View Full Version : Energy

4th Feb 2009, 04:08
My friends and I have this ongoing discussion which I hope some of you more educated folk may be able to put to bed. Consider a head of water (potential energy) and a hydro dam. Once the water flows through the dam it generates power and therfore the water must lose some of its energy. If the head of water is high enough then a number of dams could be built on the river and each dam removing energy from the water as it makes its way to the sea.
Question? If there are no dams on the river then what happens to the potential energy as it makes its way to the sea?

I have heard arguments that the energy is lost through heat, noise, and turbulance as it passes over the river bed but I am not convinced that this is any answer. Any educated answer how to convince these people that the energy is lost due to the lack of head as it reaches the sea.http://static.pprune.org/images/smilies/confused.gif

4th Feb 2009, 04:15
I remember reading somewhere that potential energy i.e. that gained from gravity works differently to other types of energy. It only has that potential energy because it is high up so as the water reduces in height the potential energy no longer has an effect.

Either that or the potential energy (gravity?) will be used in moving the water from A to B?

I, like yourself, also await an intelligent response :}

Loose rivets
4th Feb 2009, 04:56
Where does the potential energy go? Without constriction, it becomes kinetic. Just try stopping it once it's on its way.

Supposing you were asked what was the first 'force' that caused that flow down a river, what would you answer? 1,000,000 quid for the Want to be a millionaire last question, so be careful.



Atmospheric heat


Remember...the first 'force'. I put force in quotes so as to encompass any answer.

Howard Hughes
4th Feb 2009, 05:01
Potential energy doesn't have to be realised!:eek:

If I have a cup of water and hold it above my head, it has 'potential' energy, then place it on the floor does it have the same potential energy? If no where has it gone?;)

4th Feb 2009, 05:14
Hasn't the energy been expended by lifting the body of water to height in the first place (atmospheric heat/weather)? By returning to sea level the water is merely equalising the equation.

Some energy (quite a lot) will also be used in the process of erosion. Ever wondered why you sweat when you try to sandpaper a brick?

Desert Diner
4th Feb 2009, 05:16
To loose rivets: Gravity! When can I pick up my cash!

HH: The weight of the water is its potential energy. The energy you exert with your hand to hold it up is the equivalent amount that counter acts it. That is why you can hold a cup of water or a can of beer without to much effort while a large bucket of beer may cause your hand to tremble.

When you put it on the ground, the ground or floor counteracts that energy. If you put said bucket on a flimsy floor, it will fall through as the floor was not strong enough (week mechanical support) to support it.

foxy: What do you see at the bottom of a waterfall? That splashing and churning is the disipitation of that energy.

4th Feb 2009, 05:32
Wait, so in post #2 was I almost right??

Desert Diner
4th Feb 2009, 06:06
Gravity is merely the attraction between a small body (water drop) and a greater more dense particle (in this case the center of the earth) and is expressed as acceleration.

http://en.wikipedia.org/wiki/Gravity (Remember: Wikipedia is yoru friend) :)

unstable load
4th Feb 2009, 08:55
Loose rivets,

I would have said "Atmospheric heat" because unless the heat is between set limits it will not be water in the first place.:confused:

Yes? No? Maybe??

4th Feb 2009, 10:35
Head asplode

Loose rivets
4th Feb 2009, 10:35
unstable :Yes? No? Maybe??

Maybe...maybe not. Wait a little longer, but now I'm going to redefine the question a little.

Okay, so we now remove gravity in its function of causing the water to run down the hill because that's obvious, and hardly the first thing to happen. But the list remains unaltered. The very first function if you like, that takes place to cause the downhill flow of water.



Atmospheric heat


tony draper
4th Feb 2009, 10:44
Surely it is solar energy? the heat of the sun evaporates the water from the sea lifts it up against gravity later it falls as rain and makes it's way back to the level it started from thus releasing the energy on the way.

