How does evaporation generate cooling? Swamp coolers edition

In summary: Earth, but the heat that we generate from working would also be removed from the Earth, since the wealthy people are the ones who generate the most heat.
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dario2
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TL;DR Summary
The process of evaporative cooling is more complex than it first appears to be.
I looked for other related threads and read through them, but I didn't find one where I could add replies, so I made another one.

This issue came to my mind as I'm thinking about the benefits and detriments of swamp coolers given a hot climate and moderate to high relative humidity. 1) Some people say that evaporation of water generates cooling because the temperature (speed?) of the molecules of water changing from solid to liquid is hotter (is it 100C?) than the average of the solution, and so as liquid water turns into vapor, the average temperature of the remaining water must go down.

2) Others said that water "requires energy" to turn into vapor, but that this energy need not be in the form of temperature, it can be kinetic energy. I don't understand this. Does this mean the vapor molecules are moving faster? But moving where? And does this have something to do with density? Since water vapor is much less dense than liquid water.

I've been thinking about a boiling pot of water, and how it seems to take a lot less energy to heat the water up from room temperature to 100C, than it does to evaporate all of the water, something I don't entirely understand either, since it seems that once the water is at 100C (or close enough to it), it should take very little energy to push it over the edge and turn it into vapor. I assume the evaporation process itself generates cooling far beyond the average temperature drop described in 1), but I don't understand the process.

With swamp coolers, you are adding heat into the room because you're using electricity, but you're also evaporating water, which cools the surface it left behind, generating cooling, I don't understand why the vapor doesn't also heat the rest of the room and why this doesn't have a net heating effect. In any case, I suppose the answer is that since water vapor requires energy to turn into vapor, and part of this is kinetic energy, when you evaporate water you are turning heat energy into kinetic energy. But that would mean that when there's condensation of water, that kinetic energy is being turned into heat. Does that mean that on a cold day, when vapor condenses on cold windows, that is actually heating the room? Is there such a thing as condensating heaters?

All answers will be greatly appreciated! Thank you
 
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  • #2
dario2 said:
TL;DR Summary: The process of evaporative cooling is more complex than it first appears to be.

dario2 said:
I've been thinking about a boiling pot of water, and how it seems to take a lot less energy to heat the water up from room temperature to 100C, than it does to evaporate all of the water, something I don't entirely understand either, since it seems that once the water is at 100C (or close enough to it), it should take very little energy to push it over the edge and turn it into vapor. I assume the evaporation process itself generates cooling far beyond the average temperature drop described in 1), but I don't understand the process.

You are correct that it takes a lot more energy to make the water into vapor than it did to get it to 100 C, this is called the "heat of vaporization" and is 540 calories per gram (and it takes only 75 calories per gram to go from room temperature at 25 C to 100 C, which is a good thing because this means you can keep the water boiling for about 7 times longer than it takes to heat up, before it all boils off). The reason is, to increase temperature, you only have to add kinetic energy to the water molecules, you don't have to pull them away from each other. They attract each other (due to electric attraction when the positive side of one molecule approaches the negative side of another), so it takes a lot of energy to pull them apart. You can think of it like, if you had a ball hanging from a rubber band, you could easily put energy into the ball to make it move up and down, but if you wanted to actually break the rubber band, that would take a lot more energy.

When you sweat, you cool yourself, though your temperature is way below 100 C because it is only the fastest moving water molecules that have enough kinetic energy to break away from the attraction of the other molecules. So this removes energy from the rest of the molecules, because you are selecting the ones with the highest energy. It would be like if every millionaire on Earth left to go live on Mars, not only would we have a lot less money left behind on Earth, we'd also have less average money per person. Less average kinetic energy per particle means lower temperature.
dario2 said:
With swamp coolers, you are adding heat into the room because you're using electricity, but you're also evaporating water, which cools the surface it left behind, generating cooling, I don't understand why the vapor doesn't also heat the rest of the room and why this doesn't have a net heating effect. In any case, I suppose the answer is that since water vapor requires energy to turn into vapor, and part of this is kinetic energy, when you evaporate water you are turning heat energy into kinetic energy. But that would mean that when there's condensation of water, that kinetic energy is being turned into heat. Does that mean that on a cold day, when vapor condenses on cold windows, that is actually heating the room? Is there such a thing as condensating heaters?
Yes, condensation does release heat. In fact, this is the primary mechanism for energizing thunderstorms. When a plane flies through a thunderstorm, it notices that there are very high winds in there, which are powered by the heat released by condensing water vapor into water droplets. I don't know about the practical applications of this for house heating (the problem would finding enough water vapor), but it is certainly important in thunderstorms!
 
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  • #3
If water evaporates, that means it's become vapor, and vapor should be at 100C or more at normal pressure, right? So the molecules that left had to be at 100C to leave. If the average temperature was 35C, each gram of water being evaporated reduces nine other grams' average temperature by 7.22C, am I correct? 100C - 35C = 65C. And 65C / 9 = 7.22C

This could explain why evaporative cooling works so well on a solution/surface that is 35C average temperature, such as human skin, but it doesn't explain the cooling on a solution that is 99C average temperature, such as the pot of water on the stove. Because if the average temperature of the water was 99C, then each gram being evaporated would only reduce nine other grams' temperature by 0.11C. And if the water reaches 100C before evaporating, that would mean there's no cooling effect from evaporation.

You mentioned the heat of vaporization is 540 calories per gram of water, that is for water that is already at 100C, correct? That is it takes a further 540 calories per gram to fully evaporate that 1g of water that is already at 100C. And since that's enough to heat 1g of water by 540C, that would mean each gram of water evaporated also cools another remaining 9g by a further 60C from my previous calculation. 540C / 9 = 60C. Correct?

I assume this is what you meant when you said that the electrical bonds between molecules must be broken in order to cause evaporation, and presumably this would account for a much larger part of the cooling process.

But my question is, where does this energy go after it's used to break these electrical bonds? Is it contained in the vapor molecules somehow in a form other than heat? And what is the process by which it eventually turns back into heat during condensation?
 
