Mixing
Why do we have to dump the extra entropy created in a heat engine?
Clash Royale CLAN TAG#URR8PPP
up vote
3
down vote
favorite
In his introduction to thermophysics, Daniel Schroeder writes the following about the process in a heat engine:
"Only part of the energy absorbed as heat can be converted to work. The reason is, that the heat, as it flows in, brings along entropy, which must somehow be disposed of before the cycle can start over. To get rid of the entropy, every heat engine must dump some waste heat into its environment."
Why must the extra entropy be disposed?
In my understanding, we could gather more and more extra entropy, while converting all the heat into work until the entropy reaches its maximum. Then a state of equilibrium would be reached and no more energy could be withdrawn from the system according to the second law of thermodynamics. But then we could just go to the next machine and do the same, always converting all heat into work.
thermodynamics work entropy heat-engine
asked 2 hours ago
JoKli
184
New contributor
JoKli is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
add a comment |Â
up vote
3
down vote
favorite
In his introduction to thermophysics, Daniel Schroeder writes the following about the process in a heat engine:
"Only part of the energy absorbed as heat can be converted to work. The reason is, that the heat, as it flows in, brings along entropy, which must somehow be disposed of before the cycle can start over. To get rid of the entropy, every heat engine must dump some waste heat into its environment."
Why must the extra entropy be disposed?
In my understanding, we could gather more and more extra entropy, while converting all the heat into work until the entropy reaches its maximum. Then a state of equilibrium would be reached and no more energy could be withdrawn from the system according to the second law of thermodynamics. But then we could just go to the next machine and do the same, always converting all heat into work.
thermodynamics work entropy heat-engine
asked 2 hours ago
JoKli
184
New contributor
JoKli is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
add a comment |Â
up vote
3
down vote
favorite
up vote
3
down vote
favorite
In his introduction to thermophysics, Daniel Schroeder writes the following about the process in a heat engine:
"Only part of the energy absorbed as heat can be converted to work. The reason is, that the heat, as it flows in, brings along entropy, which must somehow be disposed of before the cycle can start over. To get rid of the entropy, every heat engine must dump some waste heat into its environment."
Why must the extra entropy be disposed?
In my understanding, we could gather more and more extra entropy, while converting all the heat into work until the entropy reaches its maximum. Then a state of equilibrium would be reached and no more energy could be withdrawn from the system according to the second law of thermodynamics. But then we could just go to the next machine and do the same, always converting all heat into work.
thermodynamics work entropy heat-engine
asked 2 hours ago
JoKli
184
New contributor
JoKli is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
In his introduction to thermophysics, Daniel Schroeder writes the following about the process in a heat engine:
"Only part of the energy absorbed as heat can be converted to work. The reason is, that the heat, as it flows in, brings along entropy, which must somehow be disposed of before the cycle can start over. To get rid of the entropy, every heat engine must dump some waste heat into its environment."
Why must the extra entropy be disposed?
In my understanding, we could gather more and more extra entropy, while converting all the heat into work until the entropy reaches its maximum. Then a state of equilibrium would be reached and no more energy could be withdrawn from the system according to the second law of thermodynamics. But then we could just go to the next machine and do the same, always converting all heat into work.
thermodynamics work entropy heat-engine
thermodynamics work entropy heat-engine
asked 2 hours ago
JoKli
184
New contributor
JoKli is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
asked 2 hours ago
JoKli
184
New contributor
JoKli is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
asked 2 hours ago
JoKli
184
New contributor
JoKli is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
asked 2 hours ago
JoKli
184
asked 2 hours ago
JoKli
184
184
New contributor
JoKli is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
New contributor
JoKli is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
JoKli is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
add a comment |Â
add a comment |Â
2 Answers
2
active
oldest
votes
up vote
3
down vote
accepted
We are talking about cycles here. After a complete cycle the system must be right back where it started. Since entropy is a state variable, it must then be that after one complete cycle the entropy is at its "initial" value. By the second law this must mean that the entropy has to "go somewhere else". If you "gathered more and more" entropy, then it is no longer a cycle.
If you did want to do what you propose of replacing engines then it would be extremely inefficient. You would get one "run" out of the process and then have to get a new engine (I am not sure how this would actually work). It is much better to use the same engine on a cycle.
answered 2 hours ago
Aaron Stevens
4,5461625
Why does it have to be a cycle? Can't it be more of a linear path where the entropy just increases until its maximum?
– JoKli
2 hours ago
But in theory it would be possible to convert all heat into work then. It is just inefficient, but still possible right?