Scumbag O'Riley
4th Feb 2009, 11:12
Aye, the 'first' force has got to be something going on in the sun when things fuse together. Got a book somewhere that explains it all but other things to do, one guesses the nuclear force which is sommat to do with the strong force!

4th Feb 2009, 11:36
It is atmospheric heat that raises the water, as vapour, many thousands of feet into the atmosphere where it forms clouds that then deposit the water onto the high ground as rain.

The water has mass. Gravity endows that mass with weight and thus potential energy. The river flows down the river bed to the sea converting the potential energy into kinetic energy. When the river encounters a dam, the dam converts the kinetic energy back into potential energy. Releasing the water through a turbine allows some of the resulting kinetic energy of the water to be extracted and converted into electrical energy. It exits the turbine with less energy than it entered and so flows more slowly than it would have without the presence of the dam and the turbine. (It isn't possible to extract ALL the kinetic energy as that would involve stopping the water flow and thus the turbine) When the river reaches the sea its kinetic energy pushes on the sea water and forces it aside. The energy is then dissipated in several forms - waves, tidal flows, currents and heat.

The Amazon pumps fresh water into the Atlantic Ocean for many miles surrounding the mouth of the river. The Atlantic Ocean, not being at the centre of the earth, also possess potential energy. Woe betide us if anyone accidentally pulls the plug out... :uhoh:

The bigger question for "So you want to be a multi-millionaire?" is where did all that water come from in the first place?

4th Feb 2009, 11:38
Bit like a Yo-Yo.
Potential gravitational energy converted into rotational
kinetic energy.

4th Feb 2009, 12:52
It's a fluvial transport system: it erodes the course and transports the load to the ocean. The Rouse number is a function of the sediment settling velocity and the particulate uplift velocity. A low Rouse number indicates that most of the fluvial load is suspended. A high Rouse number means that most of the load is dragged along the bed.

In systems with many dams, the Rouse number is high, and one of the things this indicates is that the dam is going to fill up with sediment.

So you can have hydroelectric schemes at the expense of filling the reservoir at somewhere between insignificant and ludicrous speed depending on the overall energy balance of the system.

tony draper
4th Feb 2009, 13:05
Think the latest theory is that twere brung here by comets Mr B,don't see it meself,if water existed on comets in the olden days why not on Earth as well.

4th Feb 2009, 15:19
I would have said "Atmospheric heat" because unless the heat is between set limits it will not be water in the first place

It is atmospheric heat that raises the water, as vapour, many thousands of feet into the atmosphere where it forms clouds that then deposit the water onto the high ground as rain.

But the question is:

Supposing you were asked what was the first 'force' that caused that flow down a river (not up it), what would you answer?

Looks gravity gets the million. A million isn't really enough these days anyway. :hmm:

unstable load
4th Feb 2009, 15:26
Then I will put my flag on MAGNETISM, 'cos of the sub-molecular forces keeping the molecules together as the flow starts off.

Hydrobolics comes into effect after that due to the effects of already moving water.
It should be moving from the peat bog to the still to become a part of Uisge Beach.

tony draper
4th Feb 2009, 15:34
Here's a question, How long does water last?, ie I mean if one filled a glass of water from the tap, covered it so it was not subject to evaporation put it on a table in a room at say 60 degrees F of course and came back in a thousand years would the glass still be full of the same water? or indeed if you came back a million years later?

Loose rivets
4th Feb 2009, 16:07
Oooh, hard one that. Since glass is a super-cooled liquid, I'm not sure what would happen in the long term. They do say that some of the fine old glasses have changes shape in a few hundred years, so a million might see a puddle...with a puddle inside - if it hadn't leached through and evaporated.

scumbag says: Aye, the 'first' force has got to be something going on in the sun when things fuse together.

scrubbed gets there but doesn't quite say why.

The answer that gets the million (write to Chris Tarrent for the cheque) is gravity. Causes the sun to light up and generate the heat that pumps the water uphill so to speak.