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  • #4
dario2 said:
If water evaporates, that means it's become vapor, and vapor should be at 100C or more at normal pressure, right?

Individual particles do not have a temperature, they have a kinetic energy. To have a temperature, you need a very large number of particles acting as a single system. However, you are correct that the characteristic kinetic energy of the particles that evaporated were likely not too far below the average kinetic energy of particles at 100 C.
dario2 said:
So the molecules that left had to be at 100C to leave. If the average temperature was 35C, each gram of water being evaporated reduces nine other grams' average temperature by 7.22C, am I correct? 100C - 35C = 65C. And 65C / 9 = 7.22C

You left out the fact that it takes about 7 times more energy than what you mentioned to get the evaporation, i.e., to break away from the forces of attraction. So I think you are on the right track, but you should multiply your result by something like 7, approximately. That means you don't have to evaporate a significant fraction of the water to get a lot of cooling.
dario2 said:
This could explain why evaporative cooling works so well on a solution/surface that is 35C average temperature, such as human skin, but it doesn't explain the cooling on a solution that is 99C average temperature, such as the pot of water on the stove. Because if the average temperature of the water was 99C, then each gram being evaporated would only reduce nine other grams' temperature by 0.11C. And if the water reaches 100C before evaporating, that would mean there's no cooling effect from evaporation.

Again, you are leaving out the heat of vaporization.
dario2 said:
You mentioned the heat of vaporization is 540 calories per gram of water, that is for water that is already at 100C, correct?

Yes.
dario2 said:
That is it takes a further 540 calories per gram to fully evaporate that 1g of water that is already at 100C. And since that's enough to heat 1g of water by 540C, that would mean each gram of water evaporated also cools another remaining 9g by a further 60C from my previous calculation. 540C / 9 = 60C. Correct?

Yes.
dario2 said:
I assume this is what you meant when you said that the electrical bonds between molecules must be broken in order to cause evaporation, and presumably this would account for a much larger part of the cooling process.

You have it now.
dario2 said:
But my question is, where does this energy go after it's used to break these electrical bonds? Is it contained in the vapor molecules somehow in a form other than heat? And what is the process by which it eventually turns back into heat during condensation?
You've seen sneakers thrown over power lines, right? Someone gave those sneakers a lot of kinetic energy when they threw them, then as they rose into the air, they lost that kinetic energy. Where did it go? When kinetic energy is used to oppose the attraction of the Earth's gravity, it is lost when the object goes up. A similar situation holds for the water molecules, but it's not gravity it's electric fields. Since we prefer to imagine that energy is a conserved quantity (it simplifies the bookkeeping), we invent the concept of "potential energy", which provides the answer to your question. If the sneakers fall back down, poof, the kinetic energy returns-- same for condensation.
 
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  • #5
Ken G said:
You've seen sneakers thrown over power lines, right? Someone gave those sneakers a lot of kinetic energy when they threw them, then as they rose into the air, they lost that kinetic energy. Where did it go? When kinetic energy is used to oppose the attraction of the Earth's gravity, it is lost when the object goes up. A similar situation holds for the water molecules, but it's not gravity it's electric fields. Since we prefer to imagine that energy is a conserved quantity (it simplifies the bookkeeping), we invent the concept of "potential energy", which provides the answer to your question. If the sneakers fall back down, poof, the kinetic energy returns-- same for condensation.

Thank you! I still don't know anything about this "potential energy" concept, but had been reading about it in one of the gravity threads. As a layman my first impression is that it seems to be a proxy for mechanisms we don't fully understand yet.

I also thought that water vapor rises not because its electrical field fights gravity but simply because water vapor has a lower density than air, so in my mind it's the air that's pushing water vapor up by taking up its space, in the same way that dropping a rock into a water bucket pushes water up. But perhaps these two are related somehow.

During thunderstorms, are lightning bolts caused by this energy that is released from the water vapor condensing?
 
  • #6
If you want a simple example of potential energy, try this

You carry a 1kg lump of rock up a ladder and place it on a small ledge. You expended energy to move the rock up (you also moved your body up, but we will ignore that for simplicity.) The rock is placed on a ledge, with potential energy.

You climb down the lader, look up at the rock, and a friend accidentally opens a window and nudges the rock off the ledge, and it falls down, converting the potential energy into kinetic energy, and it hits your foot. You scream as it hurts. The higher up the rock was, the more potential energy it has, and when nudged the more kinetic energy it has when it hits your foot (and the more it hurts).

Here the potential energy - PE = mhg - mass times height (above the unfortunate foot) times acceleration due to gravity. Where does it come from? The energy required to raise it up.
The kinetic energy is half m v squared when it hits your foot. This is converted into bone damage, of course, which you sense as pain...

Potential energy is stored energy.

As for water, at any temperature, the molecules are moving around and colliding with each other and the container walls. (They can gain or lose some speed / velocity ie some energy in these collisions, so they all have different velocities and hence different kinetic energy.) Some are moving faster than others and have more kinetic energy, and can burst through the surface into the air, to become water vapour. As the faster, more energetic molecules are being removed in this evaporation, the average energy of the remaining water is reduced, and the temperature is reduced. Individual molecules don't have a temperature, they have kinetic energy. Temperature is a bulk property, directly related to the average velocity of all the molecules in the sample.

And as you can now see, it is not related to some mechanism we don't yet understand as we can calculate it exactly using three known values.
 
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  • #7
dario2 said:
Thank you! I still don't know anything about this "potential energy" concept, but had been reading about it in one of the gravity threads. As a layman my first impression is that it seems to be a proxy for mechanisms we don't fully understand yet.
Potential energy is not a place holder for something unknown, it is a tried and true concept of physics.
dario2 said:
I also thought that water vapor rises not because its electrical field fights gravity but simply because water vapor has a lower density than air, so in my mind it's the air that's pushing water vapor up by taking up its space, in the same way that dropping a rock into a water bucket pushes water up. But perhaps these two are related somehow.
I think that you misunderstood what @Ken G was saying. You are correct in saying that water vapor rises because it has a lower density. What @Ken G noted is that the potential energy in water comes from the interaction between molecules, which is ultimately of electromagnetic origin.

dario2 said:
During thunderstorms, are lightning bolts caused by this energy that is released from the water vapor condensing?
Lightning comes from a charge separation in clouds that comes from conviction currents. See, e.g., https://www.physicsclassroom.com/class/estatics/Lesson-4/Lightning
 
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dario2 said:
During thunderstorms, are lightning bolts caused by this energy that is released from the water vapor condensing?
Yes but not directly.
Humid air is difficult to cool because of this water content. Also it is actually less dense than "dry" air at a given temperature. So an advancing cold front will cause the warmer humid air to rise and this will continue as the moisture content is "wrung out" allowing very significant and violent updrafts before these columns of air become cold clouds. It is this motion (and perhaps the water droplets) that generates the charge separation. But the energy comes mostly from the condensation.
 