– JoKli
1 hour ago
@JoKli Yes, if you are talking about a single process an not a cycle. For example, an irreversible isothermal expansion of a gas in a container does work on the surrounding where the energy is completely from heat. This is why I specifically said in my answer you would only get one "run" out of the process. It no longer is a heat engine, since heat engines use cycles.
– Aaron Stevens
20 mins ago
@jokli - imagine you have a car going on gasoline. Probably you know, that it has around 2000 -3000 rotations per minute. it is perfect example of heat engine. So with your proposal you have to use no more than, say, 1000 cycles and change engine. Which means 3 times per minute! I am writing this because maybe it is somehow abstract for you, but this is the same for train, car, plane. But of course there are engines working exactly as you are proposing: rockets. This is single run engine.
– kakaz
35 secs ago
add a comment |Â
up vote
1
down vote
In my understanding, we could gather more and more extra entropy, while converting all the heat into work
You can't store entropy while still converting all the heat into work. Storing an amount of entropy $dS$ requires that you also store an amount of energy $TdS$, where $T$ is the temperature of the object you're storing the entropy in.
But then we could just go to the next machine and do the same, always converting all heat into work.
You can retain some of the entropy internally inside your heat engine rather than expelling it into some external reservoir such as a river or the air. Let's say you have a tank of water that stays inside your heat engine until you throw the heat engine away. You store entropy in this tank of water, which requires heating the water. There are two issues here: (1) As the water tank gets hotter, the energy cost of storing energy in it, $TdS$, gets worse and worse. (2) The tank is no different from an external heat reservoir. You can keep it inside the box that holds your engine, but that doesn't matter. Our description of a heat engine abstracts away questions like where the low-temperature reservoir is physically located. The only real difference is that we normally idealize the low-temperature reservoir as an infinite resource, whose temperature never changes, while the tank is actually finite, and therefore worse thermodynamically because it heats up.
answered 24 mins ago
Ben Crowell
45.5k3147275
I thought, that if I transfer heat from a hot object to a cold one, I create extra entropy, as the multiplicity of ways to store energy increases. How can there be an amount of energy corresponding to this extra entropy? Wouldn't that create energy out of nowhere?
– JoKli
6 mins ago
add a comment |Â
2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
3
down vote
accepted
We are talking about cycles here. After a complete cycle the system must be right back where it started. Since entropy is a state variable, it must then be that after one complete cycle the entropy is at its "initial" value. By the second law this must mean that the entropy has to "go somewhere else". If you "gathered more and more" entropy, then it is no longer a cycle.
If you did want to do what you propose of replacing engines then it would be extremely inefficient. You would get one "run" out of the process and then have to get a new engine (I am not sure how this would actually work). It is much better to use the same engine on a cycle.
answered 2 hours ago
Aaron Stevens
4,5461625
Why does it have to be a cycle? Can't it be more of a linear path where the entropy just increases until its maximum?
– JoKli
2 hours ago
But in theory it would be possible to convert all heat into work then. It is just inefficient, but still possible right?
– JoKli
1 hour ago
@JoKli Yes, if you are talking about a single process an not a cycle. For example, an irreversible isothermal expansion of a gas in a container does work on the surrounding where the energy is completely from heat. This is why I specifically said in my answer you would only get one "run" out of the process. It no longer is a heat engine, since heat engines use cycles.
– Aaron Stevens
20 mins ago
@jokli - imagine you have a car going on gasoline. Probably you know, that it has around 2000 -3000 rotations per minute. it is perfect example of heat engine. So with your proposal you have to use no more than, say, 1000 cycles and change engine. Which means 3 times per minute! I am writing this because maybe it is somehow abstract for you, but this is the same for train, car, plane. But of course there are engines working exactly as you are proposing: rockets. This is single run engine.
– kakaz
35 secs ago
add a comment |Â
up vote
3
down vote
accepted
We are talking about cycles here. After a complete cycle the system must be right back where it started. Since entropy is a state variable, it must then be that after one complete cycle the entropy is at its "initial" value. By the second law this must mean that the entropy has to "go somewhere else". If you "gathered more and more" entropy, then it is no longer a cycle.
If you did want to do what you propose of replacing engines then it would be extremely inefficient. You would get one "run" out of the process and then have to get a new engine (I am not sure how this would actually work). It is much better to use the same engine on a cycle.
answered 2 hours ago
Aaron Stevens
4,5461625
Why does it have to be a cycle? Can't it be more of a linear path where the entropy just increases until its maximum?
– JoKli
2 hours ago
But in theory it would be possible to convert all heat into work then. It is just inefficient, but still possible right?
