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View Full Version : Why Doesn't Tidal Energy Violate the Laws of Thermodynamics?

Shagnasty
12-15-2006, 07:48 PM
This is a piece I am still missing. Most of the other energy on earth comes from the sun in some form or another. I understand how that works and how solar energy gets stored. Then, we have nuclear energy. Got that as well.

Tidal energy falls into its own category as I understand. The moon's gravity moves massive amounts of water all across the earth all day, every day. We know how to tap a tiny part of that but the tapping isn't the key point. It seems that this system is doing massive amounts of work all the time.

Where does the energy come from?

groman
12-15-2006, 07:56 PM
As the result of the tidal effects the moons rotation around the earth is slowing down.

Bryan Ekers
12-15-2006, 08:01 PM
Well, it's all gravity and two objects that have a mutual gravitational attraction already have potential energy to start accelerating toward each other. Tidal flow may look impressive but it's the merest rounding error of how much potential energy the Earth has in relation to the moon and (to a lesser extent) the sun. Picture how much kinetic energy (potential energy expended) the moon would have if it suddenly stopped in its orbit and splashed down in the mid-Pacific. Tides are trivial by comparison.

Q.E.D.
12-15-2006, 08:22 PM
Tidal energy isn't gravitational or tidal at all; it's rotational. As pointed out, that rotational energy comes from the Earth.

Kevbo
12-15-2006, 09:03 PM
Tidal forces are thought to be resposible for the fact that the moons rotation rate matches it's orbital period.

So not only does the tidal energy slow the moons orbit, but also slows the earth's rotation.

CookingWithGas
12-15-2006, 09:09 PM
Tidal energy isn't gravitational or tidal at all; it's rotational. As pointed out, that rotational energy comes from the Earth.I don't doubt your veracity but I don't follow this. The tides are the result of the gravitational attraction between the water and the moon being higher for the water that faces the moon. The fact that the tides go in an out are a result of the rotation. That is, it takes two to tango. There would be no tides without the earth's rotation or the moon's orbit (set aside for a moment that the moon couldn't be there if it weren't in orbit), but neither would they be there without the gravitational pull.

brossa
12-15-2006, 10:29 PM
When the Moon was young, presumably it spun at a faster rate than it does now, and therefore it exposed (almost) all of its surface to the Earth. The gravitational pull of the Earth caused tides on the Moon, but because the Moon is a solid body the tidal 'bulge' was small. The bulge also tended to get pushed "ahead" of the Earth's position in the sky (from the Moon's perspective) because the Moon was turning and did not immediately 'snap back' once the Earth wasn't overhead. The Earth's pull on this tiny bulge of Moon tended to slow the Moon's rotation down, to the point that the Moon now presents only one side to the Earth (give or take).

The Moon is doing the same thing to the Earth - it pulls up a tidal bulge on Earth, which 'leads' the Moon a bit. The Moon's pull on this off-axis bulge tends to slow down the Earth's rotation over time. Meanwhile, the Earth bulge adds a bit of off-axis pull to the Moon and slowly but persistently boosts it into a higher orbit. The end result over time is that the Moon and Earth become tidally locked to each other, with the Moon in a much higher orbit, and the each presenting only one face to the other.

Overall, though, the angular momentum of the whole system is preserved, except for the energy that is radiated away as heat because of the frictional heating of all that flexing rock and water. An explanation with diagrams is here: http://www.astronomynotes.com/gravappl/s10.htm

Q.E.D.
12-15-2006, 11:03 PM
There would be no tides without the earth's rotation or the moon's orbit.
There would still be a tidal bulge; the effects of gravity wouldn't go away. But remember that gravity is NOT a form of energy, it is merely a transfer mechanism in the same way that magnetism is.

Bryan Ekers
12-15-2006, 11:08 PM
Tidal energy isn't gravitational or tidal at all; it's rotational.
Equatorial bulge is rotational. Tidal... isn't. I don't know what you're talking about.

Q.E.D.
12-15-2006, 11:14 PM
Equatorial bulge is rotational.
I'm not talking about the equatorial bulge, I'm talking about the tidal bulge--well, really, the TWO tidal bulges, on opposite sides of the planet.

Q.E.D.
12-16-2006, 11:49 AM
Equatorial bulge is rotational. Tidal... isn't. I don't know what you're talking about.
Oh, I see where I've managed to confuse you. Yes, the tidal bulge is caused by the difference in gravity on opposite sides of the Earth, but in order to extract any useful energy from it, there must be rotation. As I said, gravity isn't a form of energy, but you can use it to get energy from one place to another. The same is true of an electric generator; the rotating magnets induce an electric current in a coil of wire, but the energy doesn't come from the magnetism, it comes from the rotation.