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Yes, my sneakers analogy was not to say that the potential energy in water vapor comes from going up (that was just the analogy with gravity), it comes from separating the molecules from the water, before there is anything going up. In a thunderstorm, the heat of condensation warms the air, and warm air rises because warm air is lower density than cool air (to be in pressure equilibrium, as per the ideal gas law). So gravitational potential energy does get involved at that point, but not in the condensation part. As to whether we should say that potential energy is something that is a kind of placeholder of things we don't understand, I would say that a healthy way to look at all of science is that it is all placeholders for things we don't understand-- but that doesn't mean we don't know anything, or that the theories and models are not of proven value, it just means we always have a lot more to learn (which is what makes science fun). When a theory has proven useful enough times, we start to think we "know it is true," but that's only because "true" in science has no other meaning beyond "is of demonstrated value in various contexts."

Still, it was perhaps a little glib for me to say that the potential energy concept only serves us in energy bookkeeping. Mass is also associated with "rest energy", so counting that is yet another way that energy can be made to be conserved, yet all forms of internal energy (including potential energy) will count toward the rest energy, and hence will be exhibited in the mass. We would not say we invented the concept of mass simply because it is useful for bookkeeping, it has important consequences of its own that relate to the kinematic relationships between momentum and energy. So even though we invented all these concepts, the fact that they are so useful and so interrelated is why they are successful enough to be considered "real." What is reality but a demonstrably proven concept?

The cited article on lightning is a good example of how the scientist manipulates concepts s/he does not yet fully understand. It's pretty clear from that article that scientists understand a lot about lightning, but there is plenty of room for new discoveries, which is why it contains careful language like "are believed to be". Scientists don't really "believe" things in the usual sense, instead they form working hypotheses that have already passed a series of tests, but might always fail the very next one. So that's not really "belief", as there is always room for surprises. But there are also useful and proven concepts, that can at the same time be regarded as a kind of placeholder, as has been true throughout the remarkable history of science. In the case of lightning, the fundamental cause appears to be that electrons tend to gather at the base of clouds, in ways that have a lot to do with the air movements (and some freezing), powered by the heat of condensation (and the heat released by freezing), so it is both electrical in nature, and also made possible by these microscopic releases of potential energy. The pileup of electrons then produces a macroscopic version of electric potential energy, as charge separation always produces electric fields and associated potential energy as that field interacts with moveable charges.

Thus lightning can be seen as a release of that macroscopic potential energy, traced back to the initial release of microscopic potential energy during condensation and freezing. And someone who gets struck by lightning is not going to have any doubt that this potential energy has very real consequences, placeholder or not!
 
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  • #10
DrJohn said:
If you want a simple example of potential energy, try this

You carry a 1kg lump of rock up a ladder and place it on a small ledge. You expended energy to move the rock up (you also moved your body up, but we will ignore that for simplicity.) The rock is placed on a ledge, with potential energy.

You climb down the lader, look up at the rock, and a friend accidentally opens a window and nudges the rock off the ledge, and it falls down, converting the potential energy into kinetic energy, and it hits your foot. You scream as it hurts. The higher up the rock was, the more potential energy it has, and when nudged the more kinetic energy it has when it hits your foot (and the more it hurts).

Here the potential energy - PE = mhg - mass times height (above the unfortunate foot) times acceleration due to gravity. Where does it come from? The energy required to raise it up.
The kinetic energy is half m v squared when it hits your foot. This is converted into bone damage, of course, which you sense as pain...

Potential energy is stored energy.

What about an asteroid coming from outer space and entering Earth's gravitational pull, then crashing down? Is its potential energy derived from the energy required to lift it up from Earth? Or maybe from the energy required to lift it up from wherever it came from before reaching Earth? What if the same asteroid got pulled in by the Sun's gravitational force instead? Would its potential energy now be different?

I have to leave for work now, but will read the rest of your and others' replies and add in my replies later, thanks everybody!
 
  • #11
dario2 said:
What about an asteroid coming from outer space and entering Earth's gravitational pull, then crashing down? Is its potential energy derived from the energy required to lift it up from Earth? Or maybe from the energy required to lift it up from wherever it came from before reaching Earth? What if the same asteroid got pulled in by the Sun's gravitational force instead? Would its potential energy now be different?

I have to leave for work now, but will read the rest of your and others' replies and add in my replies later, thanks everybody!
Asteroids are the remains from the formation of the universe, they are not "lifted up" from somewhere. Gravity pulls them around space, so they have some potential energy, and because they are moving some (lots ;) ) of kinteic energy. Now whether its potential energy increases as it is pulled towards the earth or sun is a trickier one to answer - I am not an astronomer. But it's kinetic energy is increased as it accelerates due to gravitational forces.

So when an object falls from a height towards the earth, it is converting its potential energy into kinetic energy. Potential energy is decreasing and kinetic energy is increasing. Same is true if it gets pulled towards the sun. More kinetic and less potential energy.
 