– JoKli
1 hour ago
@JoKli Yes, if you are talking about a single process an not a cycle. For example, an irreversible isothermal expansion of a gas in a container does work on the surrounding where the energy is completely from heat. This is why I specifically said in my answer you would only get one "run" out of the process. It no longer is a heat engine, since heat engines use cycles.
– Aaron Stevens
20 mins ago
@jokli - imagine you have a car going on gasoline. Probably you know, that it has around 2000 -3000 rotations per minute. it is perfect example of heat engine. So with your proposal you have to use no more than, say, 1000 cycles and change engine. Which means 3 times per minute! I am writing this because maybe it is somehow abstract for you, but this is the same for train, car, plane. But of course there are engines working exactly as you are proposing: rockets. This is single run engine.
– kakaz
35 secs ago
add a comment |Â
up vote
3
down vote
accepted
up vote
3
down vote
accepted
We are talking about cycles here. After a complete cycle the system must be right back where it started. Since entropy is a state variable, it must then be that after one complete cycle the entropy is at its "initial" value. By the second law this must mean that the entropy has to "go somewhere else". If you "gathered more and more" entropy, then it is no longer a cycle.
If you did want to do what you propose of replacing engines then it would be extremely inefficient. You would get one "run" out of the process and then have to get a new engine (I am not sure how this would actually work). It is much better to use the same engine on a cycle.
answered 2 hours ago
Aaron Stevens
4,5461625
We are talking about cycles here. After a complete cycle the system must be right back where it started. Since entropy is a state variable, it must then be that after one complete cycle the entropy is at its "initial" value. By the second law this must mean that the entropy has to "go somewhere else". If you "gathered more and more" entropy, then it is no longer a cycle.
If you did want to do what you propose of replacing engines then it would be extremely inefficient. You would get one "run" out of the process and then have to get a new engine (I am not sure how this would actually work). It is much better to use the same engine on a cycle.
answered 2 hours ago
Aaron Stevens
4,5461625
edited 19 mins ago
answered 2 hours ago
Aaron Stevens
4,5461625
answered 2 hours ago
Aaron Stevens
4,5461625
answered 2 hours ago
Aaron Stevens
4,5461625
4,5461625
Why does it have to be a cycle? Can't it be more of a linear path where the entropy just increases until its maximum?
– JoKli
2 hours ago
But in theory it would be possible to convert all heat into work then. It is just inefficient, but still possible right?
– JoKli
1 hour ago
@JoKli Yes, if you are talking about a single process an not a cycle. For example, an irreversible isothermal expansion of a gas in a container does work on the surrounding where the energy is completely from heat. This is why I specifically said in my answer you would only get one "run" out of the process. It no longer is a heat engine, since heat engines use cycles.
– Aaron Stevens
20 mins ago
@jokli - imagine you have a car going on gasoline. Probably you know, that it has around 2000 -3000 rotations per minute. it is perfect example of heat engine. So with your proposal you have to use no more than, say, 1000 cycles and change engine. Which means 3 times per minute! I am writing this because maybe it is somehow abstract for you, but this is the same for train, car, plane. But of course there are engines working exactly as you are proposing: rockets. This is single run engine.
– kakaz
35 secs ago
add a comment |Â
Why does it have to be a cycle? Can't it be more of a linear path where the entropy just increases until its maximum?
– JoKli
2 hours ago
But in theory it would be possible to convert all heat into work then. It is just inefficient, but still possible right?
– JoKli
1 hour ago
@JoKli Yes, if you are talking about a single process an not a cycle. For example, an irreversible isothermal expansion of a gas in a container does work on the surrounding where the energy is completely from heat. This is why I specifically said in my answer you would only get one "run" out of the process. It no longer is a heat engine, since heat engines use cycles.
– Aaron Stevens
20 mins ago
@jokli - imagine you have a car going on gasoline. Probably you know, that it has around 2000 -3000 rotations per minute. it is perfect example of heat engine. So with your proposal you have to use no more than, say, 1000 cycles and change engine. Which means 3 times per minute! I am writing this because maybe it is somehow abstract for you, but this is the same for train, car, plane. But of course there are engines working exactly as you are proposing: rockets. This is single run engine.
– kakaz
35 secs ago
Why does it have to be a cycle? Can't it be more of a linear path where the entropy just increases until its maximum?
– JoKli
2 hours ago
Why does it have to be a cycle? Can't it be more of a linear path where the entropy just increases until its maximum?
– JoKli
2 hours ago
But in theory it would be possible to convert all heat into work then. It is just inefficient, but still possible right?
