- #1
Eric Bretschneider
- 92
- 51
Physics students are taught about Maxwell's demon and how by opening and closing a door between two chambers filled with gas. If the demon opens and closes the door to "concentrate" low energy molecules on one side and high energy molecules on the other side he can create a temperature difference from which work can be extracted.
My question is, why is the mass of the door and the energy required to open/close the door ignores? At about room temperature and atmospheric pressure, the collisional rate is going to be on the order of 10 GHz. If the gas is helium (diameter ~62 pm) then the tunnel between the chambers would realistically need to be at least 100 pm wide to allow an atom through.
That implies that the door would have to move at least 100 pm in order to open or close the passage and would have to operate at about 10 GHz. That requires an acceleration/deceleration rate on the order of 8.0E+10 m/sec^2. Given the distance and the mass of the door, the demon needs to expend at least 1.07E-15W to create a temperature difference.
This ignores any energy needed for sensing the speed of an approaching helium atom.
More realistically the tunnel would need to be wider and the door far more massive than a single helium atom. It would also have to withstand accelerations of about 8 billion times that of gravity.
I really question if the energy extracted by results of Maxwell's demon could actually exceed the energy needed to create the temperature difference.
My question is, why is the mass of the door and the energy required to open/close the door ignores? At about room temperature and atmospheric pressure, the collisional rate is going to be on the order of 10 GHz. If the gas is helium (diameter ~62 pm) then the tunnel between the chambers would realistically need to be at least 100 pm wide to allow an atom through.
That implies that the door would have to move at least 100 pm in order to open or close the passage and would have to operate at about 10 GHz. That requires an acceleration/deceleration rate on the order of 8.0E+10 m/sec^2. Given the distance and the mass of the door, the demon needs to expend at least 1.07E-15W to create a temperature difference.
This ignores any energy needed for sensing the speed of an approaching helium atom.
More realistically the tunnel would need to be wider and the door far more massive than a single helium atom. It would also have to withstand accelerations of about 8 billion times that of gravity.
I really question if the energy extracted by results of Maxwell's demon could actually exceed the energy needed to create the temperature difference.