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  • #12
What's more, your question is demonstrating why there is always more "beneath the surface" of any simple idea, like potential energy. Above I mentioned that potential energy can be included in the mass of an object, which makes it sound like potential energy has a given value and a given source or history. But in most contexts, the actual value of the potential energy never enters, only the changes in potential energy. So we don't need to know what the potential energy is, or what it's ultimate source was, we only need to know how it changes when the object in question moves from place to place. That change accounts for the appearance or loss of kinetic energy, and it has a particular source, be that the Earth's gravity or the Sun's (or both added together). It is of course necessary that changes in potential energy must be additive like that, because even the change from the Earth's gravity is also the sum of the changes from all the tiny parts of the Earth. So you can see how wonderfully useful is the notion, it is very well founded (but deeper theories treat it differently, any description you give of anything must always be tailored to the sophistication and accuracy of the current need-- a point that is perhaps never stressed enough in science education).
 
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  • #13
DrJohn said:
As for water, at any temperature, the molecules are moving around and colliding with each other and the container walls. (They can gain or lose some speed / velocity ie some energy in these collisions, so they all have different velocities and hence different kinetic energy.) Some are moving faster than others and have more kinetic energy, and can burst through the surface into the air, to become water vapour. As the faster, more energetic molecules are being removed in this evaporation, the average energy of the remaining water is reduced, and the temperature is reduced. Individual molecules don't have a temperature, they have kinetic energy. Temperature is a bulk property, directly related to the average velocity of all the molecules in the sample.

And as you can now see, it is not related to some mechanism we don't yet understand as we can calculate it exactly using three known values.
Is temperature simply the average kinetic energy in a group of molecules? If so, then this potential energy that vapor gains is not kinetic energy, but something else, correct? So the faster molecules being removed simply account for the amount of cooling that the difference between 100C and whatever the temperature of the water/surface is. But it doesn't account for the additional 540 calories per gram.

DrClaude said:
Potential energy is not a place holder for something unknown, it is a tried and true concept of physics.

I think that you misunderstood what @Ken G was saying. You are correct in saying that water vapor rises because it has a lower density. What @Ken G noted is that the potential energy in water comes from the interaction between molecules, which is ultimately of electromagnetic origin.

Lightning comes from a charge separation in clouds that comes from conviction currents. See, e.g., https://www.physicsclassroom.com/class/estatics/Lesson-4/Lightning
Is all potential energy electromagnetic, or only the kind that vapor molecules have? Does vapor have an electric charge?

hutchphd said:
Yes but not directly.
Humid air is difficult to cool because of this water content. Also it is actually less dense than "dry" air at a given temperature. So an advancing cold front will cause the warmer humid air to rise and this will continue as the moisture content is "wrung out" allowing very significant and violent updrafts before these columns of air become cold clouds. It is this motion (and perhaps the water droplets) that generates the charge separation. But the energy comes mostly from the condensation.
I still think for humid areas it would make sense to make a condensation heater, to some extent I suppose regular AC already does this, but perhaps less efficiently than a specifically designed one, since I suspect much of the heat from condensation is lost in the outside part of the AC.

Ken G said:
Yes, my sneakers analogy was not to say that the potential energy in water vapor comes from going up (that was just the analogy with gravity), it comes from separating the molecules from the water, before there is anything going up.
Something else I don't understand is, in your sneakers example, aren't the sneakers constantly applying a downward force on the power line, even if nothing is moving? If I hold an object, that object is applying a downward force on my hand, which is applying an equal upward force on the object, and this force is transmitted down from my feet into the ground, correct? So this potential energy isn't in limbo, it's being used to apply these forces, isn't it?
Ken G said:
As to whether we should say that potential energy is something that is a kind of placeholder of things we don't understand, I would say that a healthy way to look at all of science is that it is all placeholders for things we don't understand-- but that doesn't mean we don't know anything, or that the theories and models are not of proven value, it just means we always have a lot more to learn (which is what makes science fun). When a theory has proven useful enough times, we start to think we "know it is true," but that's only because "true" in science has no other meaning beyond "is of demonstrated value in various contexts."
I still think there's a difference between knowing and understanding how a battery works, and simply being able to measure the energy going in and out, but with no idea of how the battery does its magic. Particularly when this potential energy concept is being applied to apparently completely different phenomena. I could of course be wrong about that.
Ken G said:
Mass is also associated with "rest energy", so counting that is yet another way that energy can be made to be conserved, yet all forms of internal energy (including potential energy) will count toward the rest energy, and hence will be exhibited in the mass. We would not say we invented the concept of mass simply because it is useful for bookkeeping, it has important consequences of its own that relate to the kinematic relationships between momentum and energy.
It seems to me that mass is another form of energy and has different properties that apply to it. For example, if you convert oil into heat by burning it, you've gained heat but lost weight. So I don't think it would fit this model of potential energy. With water and the discussion we're having here, I don't know what type of energy the water/vapor gains or loses during evaporation and condensation, where this energy goes, or what it's doing while it's there.

Ken G said:
The cited article on lightning is a good example of how the scientist manipulates concepts s/he does not yet fully understand. It's pretty clear from that article that scientists understand a lot about lightning, but there is plenty of room for new discoveries, which is why it contains careful language like "are believed to be". Scientists don't really "believe" things in the usual sense, instead they form working hypotheses that have already passed a series of tests, but might always fail the very next one. So that's not really "belief", as there is always room for surprises. But there are also useful and proven concepts, that can at the same time be regarded as a kind of placeholder, as has been true throughout the remarkable history of science. In the case of lightning, the fundamental cause appears to be that electrons tend to gather at the base of clouds, in ways that have a lot to do with the air movements (and some freezing), powered by the heat of condensation (and the heat released by freezing), so it is both electrical in nature, and also made possible by these microscopic releases of potential energy. The pileup of electrons then produces a macroscopic version of electric potential energy, as charge separation always produces electric fields and associated potential energy as that field interacts with moveable charges.

Thus lightning can be seen as a release of that macroscopic potential energy, traced back to the initial release of microscopic potential energy during condensation and freezing. And someone who gets struck by lightning is not going to have any doubt that this potential energy has very real consequences, placeholder or not!
I'll read that article next. So you're saying we don't entirely understand how lightning is produced, either?
DrJohn said:
Asteroids are the remains from the formation of the universe, they are not "lifted up" from somewhere. Gravity pulls them around space, so they have some potential energy, and because they are moving some (lots ;) ) of kinteic energy. Now whether its potential energy increases as it is pulled towards the earth or sun is a trickier one to answer - I am not an astronomer. But it's kinetic energy is increased as it accelerates due to gravitational forces.

