# The Naked Scientists Forum

### Author Topic: Tidal energy - where it come from?  (Read 13107 times)

#### jccc

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##### Re: Tidal energy - where it come from?
« Reply #25 on: 20/01/2015 06:29:35 »
Every time a see the word quantum, I want to run, like now.

#### PmbPhy

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##### Re: Tidal energy - where it come from?
« Reply #26 on: 20/01/2015 06:55:52 »
Quote from: jccc
Every time a see the word quantum, I want to run, like now.
That's only because you have chosen not to study it and learn it. Had you chosen learn it the proper way then things would be much different. But you're not willing to do the work. It takes a lot of work, learning a lot of math, long hours of study and years of learning.

#### CliffordK

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##### Re: Tidal energy - where it come from?
« Reply #27 on: 20/01/2015 08:27:36 »
Hmmm...
So the tides are pushing the moon (7.3 × 1022 kg) away by about 3.8cm a year.
That seems like quite a bit to me.

Say the area of the oceans are about  (510,066,000 sq km)
What is the average height of the tides, maybe 2 meters, although perhaps one should think of half of it being raised, and half lowered.  Are polar tides less?

So, for moving: 5.1  ×  108 km2 x (1m depth) x (1000m/km)2 x 1000 dm3/m3 = 5.1 × 1017 kg of water moved by the tides twice a day.

So, 5.1 × 1017 kg * 365 * 2  = 3.7 × 1020 kg of water raised by about 1m per year.

So...
The moon weighs about 100x the annual tides.
And the orbit is pushed out by about 1/100 the depth of the tides.

There are probably many different ways to do the calculation.  Perhaps it would be better to calculate velocity and momentum changes of the moon equivalent to the 3.8 cm change in the orbit.  But, my quick estimate for the amount of water in the tides came quite close to the orbital change.

Did you do any calculations to indicate it isn't possible, or did you just conclude that there is a lot of water sloshing around in the oceans?

#### Ophiolite

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##### Re: Tidal energy - where it come from?
« Reply #28 on: 20/01/2015 08:42:18 »
Why can't we build a power plant use gravity like the sea does if gravity causes tide?
We can. http://en.wikipedia.org/wiki/Rance_Tidal_Power_Station

#### syhprum

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##### Re: Tidal energy - where it come from?
« Reply #29 on: 20/01/2015 09:14:13 »
You cannot do anything in the UK that impacts on wild life we would rather get our electricity from coal burning plants than have some sand eels or wading birds inconvenienced.
« Last Edit: 20/01/2015 09:20:32 by syhprum »

#### Ophiolite

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##### Re: Tidal energy - where it come from?
« Reply #30 on: 20/01/2015 10:32:43 »
You cannot do anything in the UK that impacts on wild life we would rather get our electricity from coal burning plants than have some sand eels or wading birds inconvenienced.
So you decry the notion that biodiversity is important. Is this related to your apparent attraction for false dichotomies?

#### jccc

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##### Re: Tidal energy - where it come from?
« Reply #31 on: 20/01/2015 15:51:00 »
Hmmm...
So the tides are pushing the moon (7.3 × 1022 kg) away by about 3.8cm a year.
That seems like quite a bit to me.

Say the area of the oceans are about  (510,066,000 sq km)
What is the average height of the tides, maybe 2 meters, although perhaps one should think of half of it being raised, and half lowered.  Are polar tides less?

So, for moving: 5.1  ×  108 km2 x (1m depth) x (1000m/km)2 x 1000 dm3/m3 = 5.1 × 1017 kg of water moved by the tides twice a day.

So, 5.1 × 1017 kg * 365 * 2  = 3.7 × 1020 kg of water raised by about 1m per year.

So...
The moon weighs about 100x the annual tides.
And the orbit is pushed out by about 1/100 the depth of the tides.

There are probably many different ways to do the calculation.  Perhaps it would be better to calculate velocity and momentum changes of the moon equivalent to the 3.8 cm change in the orbit.  But, my quick estimate for the amount of water in the tides came quite close to the orbital change.

Did you do any calculations to indicate it isn't possible, or did you just conclude that there is a lot of water sloshing around in the oceans?

F=ma, V=at. The moon pushed away by tidal force for so long, it should be away way faster by now.

#### PmbPhy

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##### Re: Tidal energy - where it come from?
« Reply #32 on: 20/01/2015 18:25:51 »
Quote from: jccc
F=ma, V=at. The moon pushed away by tidal force for so long, it should be away way faster by now.
F = dp/dt = d(mv)/dt = mdv/dt = ma is correct but you didn't mention what V is supposed to stand for. Assuming its velocity then what is in the velocity of? It's show below that V = at.

Note: The relationship between V and a is a = dV/dt or dV = adt. Let us integrate both sides dV = adt to obtain V = at < c.

#### jccc

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##### Re: Tidal energy - where it come from?
« Reply #33 on: 20/01/2015 18:37:19 »
S=vt, t=5 billion years, how far away should be the moon now? If tidal force came from gravity.