Not a very good question, cos it gives the game away talking about disqualifying the downhill effects of gravity.

tony draper
4th Feb 2009, 16:19
Ah!:= but did not gravity case a cloud of interstella dust and hydrogen to collapse in on itself until such time as it reached sufficient density and temperature to commence the fusion process in order to produce this heat?

4th Feb 2009, 20:38
Errrrrrr Hang on, we are getting a bit off thread here.

I will re-phrase the question... Let us suppose that we have 1000ft head of water. We require a 100ft of head to drive the turbines in a dam to produce 1000 Mega watts of energy. one dam = 1000 Mega watts. if we have 10 dams on the river we can product 10,000 Mega watts in total.

Now if we go back to having only one dam what happens to the other 9,000 Mega watts...what has happened to this potential energy because it must dissapate somewhere as it makes its way to the sea.

4th Feb 2009, 22:12
In the first instance it becomes kinetic energy, that is to say that the water falling x distance will be moving faster if there is no dam in the way. As it decelerates, this energy becomes heat both sensible and latent. the temperature rise being limited by evaporative loss.

After an excellent landing you can use the airplane again.

4th Feb 2009, 22:33
Forget the physics lectures. All you need to consider is that the 'energy' your talking about is actually about the conversion of energy to the supply of electrickery, and that can't be stored efficiently. This is about 'easily turn off-an-onable' energy based on uncertain demand. One big dam at the top makes that more practical and efficient than lots of little ones downstream.

On the physics front a litre of water sitting at the bottom of the ocean has pretty much the same potential energy content as one held aloft on the summit of Everest. The amount of potential energy it has is dependent on the physical system you consider it's in. In this case the gravity of the Earth is the predominant parameter. It's more practical and efficient to build dams up big hills than tunnel caverns under the bottom of the ocean. And given you can't store the product anyway, one big one up a big hill is cheaper and more effective than lots of little ones.

5th Feb 2009, 01:51
The other 9000MW is available to erode the river bed and transport the material.

The flow rate in your river is just over 4000 cubic meters per second, if we assume extremely efficient generators. That's 50 times the Colorado, so you'd be able to see the other 9MW because the river would be very, very brown.

5th Feb 2009, 10:11
'spose technically its not lost, (some spark in the brain from a past physics lesson suggests it can't be created or destroyed,) it's just dissipated in other forms.

Always wondered where all the energy went in a head on collision-guess its a simillar thing-turned into heat, sound etc??:confused:

5th Feb 2009, 10:44
Always wondered where all the energy went in a head on collision-guess its a simillar thing-turned into heat, sound etc

I'd say a fair bit is expended in converting the vehicles into twisted wreckage! Also, apparently it takes a lot of energy to send the (ah) souls off to heaven to be with Elvis and Col. Sanders. The faster you crash, the faster you get to heaven, which is uphill. Crash slow enough and you fall short and land in hell. Which is probably a much more fun place to spend eternity, anyway.

scrubbed gets there but doesn't quite say why.

Because you said, "The very first function if you like, that takes place to cause the downhill flow of water."

Gravity. Steam or anything else drives it uphill. I like to think outside the box.

5th Feb 2009, 23:38
It might help to point out that potential energy is relative. You raise an object up, you give it potential energy relative to its previous position.

The explanations given at the bottom of the original post were pretty much on target. Water has internal friction at slow flow rates, but it has a lot more internal friction at higher flow rates due to turbulence - it's non-linear. There are whole shelves of books on this kind of thing, filed under Fluid Mechanics; students are expected to explain, in detail, what happens to the energy, both in pipes (internal flow) and in open channels such as rivers (external flow).

I took an introductory Fluid Mechanics course late last year, and I never want to hear about boundary layer separation (http://en.wikipedia.org/wiki/Boundary_layer_separation) ever again - but that's an example of how the energy gets released from the water.