So when an object falls from a height towards the earth, it is converting its potential energy into kinetic energy. Potential energy is decreasing and kinetic energy is increasing. Same is true if it gets pulled towards the sun. More kinetic and less potential energy.
They had to come from somewhere, but that's not my point. Instead think of an object being launched or expelled from another planet some distance away from Earth. As this object approaches Earth, or Mars, or the Sun, the potential energy of this object would have little to do with how much energy was spent to lift it up from whatever planet it was on before, it would instead depend on its mass and the gravitational field of whatever planet it happened to land on.

What's more, your question is demonstrating why there is always more "beneath the surface" of any simple idea, like potential energy. Above I mentioned that potential energy can be included in the mass of an object, which makes it sound like potential energy has a given value and a given source or history. But in most contexts, the actual value of the potential energy never enters, only the changes in potential energy. So we don't need to know what the potential energy is, or what it's ultimate source was, we only need to know how it changes when the object in question moves from place to place. That change accounts for the appearance or loss of kinetic energy, and it has a particular source, be that the Earth's gravity or the Sun's (or both added together). It is of course necessary that changes in potential energy must be additive like that, because even the change from the Earth's gravity is also the sum of the changes from all the tiny parts of the Earth. So you can see how wonderfully useful is the notion, it is very well founded (but deeper theories treat it differently, any description you give of anything must always be tailored to the sophistication and accuracy of the current need-- a point that is perhaps never stressed enough in science education).

Yes, it seems very useful to know that the energy required to lift up an object is the same as that which the object will exert as downward force when it falls down. I still would not call this stored energy, but for practical applications this concept is useful. Like a water tank that you fill up once a day, using energy to pump the water up, in order to have water pressure throughout the day on demand.
 
  • #14
dario2 said:
I still think for humid areas it would make sense to make a condensation heater, to some extent I suppose regular AC already does this, but perhaps less efficiently than a specifically designed one, since I suspect much of the heat from condensation is lost in the outside part of the AC.
One of the problems with heat pumps (for heating) is they condense water on the cold (outside) coils. This is good for heat transfer into the coils as you note. But if the ambient outside temperature approaches freezing this turns into ice buildup.
 
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  • #15
hutchphd said:
One of the problems with heat pumps (for heating) is they condense water on the cold (outside) coils. This is good for heat transfer into the coils as you note. But if the ambient outside temperature approaches freezing this turns into ice buildup.
Yes, and even if not freezing, you still have to recapture that heat before it's lost to evaporation and convection. But if instead both units were inside, you'd capture all of that condensation heat, plus the heat from the electricity spent in running the system.
 
  • #16
I think you need to rethink that.....what you have described is a dehumidifier.
 
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  • #17
dario2 said:
Yes, and even if not freezing, you still have to recapture that heat before it's lost to evaporation and convection. But if instead both units were inside, you'd capture all of that condensation heat, plus the heat from the electricity spent in running the system.

dario2 said:
[prior post] ...since I suspect much of the heat from condensation is lost in the outside part of the AC.
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.

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. Either way, this is already a "feature" of all air source heat pumps.

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.


dario2 said:
Is temperature simply the average kinetic energy in a group of molecules?
For gases, pretty much yes.

dario2 said:
If so, then this potential energy that vapor gains is not kinetic energy, but something else, correct?
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.

dario2 said:
So the faster molecules being removed simply account for the amount of cooling that the difference between 100C and whatever the temperature of the water/surface is. But it doesn't account for the additional 540 calories per gram.
The 540 is the energy required to break the chemical bond.
dario2 said:
Is all potential energy electromagnetic, or only the kind that vapor molecules have? Does vapor have an electric charge?
In this case the potential energy is chemical bond energy, which is electromagnetic in nature.
dario2 said:
Something else I don't understand is, in your sneakers example, aren't the sneakers constantly applying a downward force on the power line, even if nothing is moving? If I hold an object, that object is applying a downward force on my hand, which is applying an equal upward force on the object, and this force is transmitted down from my feet into the ground, correct? So this potential energy isn't in limbo, it's being used to apply these forces, isn't it?
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.
dario2 said:
Particularly when this potential energy concept is being applied to apparently completely different phenomena. I could of course be wrong about that.
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.
dario2 said:
It seems to me that mass is another form of energy and has different properties that apply to it. For example, if you convert oil into heat by burning it, you've gained heat but lost weight. So I don't think it would fit this model of potential energy.
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.
dario2 said:
With water and the discussion we're having here, I don't know what type of energy the water/vapor gains or loses during evaporation and condensation, where this energy goes, or what it's doing while it's there.
Chemical bond energy. It's very similar to the energy required to pull apart two magnets.
dario2 said:
Yes, it seems very useful to know that the energy required to lift up an object is the same as that which the object will exert as downward force when it falls down. I still would not call this stored energy, but for practical applications this concept is useful. Like a water tank that you fill up once a day, using energy to pump the water up, in order to have water pressure throughout the day on demand.
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.
 
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dario2 said:
Is all potential energy electromagnetic, or only the kind that vapor molecules have? Does vapor have an electric charge?
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.)

There is also gravitational interaction between atoms and molecules, so there is gravitational potential energy, but it is negligible in this case.
 