– JoKli
1 hour ago
But in theory it would be possible to convert all heat into work then. It is just inefficient, but still possible right?
– JoKli
1 hour ago
@JoKli Yes, if you are talking about a single process an not a cycle. For example, an irreversible isothermal expansion of a gas in a container does work on the surrounding where the energy is completely from heat. This is why I specifically said in my answer you would only get one "run" out of the process. It no longer is a heat engine, since heat engines use cycles.
– Aaron Stevens
20 mins ago
@JoKli Yes, if you are talking about a single process an not a cycle. For example, an irreversible isothermal expansion of a gas in a container does work on the surrounding where the energy is completely from heat. This is why I specifically said in my answer you would only get one "run" out of the process. It no longer is a heat engine, since heat engines use cycles.
– Aaron Stevens
20 mins ago
@jokli - imagine you have a car going on gasoline. Probably you know, that it has around 2000 -3000 rotations per minute. it is perfect example of heat engine. So with your proposal you have to use no more than, say, 1000 cycles and change engine. Which means 3 times per minute! I am writing this because maybe it is somehow abstract for you, but this is the same for train, car, plane. But of course there are engines working exactly as you are proposing: rockets. This is single run engine.
– kakaz
35 secs ago
@jokli - imagine you have a car going on gasoline. Probably you know, that it has around 2000 -3000 rotations per minute. it is perfect example of heat engine. So with your proposal you have to use no more than, say, 1000 cycles and change engine. Which means 3 times per minute! I am writing this because maybe it is somehow abstract for you, but this is the same for train, car, plane. But of course there are engines working exactly as you are proposing: rockets. This is single run engine.
– kakaz
35 secs ago
add a comment |Â
up vote
1
down vote
In my understanding, we could gather more and more extra entropy, while converting all the heat into work
You can't store entropy while still converting all the heat into work. Storing an amount of entropy $dS$ requires that you also store an amount of energy $TdS$, where $T$ is the temperature of the object you're storing the entropy in.
But then we could just go to the next machine and do the same, always converting all heat into work.
You can retain some of the entropy internally inside your heat engine rather than expelling it into some external reservoir such as a river or the air. Let's say you have a tank of water that stays inside your heat engine until you throw the heat engine away. You store entropy in this tank of water, which requires heating the water. There are two issues here: (1) As the water tank gets hotter, the energy cost of storing energy in it, $TdS$, gets worse and worse. (2) The tank is no different from an external heat reservoir. You can keep it inside the box that holds your engine, but that doesn't matter. Our description of a heat engine abstracts away questions like where the low-temperature reservoir is physically located. The only real difference is that we normally idealize the low-temperature reservoir as an infinite resource, whose temperature never changes, while the tank is actually finite, and therefore worse thermodynamically because it heats up.
answered 24 mins ago
Ben Crowell
45.5k3147275
I thought, that if I transfer heat from a hot object to a cold one, I create extra entropy, as the multiplicity of ways to store energy increases. How can there be an amount of energy corresponding to this extra entropy? Wouldn't that create energy out of nowhere?
– JoKli
6 mins ago
add a comment |Â
up vote
1
down vote
In my understanding, we could gather more and more extra entropy, while converting all the heat into work
You can't store entropy while still converting all the heat into work. Storing an amount of entropy $dS$ requires that you also store an amount of energy $TdS$, where $T$ is the temperature of the object you're storing the entropy in.
But then we could just go to the next machine and do the same, always converting all heat into work.
You can retain some of the entropy internally inside your heat engine rather than expelling it into some external reservoir such as a river or the air. Let's say you have a tank of water that stays inside your heat engine until you throw the heat engine away. You store entropy in this tank of water, which requires heating the water. There are two issues here: (1) As the water tank gets hotter, the energy cost of storing energy in it, $TdS$, gets worse and worse. (2) The tank is no different from an external heat reservoir. You can keep it inside the box that holds your engine, but that doesn't matter. Our description of a heat engine abstracts away questions like where the low-temperature reservoir is physically located. The only real difference is that we normally idealize the low-temperature reservoir as an infinite resource, whose temperature never changes, while the tank is actually finite, and therefore worse thermodynamically because it heats up.
answered 24 mins ago
Ben Crowell
45.5k3147275
I thought, that if I transfer heat from a hot object to a cold one, I create extra entropy, as the multiplicity of ways to store energy increases. How can there be an amount of energy corresponding to this extra entropy? Wouldn't that create energy out of nowhere?