Bryan Ekers
12-16-2006, 12:12 PM
I think you have it backward, actually. If Earth was to suddenly stop its own rotation, the moon in its orbit would still have a tendency to cause tides as its gravity causes the Earth's surface to deform slightly. Friction from this would start the Earth rotating again, albeit very slowly. Its not the rotation of Earth that causes tides, but the gravity of the moon in combination with the rotation of Earth, though I admit at this point we're getting pretty chicken-and-egg.

Q.E.D.
12-16-2006, 12:18 PM
You misunderstand. I'm not talking about the cause of tides, I'm talking about the source of so-called tidal energy. That source is ultimately rotation, not gravitation.

Bryan Ekers
12-16-2006, 12:30 PM
No, you misunderstand that without gravitation, the entire idea of tides is moot and the tides can exist even without rotation, given the revolution of an orbiting body.

I'm willing to concede, however, that this comes down to how certain terms are defined.

Q.E.D.
12-16-2006, 12:36 PM
No, you misunderstand that without gravitation, the entire idea of tides is moot and the tides can exist even without rotation, given the revolution of an orbiting body.
No, I understand that completely. To go back to my previous analogy, generators won't work without magnets in much the same way that tidal energy can't be extracted without the Moon's gravity. But, neither the Moon's gravity nor the magnetic field are sources of energy. That is the only point I am making here.

If your definitions allow you to call gravity a form of energy, then your definitions are wrong.

Bryan Ekers
12-16-2006, 12:46 PM
If your definitions allow you to call gravity a form of energy, then your definitions are wrong.
Actually, I was trying to be polite and avoid calling you completely out to lunch on this subject, but so be it.

The electromagnetic analogies are completely misplaced, incidentally.

The source of this energy is arguably whatever supernova exploded to create the nebula from which our solar system gradually condensed into its current form, including a rocky planet that got hit with something massive, creating a rocky moon and setting up an orbital system that has been fairly stable for some five billion years so far, graaaadually losing its energy to waste heat.

Q.E.D.
12-16-2006, 12:51 PM
The electromagnetic analogies are completely misplaced, incidentally.
No, they're not. Gravity and electromagnetism are both fundamental forces. A force is not energy, it's a medium for transferring energy from one place (or thing) to another. I'm not sure why you can't understand that, but perhaps I'm explaining it poorly.

Yes, you can trace the source of tidal (and indeed, all other forms of) energy back to the Big Bang, but that's not really germane to the subject.

Cheesesteak
12-16-2006, 01:02 PM
But, neither the Moon's gravity nor the magnetic field are sources of energy. That is the only point I am making here.

If your definitions allow you to call gravity a form of energy, then your definitions are wrong.You've never heard of gravitational potential energy? You can't seriously be saying that there is no energy stored up in the moon by virtue of it orbiting the earth.

Clearly the rotational energy stored in each body is affected by the tide, but if neither body were rotating, rotational energy would be zero, and the tides would still exist.

JustAnotherGeek
12-16-2006, 01:19 PM
You've never heard of gravitational potential energy? You can't seriously be saying that there is no energy stored up in the moon by virtue of it orbiting the earth.

Clearly the rotational energy stored in each body is affected by the tide, but if neither body were rotating, rotational energy would be zero, and the tides would still exist.

Gravitational energy is not yet energy. It is the potential for a force to act on an object to give it energy, or reduce energy. If you are motionless in the middle of a ladder, you have this gravitational potential energy reletive to the ground. There's no energy there until you jump off of the ladder and allow the force of gravity to pull you down. Conversely, if you decide to climb higher, you must use energy to overcome the work done to you by gravity.

(Kinetic) Energy = 1/2 m v^2
Work = force . distance

If you allow gravity (the force) to act on you over a distance it will do work on you, possibly increasing your speed (if you are going with gravity, possibly acting against your motion if you go opposite the tug of gravity).

Potential Gravitational Energy is a convenient way to avoid doing work calculations, but is not the "full" truth. It's not a "real" energy in the same way that the Coriolis force is not a "real" force. They are merely physics shortcuts.

Tides exist because Earth is spinning under (or around) two bodies: Luna and Sol. If Earth, Luna and Sol were in lock-step, with rotations and revolutions matched, there would be tidal bulges, but no tides.

For comparison, Pluto and Charon rotate in sync with each other. (If you stand on one side of Pluto, you will either a. always see Charon in the same place, or b. never see it.) Thus, there are no tides, merely tidal bulges.