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  • #19
dario2 said:
Is temperature simply the average kinetic energy in a group of molecules? If so, then this potential energy that vapor gains is not kinetic energy, but something else, correct?
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.
dario2 said:
So the faster molecules being removed simply account for the amount of cooling that the difference between 100C and whatever the temperature of the water/surface is. But it doesn't account for the additional 540 calories per gram.
Right, what accounts for that is potential energy.
dario2 said:
Is all potential energy electromagnetic, or only the kind that vapor molecules have? Does vapor have an electric charge?
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. 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).
dario2 said:
Something else I don't understand is, in your sneakers example, aren't the sneakers constantly applying a downward force on the power line, even if nothing is moving? If I hold an object, that object is applying a downward force on my hand, which is applying an equal upward force on the object, and this force is transmitted down from my feet into the ground, correct? So this potential energy isn't in limbo, it's being used to apply these forces, isn't it?
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.
dario2 said:
I still think there's a difference between knowing and understanding how a battery works, and simply being able to measure the energy going in and out, but with no idea of how the battery does its magic. Particularly when this potential energy concept is being applied to apparently completely different phenomena. I could of course be wrong about that.
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.
dario2 said:
It seems to me that mass is another form of energy and has different properties that apply to it. For example, if you convert oil into heat by burning it, you've gained heat but lost weight. So I don't think it would fit this model of potential energy.
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.

dario2 said:
I'll read that article next. So you're saying we don't entirely understand how lightning is produced, either?
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.
dario2 said:
They had to come from somewhere, but that's not my point. Instead think of an object being launched or expelled from another planet some distance away from Earth. As this object approaches Earth, or Mars, or the Sun, the potential energy of this object would have little to do with how much energy was spent to lift it up from whatever planet it was on before, it would instead depend on its mass and the gravitational field of whatever planet it happened to land on.
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.
dario2 said:
Yes, it seems very useful to know that the energy required to lift up an object is the same as that which the object will exert as downward force when it falls down. I still would not call this stored energy, but for practical applications this concept is useful. Like a water tank that you fill up once a day, using energy to pump the water up, in order to have water pressure throughout the day on demand.
Yes, I would say your example is exactly the kind of situation where we do like to regard that as stored energy.
 
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  • #20
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.
 
  • #21
dario2 said:
Does this have something to do with surface tension? And do vapor and ice also have this polar quality?

No doubt the polar nature increases the surface tension, though nonpolar liquids also have surface tension because of other types of attraction between the molecules. Anything that causes attraction between similar molecules will make the liquid try to minimize its surface area given its volume, as that tends to minimize (meaning, largest negative number) the potential energy we have been talking about (which is also the reason that gravity makes things fall, to minimize the potential energy). So we see that the potential energy concept also motivates the idea of surface tension!
dario2 said:
When you say it lowers its energy, do you mean kinetic energy, potential energy, electromagnetic energy, or all of the above?

The total of all. The energy is being lowered by the loss of heat to the environment, and energy conservation requires that we be talking about total energy.
dario2 said:
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.
The total energy is conserved, so anything lost to the environment must show up as a drop in the energy within the substance. That includes kinetic energy and potential energy. So the heat that must be lost to make water go from 80C to 20C must show up as a combination of a drop in the kinetic energy of jiggling in the molecules, and a drop in the potential energy (where the former is a drop in a positive number, the latter is often regarded as increasing the absolute value of a negative number). But since the density of water does not change much with temperature, it's mostly all coming from kinetic energy. The opposite is true during condensation at fixed temperature.

dario2 said:
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.

The gravitational potential energy cares only about the distance from the center of the Earth, be it a hill or a power line. The distance to the ground is by itself irrelevant, we just normally consider a horizontal ground for simplicity.
dario2 said:
Is there any form of energy transfer, storage or conversion, that would not be considered potential energy?
Yes. To be considered potential energy, the conversion process must be able to be regarded as "work", which is a force times a distance moved, and the force must depend only on the location in such a way that the total work depends only on the initial and final location, not the path taken. This is the great simplification of potential energy, it doesn't matter the details of how you separate the water molecules into a vapor, only that you did it. A classic type of energy conversion that does not have this property is friction. You create much more friction by pushing your hands together when you rub them, it's not just about how far you move your hands (like it would be for throwing sneakers on a power line). This other type of energy conversion is called "heat", which is energy being transported spontaneously by the complexity of the situation (which says that when something can happen in staggeringly many different ways, what does happen will be simply what can happen in the most ways), and it depends on so many details of how it is done that we either simply track it without trying to understand how it happened, or we average over all its complex behavior and treat it in terms of constants that depend only on the material, like thermal conductivity. This all leads in its simplest form to the "first law of thermodynamics", which says that the change in internal energy of a system is the sum of the work done on it (which can often come in the form of a potential energy, though it could also just be manually applied in other ways), and the heat added to it.

By the way, when you hold a heavy object in place, you get tired, so you think you are "doing work." But you are not doing work on the object, because the object is not moving, and its energy is not changing. What is happening is you are releasing chemical energy in your muscles, for pretty complicated biological reasons. You are doing work to create heat, which is essentially friction in your muscles (though just what exactly is doing this cannot be too simple, one would have to ask a biologist!). Since the work is going straight into heat, none of it gets to the object. The same is true if you carry the object sideways from one point to another-- it ends up with no change in either kinetic energy or gravitational potential energy, so no work is done on it, because the motion of the object is perpendicular to the force you are applying. Work requires a force in the direction of the motion, so you have to lift the object or impel it with some speed, if you want to do work on the object and not just in your muscles.
 
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  • #22
I can respond more later, but one thing:
dario2 said:
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.

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.

If both units were inside, as with a dehumidifier, this wouldn't happen.
No, you're describing the system backwards. You're describing an air conditioner, or dehumidifier not a heater. The cold coil is absorbing heat from the air around it, not heating the air around it. The coil is cold, so it is getting warmer but the air is getting colder (and drier).

A dehumidifier has both coils inside so that it doesn't change the room temperature much.
 
  • #23
russ_watters said:
I can respond more later, but one thing:

No, you're describing the system backwards. You're describing an air conditioner, or dehumidifier not a heater. The cold coil is absorbing heat from the air around it, not heating the air around it. The coil is cold, so it is getting warmer but the air is getting colder (and drier).

A dehumidifier has both coils inside so that it doesn't change the room temperature much.
Correct me if I'm wrong, but a space heater is simply a device that uses up electricity, with a mechanism that prevents overheating. For all intents and purposes, all electric devices generate heat equal to the amount of electricity they consume. The advantage of an AC is it can pull heat from outdoors, and bring it inside, so it can generate more heat indoors than the electricity it consumes, but when the outdoor temperature is too low, there are problems with this system. The dehumidifier as was described here is simply an AC where both units are indoors. So you're taking heat from one side of the device, and transferring it to the other side. The average effect this has on the total room temperature is zero. But first, the device generates heat due to the energy it's consuming, and second, by condensating vapor into liquid water, more heat is released, as was described in this thread. That heat has to go somewhere, and that somewhere in this case is raising the indoors temperature.
 