– JoKli
6 mins ago
add a comment |Â
up vote
1
down vote
up vote
1
down vote
In my understanding, we could gather more and more extra entropy, while converting all the heat into work
You can't store entropy while still converting all the heat into work. Storing an amount of entropy $dS$ requires that you also store an amount of energy $TdS$, where $T$ is the temperature of the object you're storing the entropy in.
But then we could just go to the next machine and do the same, always converting all heat into work.
You can retain some of the entropy internally inside your heat engine rather than expelling it into some external reservoir such as a river or the air. Let's say you have a tank of water that stays inside your heat engine until you throw the heat engine away. You store entropy in this tank of water, which requires heating the water. There are two issues here: (1) As the water tank gets hotter, the energy cost of storing energy in it, $TdS$, gets worse and worse. (2) The tank is no different from an external heat reservoir. You can keep it inside the box that holds your engine, but that doesn't matter. Our description of a heat engine abstracts away questions like where the low-temperature reservoir is physically located. The only real difference is that we normally idealize the low-temperature reservoir as an infinite resource, whose temperature never changes, while the tank is actually finite, and therefore worse thermodynamically because it heats up.
answered 24 mins ago
Ben Crowell
45.5k3147275
In my understanding, we could gather more and more extra entropy, while converting all the heat into work
You can't store entropy while still converting all the heat into work. Storing an amount of entropy $dS$ requires that you also store an amount of energy $TdS$, where $T$ is the temperature of the object you're storing the entropy in.
But then we could just go to the next machine and do the same, always converting all heat into work.
You can retain some of the entropy internally inside your heat engine rather than expelling it into some external reservoir such as a river or the air. Let's say you have a tank of water that stays inside your heat engine until you throw the heat engine away. You store entropy in this tank of water, which requires heating the water. There are two issues here: (1) As the water tank gets hotter, the energy cost of storing energy in it, $TdS$, gets worse and worse. (2) The tank is no different from an external heat reservoir. You can keep it inside the box that holds your engine, but that doesn't matter. Our description of a heat engine abstracts away questions like where the low-temperature reservoir is physically located. The only real difference is that we normally idealize the low-temperature reservoir as an infinite resource, whose temperature never changes, while the tank is actually finite, and therefore worse thermodynamically because it heats up.
answered 24 mins ago
Ben Crowell
45.5k3147275
answered 24 mins ago
Ben Crowell
45.5k3147275
answered 24 mins ago
Ben Crowell
45.5k3147275
answered 24 mins ago
Ben Crowell
45.5k3147275
45.5k3147275
I thought, that if I transfer heat from a hot object to a cold one, I create extra entropy, as the multiplicity of ways to store energy increases. How can there be an amount of energy corresponding to this extra entropy? Wouldn't that create energy out of nowhere?
– JoKli
6 mins ago
add a comment |Â
I thought, that if I transfer heat from a hot object to a cold one, I create extra entropy, as the multiplicity of ways to store energy increases. How can there be an amount of energy corresponding to this extra entropy? Wouldn't that create energy out of nowhere?
– JoKli
6 mins ago
I thought, that if I transfer heat from a hot object to a cold one, I create extra entropy, as the multiplicity of ways to store energy increases. How can there be an amount of energy corresponding to this extra entropy? Wouldn't that create energy out of nowhere?
– JoKli
6 mins ago
I thought, that if I transfer heat from a hot object to a cold one, I create extra entropy, as the multiplicity of ways to store energy increases. How can there be an amount of energy corresponding to this extra entropy? Wouldn't that create energy out of nowhere?
– JoKli
6 mins ago
add a comment |Â
JoKli is a new contributor. Be nice, and check out our Code of Conduct.
JoKli is a new contributor. Be nice, and check out our Code of Conduct.
JoKli is a new contributor. Be nice, and check out our Code of Conduct.
JoKli is a new contributor. Be nice, and check out our Code of Conduct.
Sign up or log in
StackExchange.ready(function ()
StackExchange.helpers.onClickDraftSave('#login-link');
);
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
StackExchange.ready(
function ()
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fphysics.stackexchange.com%2fquestions%2f434683%2fwhy-do-we-have-to-dump-the-extra-entropy-created-in-a-heat-engine%23new-answer', 'question_page');
);
Post as a guest
Sign up or log in
StackExchange.ready(function ()
StackExchange.helpers.onClickDraftSave('#login-link');
);
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Sign up or log in
StackExchange.ready(function ()
StackExchange.helpers.onClickDraftSave('#login-link');
);
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Sign up or log in
StackExchange.ready(function ()
StackExchange.helpers.onClickDraftSave('#login-link');
);
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
