When a man made satellite falls towards earth its energy can be collected by fixing a wind turbine generator above or below it.It is so obvious but how does 2nd law of thermodynamics explain it.

When a man made satellite falls towards earth its energy can be collected by fixing a wind turbine generator above or below it.It is so obvious but how does 2nd law of thermodynamics explain it.

You are wondering where the entropy increase is? The gravitational potential energy and kinetic energy is converted to electrical energy (stored or used), and waste thermal energy.

The wind turbine will generate less electric energy than the satellite has kinetic and potential energy at the start. In practice, it will probably be a lot less.

The question is about reversibility,2nd law assumes it is absolutely not reversible. The rocket energy that hurls the satellite in to space becomes partially reversible on return path to earth.

Why does the 2nd law assume that it is not reversible? Part of the rocket energy is converted to entropy (which you cannot recover), the other part is converted to kinetic and potential energy of the satellite (which you can recover partially).

In case of 100% reversibility a perpetual machine becomes,in theory,possible.In case of partial reversibility it can lead to energy efficiency in the sense that a part of the same energy can be used again.

In case of 100% reversibility a perpetual machine becomes,in theory,possible.In case of partial reversibility it can lead to energy efficiency in the sense that a part of the same energy can be used again.

 

 

But there's nothing about recovering a fraction of the energy that's inconsistent with the 2nd law.

 

 

But there's nothing about recovering a fraction of the energy that's inconsistent with the 2nd law.

How would you propose that a satellite re-entering the atmosphere could generate any energy at all with a fan or propeller? It would seem to me that the rotation of the prop would be countered by a counter motion of the satellite.

How would you propose that a satellite re-entering the atmosphere could generate any energy at all with a fan or propeller? It would seem to me that the rotation of the prop would be countered by a counter motion of the satellite.

two turbines...counter rotating...plus of course heat resistance and control

How would you propose that a satellite re-entering the atmosphere could generate any energy at all with a fan or propeller? It would seem to me that the rotation of the prop would be countered by a counter motion of the satellite.

The satellite would rotate in the same direction as the prop without countermeasures, like what MacSwell said.

 

Anyway, there would be some motion between fan and satellite, so some energy could be generated. It is not like it is a very practical idea anyway.

The satellite would rotate in the same direction as the prop without countermeasures, like what MacSwell said.

 

Anyway, there would be some motion between fan and satellite, so some energy could be generated. It is not like it is a very practical idea anyway.

 

Oops, of course you and MacSwell are correct. Having a pretty serious infection from a broken tooth and having to take strong pain medication while the antibiotics are working does tend to dampen one's ability to visualize.

I feel your pain (at least I did last time I went to the dentist who successfully anaesthetised half my face, but somehow not my tooth.)