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0 pointsmannykannot5y ago0 comments

> ...the problem is more subtle.

What is 'the problem' that you are apparently trying to address here? Everyone in the discussion is well aware that the 2nd. law of thermodynamics prevents the continuous extraction of work from ambient heat without there being a temperature difference. Are you saying that the paper is claiming that this has been done? If not that, are you claiming that the device, as described in the paper, is incapable of generating power from fluctuations in the ambient temperature? Have you identified some other problem in the paper? Alternatively - I don't think you are saying this, but I will put it in for completeness - could it be that you are saying that the paper shows that this team really has invented a Maxwell's diode, capable of continuously extracting work from the ambient heat of a closed system in thermal equilibrium?

If you are still only taking issue with the press release, you are just nerdsplaining to a bunch of people who already get it, thank you very much.

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gus_massa5y ago· 2 in thread

> Everyone in the discussion is well aware that the 2nd. law

Probably not, but it is good to know that you are.

The paper is not about a device that harvest energy from the thermal variations of the environment. There is no mention of macroscopic thermal variations in the press release or in the abstract of the paper. Also, the graphene is inside a ultra high vacuum chamber, that is a weird place to put a device that that depends on the macroscopic thermal variations of the environment.

From the abstract:

> However, there is power dissipated by the load resistor, and its time average is exactly equal to the power supplied by the thermal bath.

My interpretation is that the resistor is dissipating some power (from the graphics ~1pW), but it is also absorbing the same amount of power due to the 2nd law. It is not a Carnot engine that produces work from heat.

mannykannotOP5y ago

I am not sure what your claim is in that last paragraph. If it takes heat energy from the thermal bath and produces electrical energy (which is then dissipated in a load resistor), in what way is it not a Carnot engine?

Are you taking the quoted passage as implying it is performing 100% conversion? This would indeed be a problem, as there is no heat sink at 0K here. That quote, however, is ambiguous, as it says power supplied by the heat bath and not lost from it, and may merely be a statement that it obeys the conservation of energy (i.e. the point being made here may be that the power being supplied to the load comes from the heat bath, as opposed to coming from another possible source, such as the bias voltage supply.)

I really should buy and read the paper, but I am not that motivated yet.

gus_massa5y ago

It is not producing a macroscopic current, it is producing something like Johnson noise https://en.wikipedia.org/wiki/Johnson%E2%80%93Nyquist_noise (The interesting part seams to be that it has another frequency distribution, but I don't understand the details.)

A real engine transform heat into electric energy that can be used to dissipate heat inside a container at any temperature. In particular at a higher temperature that the graphene and the support and the circuit. In the PR they claim that it produce "something useful" that I interpret as "work" or "electrical energy", but in that case it breaks the Second Law even if in their setup the resistor is at the same temperature.

After reading the paragraph again, I'm not sure if your interpretation is right and they are claiming that the device is transferring energy from the graphene to the resistor, but that breaks the second law, in spite in the PR they claim it doesn't.

I think they never claim a 100% conversion anyway. They just don't have a temperature difference to operate the engine.

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