Cheesesteak
12-16-2006, 01:29 PM
There's no energy there until you jump off of the ladder and allow the force of gravity to pull you down. So, when I jump off the ladder, energy is created?

Q.E.D.
12-16-2006, 01:34 PM
So, when I jump off the ladder, energy is created?
No, it's being released. You stored that energy in your body when you climbed up the ladder in the first place. The energy comes from your muscles, which get their energy from your food (which gets it from the Sun, which gets it....blah blah blah, ad nauseum), not from the Earth's gravity. Gravity is the force which accelerates your body towards the center of the Earth, turning potential energy into kinetic energy.

DanBlather
12-16-2006, 01:40 PM
Interesting thread. I'm not sure about rotation being the source of tidal energy. If neither the earth nor the moon were rotating, but the moon still orbited the earth, then there would still be tides. Indeed the only way there cold be no tides is if the earth were to rotate at the same rate that the moon orbited the earth. In that case you could say that rotation prevented tidal energy.

One could argue that the earth rotating once per-day increases the rate of tides from every two weeks to twice a day, and that effects the tidal energy. I'm not sure if that increases the total amount of energy or just changes its nature.

Stranger On A Train
12-16-2006, 02:55 PM
Interesting thread. I'm not sure about rotation being the source of tidal energy. If neither the earth nor the moon were rotating, but the moon still orbited the earth, then there would still be tides. Indeed the only way there cold be no tides is if the earth were to rotate at the same rate that the moon orbited the earth. In that case you could say that rotation prevented tidal energy.The energy that generates the ocean tides that flow into and out of coastal areas and harbors is due to the difference between the rotation of the Earth and the orbital period of the Moon (and to a lesser extent, the Sun). If the Earth were not rotating at all (tidally locked to the Sun), there would still be the orbital energy (although much less) between the Moon and the Earth. Given sufficient time, the Moon would lose the kinetic energy to hysteresis in the ocean and crust, slowing down (and via the perverse laws of orbital mechanics, speeding up), spiraling inward and eventually crashing into the Earth, though over a timespan that is much longer than the predicted lifetime of the Universe. If the Earth and Moon were tidally locked (always facing each other) there would be no tidal movement between the Earth and the Moon (though there would still be tides from gravity gradient and the velocity differential, just as the centrifugal force on someone on the outside of a merry-go-round is greater than someone on the inside). Every time a tide comes in, the Earth loses rotational momentum, and thus energy. (You can see this in the sound of the ocean crashing, the erosion of beaches and piers, and surfers who find the Big Wave.) Fortunately, the Earth has rotational momentum to spare.

As Q.E.D. and brossa have already correctly and repeatedly noted, gravity itself is not "energy"; it is a force. Forces are, of course, the result of directed energy at some level (in the case of magnetism, by moving electrons, or in a steam engine, by the Van der Waals forces which cause the molecules to repell one another when heated). Where does this energy come from? It comes from distortions in the plenum of spacetime itself due to the presence of mass (per General Relativity). However gravity is conserved--that is, without actually disposing of mass, the total gravitational energy of a system can't change; this is a direct result from applying Newton's Laws of Motion. So the energy can't come "from" gravity; gravitational attraction is merely the mechanism, the lever by which rotational energy of the Earth is extracted.

Stranger

Q.E.D.
12-16-2006, 07:25 PM
...I was trying to be polite and avoid calling you completely out to lunch on this subject...
You were saying...?

DanBlather
12-16-2006, 11:52 PM
The energy that generates the ocean tides that flow into and out of coastal areas and harbors is due to the difference between the rotation of the Earth and the orbital period of the Moon Good explanation

Bryan Ekers
12-17-2006, 10:49 AM
You were saying...?

I stand by my earlier statements, including an effort at politeness that was specifically not returned. The electromagnetic analogy is problematic because while opposite charges have a gravity-like attraction, the equally important like-charges repulsion has no analogous gravitational effect.

Besides, you seemed to think I was calling gravity energy. I said potential energy way back in my first post to this thread which you later supported with the ladder-climbing analogy.

Cute alternate analogy: stirring cake batter in a large bowl. The batter is viscous and as the mixing spoon makes rapid circuits, the bowl, if not secured, will start to rotate in the direction of the stirring. Eventually, the bowl and the spoon will be rotating at the same rate, achieving "tidal lock".