  • #24
dario2 said:
Is it contained in the vapor molecules somehow in a form other than heat?
I think this thread has expanded a bit more than is helpful and the basic message may have been lost. The temperature of an object is defined by the average kinetic energy of the particles in it . (This can be just translational, as in an ideal gas or vibrational in liquids and solids.)

A simple description of how heat is lost from the water surface by evaporation is as follows. The water molecules at the surface have a distribution of speeds (Kinetic Energy) and the fastest ones leave the surface temporarily. They mix with air molecules near the surface but soon return due to the intermolecular potential (attractive) energy between these escapees and the body of water. There is what could be described as a gradient of saturation with increasing distance from the surface (which is not exactly a sharp boundary but more of a fuzzy interface.).
After a while under constant conditions, and if the air above is totally stationary an equilibrium will be established. Only the fastest water molecules will be furthest from the surface and the slowest molecules will be nearest. (When the experiment was started, there will have been a slight drop in (what we call the) water temperature because its share of fast molecules will be lower than when things started.

A drift of new and 'dry' air over the surface will permanently remove the faster molecules from the surface and more fast molecules will fly out, removing KE and dropping the water temperature but increasing the air temperature. If the volume of air is limited, it will become saturated and it will deposit as many molecules back into the water as are being lost. Equilibrium will exist.
 
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  • #25
dario2 said:
The average effect this has on the total room temperature is zero.
No this can never be true as Sadi Carnot showed. You will always necessarilly produce wasted energy. This realization is a foundation of modern thermodynamics. But this is far afield
 
  • #26
dario2 said:
Correct me if I'm wrong, but a space heater is simply a device that uses up electricity, with a mechanism that prevents overheating. For all intents and purposes, all electric devices generate heat equal to the amount of electricity they consume. The advantage of an AC is it can pull heat from outdoors, and bring it inside, so it can generate more heat indoors than the electricity it consumes, but when the outdoor temperature is too low, there are problems with this system. The dehumidifier as was described here is simply an AC where both units are indoors. So you're taking heat from one side of the device, and transferring it to the other side. The average effect this has on the total room temperature is zero.
Close to zero but otherwise all of that is fine. So note: this all directly contradicts your idea that putting both coils in the room adds more heating. As you say, it doesn't.
dario2 said:
But first, the device generates heat due to the energy it's consuming, and second, by condensing vapor into liquid water, more heat is released, as was described in this thread.
Yes.
dario2 said:
That heat has to go somewhere, and that somewhere in this case is raising the indoors temperature.
Not directly and not by the cold coil. It goes to heating up the cold coil and then gets transferred to the hot coil through the refrigerant. It only gets released into the room because you have the hot coil in the room. That's why for a heater the cold coil (where the condensation occurs) says outside. The cold coil isn't directly heating the room, it's cooling the room.
 
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  • #27
dario2 said:
Very interesting. And for liquids? And solids?
For liquids and solids temperature is not proportional to kinetic energy.
dario2 said:
You mean the liquid water loses the potential energy, and the vapor gains it, correct?
The water loses potential energy and it's converted to kinetic energy in the vapor.
dario2 said:
When you say it breaks the chemical bond, you're not talking about a molecular bond, are you?
Yes. It's a hydrogen bond: https://en.wikipedia.org/wiki/Hydrogen_bond#Hydrogen_bonds_in_small_molecules
dario2 said:
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?
No. This energy is not released as electricity.
dario2 said:
From my very limited understanding, this is not strictly speaking what steam engines and power plants already do. Or is it?
No. Steam engines use generators. They do not convert steam thermal energy directly into electricity.
dario2 said:
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?
This is unlikely. There are only 4 fundamental forces and only two of them act outside the nucleus of the atom. Although I suppose electromagnetism can "hide" in other places. Spring potential energy is ultimately electromagnetic for example.
dario2 said:
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?
It doesn't go anywhere. It's being stored. That's the point.
dario2 said:
Magnets lose electromagnetic potential over time, or when something pulls magnetic objects away from them, right?
Some do, some don't, and no, that's not directly related to potential energy.
dario2 said:
Isn't gravity a form of electromagnetism?
No, gravity is its own force.
dario2 said:
I think some forms of energy are better understood than others.

Is there any form of energy transfer, storage or conversion, that would not be considered potential energy?
There's no mystery here for scientists/engineers. You just don't quite have it yet. In transfer or conversion it wouldn't be potential energy. The whole point of potential energy is that it is stationary. It's due to the position of the object in a force field.
 
  • #28
dario2 said:
Is there any form of energy transfer, storage or conversion, that would not be considered potential energy?
Try to avoid getting hung up on 'classification'. If you dig deep enough into any energy transfer process you will come across quantities that can be considered as Potential Energy. Any interaction between moving objects (or particles) will involve some sort of distortion so PE's in there. The coils of an electric motor will distort (mechanical PE) and the magnetic field will be storing Energy. etc. etc.
 
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  • #29
sophiecentaur said:
I think this thread has expanded a bit more than is helpful and the basic message may have been lost. The temperature of an object is defined by the average kinetic energy of the particles in it . (This can be just translational, as in an ideal gas or vibrational in liquids and solids.)
This may be a technical correction, but actually temperature is defined by the class of thermal reservoir with which a system will not spontaneously exchange heat (and it receives heat spontaneously from higher T reservoirs, loses heat to lower T reservoirs). These classes of reservoirs are organized by the heat that needs to be added to the system to require one more "yes/no" question to be answered in order to fully describe that system. Your point is correct that this is directly related to the kinetic energy of the particles, because that describes the energy scale of what you would need to add to get that one more degree of "yes/no" ambiguity into the system. But how many degrees of freedom for putting that energy into also counts in determining the temperature.