#### PmbPhy

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##### Re: Tidal energy - where it come from?
« Reply #34 on: 20/01/2015 18:45:21 »
Quote from: jccc
S=vt, t=5 billion years, how far away should be the moon now? If tidal force came from gravity.
Your equations are all wrong. That expression only works if the moon isn't orbiting earth and when the earth and moon are modeled as a point particles.

#### syhprum

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##### Re: Tidal energy - where it come from?
« Reply #35 on: 20/01/2015 19:02:27 »
In the UK there is an ideal site for a tidal hydro system the estuary of the river Severn and its development has been discussed for at least a hundred years during that time there has been innumerable deaths and loss of habitat due to coal burning that would have been reduced somewhat if this site had been utilised

#### CliffordK

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##### Re: Tidal energy - where it come from?
« Reply #36 on: 20/01/2015 19:21:36 »
If the moon is receding at about 3.8 cm / year for 4 billion years.
At a constant rate of change, that equals (.038 m * 4,000,000,000 y/1000 m/km) =152,000 km

The moon is currently orbiting at about 384,000 km
Geosynchronous orbit is about 36,000 km, and the moon would have to have formed above that to be in a receding orbit.

The actual calculations would be much more complicated taking into account the difference of gravity over time, the orbital/rotational speed differential between the Earth and moon, velocity, momentum, and perhaps changing depth of the oceans, and even the continental drift and glaciation of Antarctica.

However, a quick ballpark estimate puts the moon about where it is.

#### jccc

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##### Re: Tidal energy - where it come from?
« Reply #37 on: 20/01/2015 20:51:37 »
The a is always there, the v is increasing with t and not constant. Your ballpark is huge.
« Last Edit: 21/01/2015 05:33:15 by jccc »

#### Ophiolite

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##### Re: Tidal energy - where it come from?
« Reply #38 on: 21/01/2015 04:47:44 »
Ignorance is nothing to be ashamed of, unless it is a lifestyle choice.

#### CliffordK

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##### Re: Tidal energy - where it come from?
« Reply #39 on: 21/01/2015 07:25:37 »
The a is always there, the v is increasing with t and not constant. Your ballpark is huge.
The orbital velocity decreases with time.

Leave it to the physicists to write a program calculating where the moon was every day of its existence.

Suffice it to say that a few cm a year of receding from Earth isn't enough to fling the moon off towards Jupiter, at least not yet.

#### jccc

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##### Re: Tidal energy - where it come from?
« Reply #40 on: 21/01/2015 08:06:45 »
The a is always there, the v is increasing with t and not constant. Your ballpark is huge.
The orbital velocity decreases with time.

Leave it to the physicists to write a program calculating where the moon was every day of its existence.

Suffice it to say that a few cm a year of receding from Earth isn't enough to fling the moon off towards Jupiter, at least not yet.

Let our immortal minds wait and see, story is always getting better. Nature has wonder beyond minds.

#### chiralSPO

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##### Re: Tidal energy - where it come from?
« Reply #41 on: 21/01/2015 13:38:43 »
The a is always there, the v is increasing with t and not constant. Your ballpark is huge.
The orbital velocity decreases with time.

Leave it to the physicists to write a program calculating where the moon was every day of its existence.

Suffice it to say that a few cm a year of receding from Earth isn't enough to fling the moon off towards Jupiter, at least not yet.

No, the Moon's orbital velocity is increasing, not decreasing. The orbital period is increasing. It's somewhat of an oversimplification, but I would say the energy comes mostly from Earth's rotation. We spin about once every 24 hours, while the Moon orbits about once every 27 days--that the tides move are a result of this difference--through the action of the tides (one can think of them as friction) the Earth's rotation is slowing down and the Moon's orbit (velocity) is speeding up.

#### RD

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##### Re: Tidal energy - where it come from?
« Reply #42 on: 22/01/2015 02:10:59 »
Most of the kinetic energy in the Earth-Moon system is from Earth's rotation, which has measurably slowed ... http://www.lawrencehallofscience.org/pass/passv07/v7SlowingEarthRotation.html

This kinetic energy is the source of the energy in tides.

Theoretically Earth's rotation will slow until there is a double tidal lock : no more tides, or tidal power, on Earth.

However by that stage the point is moot, as the expanding sun will have boiled Earth dry.

« Last Edit: 22/01/2015 02:24:21 by RD »

#### yor_on

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##### Re: Tidal energy - where it come from?
« Reply #43 on: 23/01/2015 21:19:35 »
Jcc, as I read you you're asking 'what do you mean by saying 'energy'?
And that is a very good question to my mind. You can see it (probably:) several ways. One is the one in where we have a commonly agreed on universe, that we exist in. That one controlled by 'conservation laws', in where 'gravity' is one coin of exchange. The other one is a local definition, well, at least to me.

both definitions though, agree on that when we find something to be true locally experimented (tested) over time and space, we call it a constant.

In the first case what we call a universe is 'energy'. In the second case it's more problematic.

#### yor_on

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##### Re: Tidal energy - where it come from?
« Reply #44 on: 23/01/2015 21:25:27 »
or simpler :)

Is a vacuum 'energy', how will you prove it?

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##### Re: Tidal energy - where it come from?
« Reply #44 on: 23/01/2015 21:25:27 »