Q.E.D.
12-17-2006, 11:37 AM
I stand by my earlier statements, including an effort at politeness that was specifically not returned.
And yet you managed to call me "completely out to lunch" while at the same time claiming you were be polite and avoid just that. Sorry, but that's bullshit. Nowhere have I been impolite to you; if you perceived rudeness in my posts, that's your problem, not mine.
Besides, you seemed to think I was calling gravity energy.
You said tidal energy was "all gravity" way back in your first post, did you not? That statement is not correct, and is the statement I was responding to. In any case, I introduced the magnetism analogy as a way to illustrate the difference between a force and energy; in that respect, the analogy is an apt one. Naturally, if you try to carry the analogy too far, as you have done, it will break. All analogies will.

Stranger On A Train
12-17-2006, 11:50 AM
I stand by my earlier statements, including an effort at politeness that was specifically not returned. The electromagnetic analogy is problematic because while opposite charges have a gravity-like attraction, the equally important like-charges repulsion has no analogous gravitational effect.No, it's not really problematic; while it's true (as far as we have observed, although the principle of symmetry suggests otherwise) that there is no repulsive form of gravity; however whether the force is attractive or repulsive is not at issue. The energy stored in the field of a permanent magnet is conserved; that is to say, without breaking up the magnet there is no change in the respulsive or attractive forces delivered by the magnet. This is an exact analogy to mass and gravity.

The energy that causes ocean tides (not the equatorial bulge) and a variety of other visible tidal effects comes from the rotation of the Earth, due to which any individual plot of land sees a varying gravitational attraction to the Moon throughout the day. If the Earth and Moon were tidally locked, so that the Earth showed no rotation with respect to the Moon, there would be no tides; just an egg-shaped cross-section with the peaks nearest and opposite the Moon. (Also, real estate on the Luna-facing side would be way more pricey than that on the non-facing side.)

Whether the energy comes from the Earth's (or Moon's) gravity or whether is extracted from the rotation of the Earth via a change in rotational momentum isn't a semantic argument; it's a fundamental one. Gravity is conservative; in order to extract gravitational energy out of a planet, you'd have to break up and distribute the mass. Since that isn't happening with Lunar tides, the stored energy comes from somewhere else; specifically the rotation of the planet with respect to the Moon.

Stranger

ZenBeam
12-17-2006, 04:41 PM
Just wanted to pop in and point out that since the Moon is receeding from the Earth, the gravitational potential energy of the Earth-Moon system is increasing, not decreasing, so it can't possibly be supplying the energy that drives the tides.

Bryan Ekers
12-17-2006, 05:28 PM
And yet you managed to call me "completely out to lunch" while at the same time claiming you were be polite and avoid just that.

Well, that you missed my earlier attempt to disengage with no loss of face on either side ("I'm willing to concede, however, that this comes down to how certain terms are defined") surprises me not at all.

You said tidal energy was "all gravity" way back in your first post, did you not? That statement is not correct, and is the statement I was responding to.Then we're even because it was the off-the-cuff remark in your first post (misleading or, to be charitable, grossly incomplete) that got this started.

Ta.

Q.E.D.
12-17-2006, 05:43 PM
Well, that you missed my earlier attempt to disengage with no loss of face on either side ("I'm willing to concede, however, that this comes down to how certain terms are defined") surprises me not at all.
Well, not to carry this too far afield, but I had to address that earlier because it's not a correct statement, either. You can't pick and choose your definitions in science; if everyone is going to work together, we all have to agree on what things mean and how we define the terms we use to describe things. I'm sorry you don't like my definitions, nevertheless, most scientists use these terms just as I have here and would agree with me that the source of energy was kinetic rotational energy, not gravity.

I don't know what else to tell you, but I'm sorry if somehow I've offended you. You seem to be focused on me, even though others are saying the same thing I did. Fine, I give up. It's really not worth getting all worked up about.

Stranger On A Train
12-17-2006, 06:43 PM
Just wanted to pop in and point out that since the Moon is receeding from the Earth, the gravitational potential energy of the Earth-Moon system is increasing, not decreasing, so it can't possibly be supplying the energy that drives the tides.Actually, the energy for this comes from the Earth's rotation, too. See this thread for more explanation and links.

Stranger

David Simmons
12-17-2006, 09:16 PM
Actually, the energy for this comes from the Earth's rotation, too. See this thread for more explanation and links.

StrangerYes, the extra potential energy for the moon comes from the decreasing rotational energy of the earth.

I always thought the tidal energy came from the moon's gravitation. When the tidal bulge resulting from the moon's gravity passes my location I trap some of the high water in a basin. Then when low water comes I let it out through a turbine and recover the energy supplied by the moon.