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Q & A: sources of tidal energy

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Most recent answer: 07/29/2013
Q:
When we use tidal forces to generate energy, that energy has to come from somewhere. Doing this, does it mean that the Earth slowly escapes the sun's attraction since we use the sun's gravity as an energy source?
- Anonymous
A:
That's an interesting question. The tides do have effects on orbits, but not quite what you'd guess. For starters, the tides on earth are mostly from the moon, not the sun. Your idea about tidal friction draining energy from other forms is completely correct, however. 
 
So let's start with the effects of the moon tides. The facts are that the moon is moving away from the earth at about 3.8 cm per year and that the earth's days are getting longer at about 2 milliseconds per century. The earth's orbit around the sun changes by only a negligible amount.

 These lunar tides mainly can drain energy from two sources:
1. the rotational energies of the earth and (to a much smaller extent ) the moon
2. the orbital energy of the moon.

 
One effect is to slow the earth's rotation, gradually making days longer. That's what's happening, and that's where energy is actually being drained from.

The other effect is less obvious. Draining energy from the moon's orbit would actually cause the moon to speed up while pulling it in closer to the earth. The reason is that in a gravitational orbit like that, the change in potential energy is twice as big and opposite in sign to the change in kinetic energy. So speeding up and moving in closer is the way to lose net energy.

Adding energy to the moon's orbit actually slows its orbital speed a bit while increasing its distance to the earth and adding gravitational potential energy. Since the moon is actually moving farther away and slowing down, its gaining orbital energy. How can that be? Although the tides cause a net energy drain to heat, they're also transferring some of the energy drained from the earth's rotation to the moon's orbit. It turns out that this must happen in order for the angular momentum lost as the earth's spin slows to go somewhere. Angular momentum goes up as the distance grows.

These two effects will continue until the moon-tides stop, when the moon orbits the earth in one day.  The earth will have slowed its rotation down to the point where the same side always faces the moon.

You can see an example of something just like that. The moon rotates just fast enough to always show the same face to the earth. Tidal friction caused that.

The end result will be that both the earth's and moon's rotational speed (length of day) as well as the lunar month will be equal, about 47 of our current days.  This will happen far, far in the future, several billions of years from now. 

Sun tides would produce similar effects, but not as large. They also tend to make the days longer.

There are some nice  articles on this:
   and
   .

Mike W.
LeeH

(published on 07/04/2012)

Follow-Up #1: Tidal effects on the Moon

Q:
What is the tidal effects of the moon? And more importantly, does the moon have any effect on the earth's rotation speed? Thank you!
- Allee Robinson (age 18)
Paris France
A:

That is a great question. First, let's define what the tides are and how they are caused. A tide is the rise and fall of the water levels in the oceans caused by the combined gravity of the Moon and Sun on the oceans. Because the force of gravity of the Moon is stronger on the side of the Earth it faces and weaker on the side opposite the Moon, water forms two bulges, one closest to the Moon and one on the opposite side of Earth where water piles up. The same process applies to the tides the Sun causes but they are much smaller. The force of the Moon's gravity causes the same type of rise and fall on the Earth's crust, but because the crust is not a liquid, it does not rise and fall as much. Now, the two bulges of water (the side closest to the Moon and the side furthest opposite to the Moon) are unable to be right under or opposite the Moon because the Earth rotates faster than the Moon orbits the Earth. The Moon thus pulls back on those bulges, slightly slowing the Earth's rotation and thus lengthening the days. Because the total angular momentum of the Earth-Moon system must be conserved, the Moon must move out to a higher orbit, slowing it down  and lengthening a month.

So, to summarize the answer, the Moon, through the tides, slows down the rate of Earth's rotation lengthening the day while the Moon slowly moves away from the Earth at about 3.8 cm a year. The Earth will never lose the Moon, it will just be farther away. In fact, about 620 million years ago, the length of a day was about 21.9 hours long and the length of a year was about 400 days (21.9 hour days) long while the Moon orbited closer to the Earth.

You can find more about this by searching our site for "moon tides".

Erik


(published on 01/27/2013)

Follow-Up #2: where does tidal power come from?

Q:
We can use the power of the tide to create electrical energy. Where does this energy originate? The moon circles us and the gravitational force of the moon pulls our sea towards it during its cycle, so the sea looses gravitational potential energy as it is pulled (& 'falls') towards the moon, however when the moon pulls away from the sea, the sea is moved out of its gravitational field, increasing its gpe. But yet this doesn't attract the moon? Is this offset by the land on the other side of the planet which is now 'falling' towards the moon? If so is there no net Change of energy? Could we harvest tidal power indefinitely?
- Chris I (age 21)
Blackpool, UK
A:

It turns out that one of our old answers addresses this, so this is now a follow-up. The source of the tidal energy (The Earth's spin) is very gradually wearing down, regardless of whether we harvest some of that energy. Nonetheless, it will remain available for longer than any likely continuation of our civilization.

Mike W.


(published on 07/29/2013)

Follow-up on this answer.