Thursday, September 20, 2007

Thermodynamics

So I was reading a letter to the editor which argued that tidal turbines are the way to go for green energy, because we'll have them as a source of power as long as we have gravity. That made me wonder how the heck gravity gets around the first law of thermodynamics. We're experiencing a massive amount of energy use in the form of tides. Where is that energy coming from?

3 Comments:

Blogger Noumenon said...

The moon. The thermodynamics applies to a closed system. In the earth-moon system, the tides get pulled along by the moon and cause the moon to lose energy (angular momentum or orbital speed or something), so it rotates slower or gets closer to the earth (losing potential energy).

Tidal acceleration

10:38 AM  
Blogger Yoel Natan said...

No. The moon is orbiting the earth so fast that it has escape velocity. Every year it's two inches farther from earth. Moreover, the same moon face shows towards earth all the time, the lowest energy state the rotating moon could have. In other words, there's no energy trade-off (anymore) between rotation and orbit, unless perhaps the earth's and moon's elliptic orbit are going into a more circular, lower-energy orbit to compensate for all the gravity tugging waves on earth.

6:52 PM  
Blogger Noumenon said...

Was I tired, or dumb? The article I linked to is full of statements saying just the opposite of what I typed:

"The gravitational torque between the Moon and the tidal bulge of the Earth causes the Moon to be accelerated in its orbit, and the Earth to be decelerated in its rotation. As in any physical process, total energy and angular momentum are conserved. Effectively, energy and angular momentum are transferred from the rotation of the Earth to the orbital motion of the Moon. The Moon moves farther away from the Earth, so its potential energy (in the Earth's gravity well) increases... The Moon's orbital angular momentum also increases."

So I guess the energy comes out of the Earth's angular momentum instead.

10:34 PM  

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