molecular bond in water, separating the hydrogen and oxygen atoms, which animals and plants then reconstitute as water during cellular respiration.
russ_watters said:
The 540 is the energy required to break the chemical bond.
In this case the potential energy is chemical bond energy, which is electromagnetic in nature.
I wonder, with all of this electromagnetism going on during condensation, would it ever make sense to use this as a form of electricity generation from heat? From my very limited understanding, this is not strictly speaking what steam engines and power plants already do. Or is it?
russ_watters said:
No, force is not energy. If you have a book sitting on a table, the table is not consuming energy just by holding up the book.
This is somewhat confusing to me. As a biological being, I would be spending energy by holding up the book.
russ_watters said:
There is more than one type of potential energy. People are giving you analogies to other examples because "chemical bond energy" isn't something you can easily visualize.
Is there a limit to the types of potential energy that can exist, or is it simply a matter of what we have and haven't observed yet? And is there something connecting all forms of potential energy together, or is it just that we don't know where this energy goes while it's in limbo waiting to be released, so we call it potential energy?
russ_watters said:
No. What you are saying is sort of true in Relativity, but this isn't Relativity. All of the energy is chemical and the mass (weight) of the fuel oil is not lost.
Oh! I was under the impression that the mass/weight of the burned oil would be less than the oil itself. I don't know why I thought that. Probably because of density.
russ_watters said:
Chemical bond energy. It's very similar to the energy required to pull apart two magnets.
Magnets lose electromagnetic potential over time, or when something pulls magnetic objects away from them, right?
russ_watters said:
Gotta call it something. Since it consumes energy when you pump it up and gives you energy back when you release it, "potential energy" sounds reasonable to me. I suppose you could call it "Frank" if you want, but it wouldn't change the math.
What's confusing to me is that for other forms of energy transfer/conversion, we have different names. So I don't understand why these ones in particular are called potential energy. Maybe way more things are considered potential energy than I initially realized.
DrClaude said:
Ordinary matter is made of protons, neutrons, and electrons. The interaction between atoms or molecules is due the the Coulomb interaction between the protons in the nuclei and the electrons (including nucleus-nucleus and electron-electron interactions). It is in that sense that it is ultimately electromagnetic interaction. (To be exact, one should consider also quantum mechanics spin, but the resulting interaction will also be of a magnetic nature.)
I'll need to read a bunch more to understand what all of this means.
DrClaude said:
There is also gravitational interaction between atoms and molecules, so there is gravitational potential energy, but it is negligible in this case.
Isn't gravity a form of electromagnetism?
Ken G said:
Correct, the main purpose of the potential energy concept is to keep track of something you can add to the kinetic energy to keep the total energy constant. That sounds like pure bookkeeping, but then it turns out this total energy concept ends up having some significant meaning of its own, like E = mc^2. However, in most applications, it is only the change in potential energy that must be tracked, not its actual value.
I understand. We don't need to know what potential energy vapor could have under other conditions, as long as we can track the changes in energy under all observed conditions.
Ken G said:
Right, what accounts for that is potential energy.
It's not its overall charge, it is the fact that water is a "polar" molecule, meaning it likes to have its positive charge on one side and its negative charge on the other. By aligning these charges oppositely to its neighbors, it is able to be attracted by other water molecules.
Does this have something to do with surface tension? And do vapor and ice also have this polar quality?
Ken G said:
That lowers its energy when the density increases, to a point-- the reason the density does not continue to rise is that at some point the Pauli exclusion principle comes into play and says the electrons in the molecules won't allow the molecules to get any closer (they are "excluded" from overlapping).
When you say it lowers its energy, do you mean kinetic energy, potential energy, electromagnetic energy, or all of the above? And if this energy lowers, is that why its released as heat during condensation? Because it doesn't seem to me like water going from 80C to 20C releases extra heat, other than the amount required to lower the water temperature by that amount.
Ken G said:
It is important not to confuse force with energy. Potential energy is not just a force, it is a force applied over a distance that the object moves. So when the sneakers move upward against the force of gravity, that's when the potential energy changes, but just sitting there on the power line, they still experience the force of gravity but there is no change in potential energy associated with that force because there is no change in location.
If the power line you throw the sneakers on is on an elevated hill, but you throw them from under the hill, does the potential energy change, because the sneakers are now closer to the ground? Ignoring sideways motion, you had to spend extra energy to throw the sneakers up over the hill, but that's not accounted for if the sneakers were to fall straight down.
Ken G said:
I think it would be fairer to say that we really have no idea what energy is, even kinetic energy, we can only say that it is a useful concept. That is all we can ever say in science, the proof is in the value of the concept.
I think some forms of energy are better understood than others.
Ken G said:
On the contrary, the energy released by burning is well described by potential energy-- it is the electrical potential energy associated with the opposite charges inside molecules. One can understand quite well the energy released when flammable substances are oxidized by analyzing the electrical potential energies of all the charges that change position.
Is there any form of energy transfer, storage or conversion, that would not be considered potential energy?
Ken G said:
I'm sure that statement has truth to it, though it can also be said that we do understand some things about how lightning is produced. When is that not ever true in science? The very nature of science is to always be an unfinished work of art.
That's why it is only the changes in potential energy you need to track. Fortunately, we never need to track the complete history of every object we analyze, for we would never get to know that. Instead, we analyze its kinetic energy "initial state", starting from wherever we desire, and from that point on we track potential energy changes to understand changes in kinetic energy. This approach was an early criticism of Newton's approach to motion-- people wanted "ultimate causes." Physics became much more powerful when we dropped that requirement, and replaced it with the concept of "initial conditions" and only tried to understand how things change.
Yes, I would say your example is exactly the kind of situation where we do like to regard that as stored energy.
That does seems useful.