# Hydroelectric power plants question

1. Nov 21, 2013

### santiag0m

If we put a system that functions like the hydroelectric power plants do ( the gravity produces the movement of the water, producing electricity ) in vacuum and we put and object ( of a considerable mass ) that causes gravity and makes the "water" to orbit around it, wouldn't be a source of "infinite" electricity? ( i don't have clear if the gravity could be a source of infinite energy though)

2. Nov 21, 2013

### Staff: Mentor

No. The orbiting water will be be transferring its energy to the turbine blades, so its orbit will decay until ends up sitting still on the surface of your gravitating object.

What you're describing is somewhat similar to something that is done today: Position a turbine in such a way that the flow of the ocean tides will turn it, use that to generate electricity. The moon is orbiting the earth, its gravity is dragging the water around to create the tide, and it looks as if we getting infinite free energy from the orbiting moon. We aren't: the moon is actually losing a tiny bit of its kinetic energy in the process. Fortunately, it has plenty to spare

3. Nov 21, 2013

### jfizzix

If I read you correctly, your question is, if we put a jet of water in orbit around a planet, could we use a paddle wheel or turbine to generate electricity forever?
And
"Can gravity be a source of infinite energy?"

Gravity cannot be a source of infinite energy.
The largest amount of energy you can extract form a falling body would be the difference in the gravitational potential energy between the surface of the Earth, and infinitely far away above the Earth. This number can be big for massive falling objects, but it is still finite. You would need an infinite amount of falling objects to extract an infinite amount of energy.

As far as the second question goes:
If you were to use the momentum of the orbiting water to push a paddle wheel, that orbiting water would slow down (because the paddle wheel is pushing back on the water), and fall into a lower orbit. Eventually, if you keep using the paddle wheel, the water would slow down to the point where it will fall back into the atmosphere.

One side point:
Liquid water can't exist in a vacuum since at those pressures, the boiling point is well below room temperature. However, one could still think about using a paddle wheel against orbiting streams of rocks, like the rings of Saturn.

4. Nov 21, 2013