So what this means is, your inclusion of additional "degrees of freedom", the vibrations in liquids and solids, has an important ramification that has thus far been overlooked in the thread: the change in degrees of freedom in which to express the internal kinetic energy associated with the temperature that happens when water molecules go from liquid to vapor must serve as an energy source that mitigates somewhat the potential energy increase that comes from vaporization. In other words, if a water molecule can express three spatial directions of translational kinetic energy, plus three directions of vibration, that's twice as much energy (at least it would for a solid, in a liquid it might not be quite that much because the vibrations are probably not fully excited) as the particle is going to have at the same T when it is a gas. So we have to count that change in the modes that can hold the kinetic energy, along with whatever the potential energy change is, and it reduces the energy needed to evaporate the water.
 
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  • #30
sophiecentaur said:
A simple description of how heat is lost from the water surface by evaporation is as follows.
But this doesn't account for the extra 540C heat of vaporization that was mentioned here, it only accounts for the difference between 100C in the vapor molecules leaving and the liquid water temperature, divided by how many molecules are still left in liquid form. And I previously thought it would heat the air, like you said, but it turns out it cools the air because even though the vapor molecules are hotter than air, way more temperature is removed from the liquid water than is required to heat those molecules, in order to allow for it to vaporize. It cools the air because the now cooler liquid water pulls heat from it to reach equilibrium.
hutchphd said:
No this can never be true as Sadi Carnot showed. You will always necessarilly produce wasted energy. This realization is a foundation of modern thermodynamics. But this is far afield
Right. I was saying that the process of taking heat from one side and putting it out in the other side, in itself, doesn't generate heating nor cooling of the average room temperature, it's the energy you use to make that happen that generates the heating, and the water condensation.

russ_watters said:
That's why for a heater the cold coil (where the condensation occurs) says outside. The cold coil isn't directly heating the room, it's cooling the room.
Yes, but if the cold coil was outside, you could have a situation where it's losing more heat to the environment than it's pulling in, and in that case, you'd be better off having both inside.

Something else I'm thinking of now, is some of the condensation on the cold coil coming from vapor in the outdoor air, or is it all from vapor in the indoor air that's being recirculated?

I'll reply to the other messages when I can, thanks all
 
  • #31
dario2 said:
Yes, but if the cold coil was outside, you could have a situation where it's losing more heat to the environment than it's pulling in....
No, that doesn't even make any sense/it's a self contradiction. It's called "the cold coil" because it is colder than the environment it is in and colder than the warm coil.
Something else I'm thinking of now, is some of the condensation on the cold coil coming from vapor in the outdoor air, or is it all from vapor in the indoor air that's being recirculated?
The cold coil is outside. All of the air flowing over it is outside air so all of the condensation is from outside air. There's no way for inside air to get to it.

It may help if you draw or look at a diagram of the process. It should be immediately obvious why what you are saying makes no sense.
 
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  • #32
dario2 said:
But this doesn't account for the extra 540C heat of vaporization that was mentioned here, it only accounts for the difference between 100C in the vapor molecules leaving and the liquid water temperature, divided by how many molecules are still left in liquid form.
You are right, of course. My description misses out a significant factor. The 'energy' that air, passing over the surface, removes will consist of the KE plus the PE of each molecule.* However, I think I'm correct in saying that only the fraction of water molecules with initially high KE will be taken far enough from the surface to be removed 'into the air'. It's the loss of the KE , shared with the air, that increases the temperature of the air but, losing the PE, will require more energy to be supplied to the surface than is delivered into the air. I liked the old term "sensible heat" which is heat that's associated with the temperature rise of the steam from a boiler after it's boiled with latent heat.

I've been thinking about Orbital Physics more than Thermal Physics, lately but there's a parallel with the energy used by a rocket to 1. Escape from a planet's gravitational PE and 2. the KE left for interplanetary travel. The bonds at a water surface corresponds to the g from a massive planet where the bonds on the surface of, say Methanol, correspond to the g from a small planetoid.
 
  • #33
Evaporative coolers (i.e., "Swamp Coolers") work by having water put onto a surface and then blowing air over it so that it evaporates; the equilibrium temperature of this action is the Adiabatic Saturation Temperature or "Wet Bulb" temperature. In a high humidity environment, the wet-bulb T is close to the regular (or "Dry Bulb") temperature, so this doesn't do much, but in a low-humidity environment, the wet-bulb temperature is much lower, so it works fine, also adding in some humidity so that the controlled environment is not so dry. The difference between the dry- & wet- bulb temperatures is basically due to the heat of evaporation of water vapor - such that energy that goes into the evaporation is taken from the water on a wet surface.

The biological action of sweating basically uses this idea, as sweated skin has the equilibrium temperature as the wet-bulb (ironically, sweating in a dry environment doesn't leave any sweat as it evaporates away, while sweating in a humid environment leaves sweat drops).
 

Related to How does evaporation generate cooling? Swamp coolers edition

How does evaporation cause cooling in swamp coolers?

Evaporation causes cooling in swamp coolers by utilizing the principle that when water evaporates, it absorbs heat from its surroundings. This process lowers the temperature of the air, which is then circulated through the space to provide a cooling effect.

What is the basic mechanism behind a swamp cooler?

A swamp cooler, or evaporative cooler, works by drawing warm air through water-saturated pads. As the warm air passes through the pads, water evaporates and absorbs heat from the air, resulting in cooler, more humid air being expelled into the living space.

Why are swamp coolers more effective in dry climates?

Swamp coolers are more effective in dry climates because the lower humidity levels allow for more efficient evaporation. In humid climates, the air is already saturated with moisture, which reduces the rate of evaporation and thus the cooling effect.

Can swamp coolers lower the temperature as much as traditional air conditioners?

Swamp coolers generally do not lower the temperature as much as traditional air conditioners. They are most effective in regions with low humidity and can typically reduce the air temperature by 10-20 degrees Fahrenheit. Traditional air conditioners can achieve lower temperatures regardless of the ambient humidity.

What maintenance is required for a swamp cooler to function effectively?

Regular maintenance for a swamp cooler includes replacing the cooling pads when they become clogged or worn, ensuring the water supply is clean and adequate, and periodically cleaning the unit to prevent mold and mineral buildup. Proper maintenance ensures optimal evaporation and cooling efficiency.

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