Why isn't a hydroelectric dam considered perpetual motion?

In summary, the conversation discusses the concept of harnessing hydro energy, which is a manifestation of solar energy. This energy is obtained by using the potential energy of water, which is then replenished by the sun through evaporation and rain. The conversation also touches on the idea that solar energy is not truly renewable in the long term, as it relies on the sun's nuclear energy which will eventually deplete.
  • #1
jaydnul
558
15
I am familiar with the second law of thermo, so i realize that there is an answer. My reasoning is that the water is flowing because of gravity, which turns the dam turbines. So where is the energy being removed from the flowing water? I mean its not changing the Earth's gravity right?
 
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  • #2
You use the potential energy of water - and you need the sun to get water back to the top (via evaporation and rain).

I mean its not changing the Earth's gravity right?
You lower the total energy content of the gravitational field of earth+water.
 
  • #3
So its kind of a manifestation of "solar" energy?
 
  • #4
Harnessing hydro energy is solar energy. So is wind energy, and wave energy (and coal energy if you wish to stretch the time line for "renewable" a bit).
 
  • #5
NascentOxygen said:
Harnessing hydro energy is solar energy. So is wind energy, and wave energy (and coal energy if you wish to stretch the time line for "renewable" a bit).
Yes so it's solar, but what is the manifestation of solar energy the big bang?
You can't just stop at it's solar that does it!
Stretch the time line and it's becomes rather hard to come to a reasonable answer.
We don't know.
 
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  • #6
The sun uses nuclear energy - and this is not renewable, it will end (for our sun) in about 5 billion years. And if you wait long enough, most hydrogen in our universe will be fused, and all stars died. In that way, solar energy is not really "renewable" - but a billions of years are way beyond our timescales.
 

Related to Why isn't a hydroelectric dam considered perpetual motion?

1. Why can't a hydroelectric dam generate infinite energy?

Perpetual motion refers to a hypothetical machine that can continue to operate indefinitely without any external source of energy. While a hydroelectric dam produces energy from the natural flow of water, it still requires an external force to keep the water flowing, such as rain or melting snow. Eventually, these sources of water will run out, making it impossible for the dam to generate infinite energy.

2. Can't a hydroelectric dam produce energy as long as there is water flowing?

While it is true that a hydroelectric dam can produce energy as long as there is water flowing, it is not perpetual motion. The dam relies on the natural cycle of water to continue producing energy, and this cycle is not infinite. Eventually, the water sources will run dry, and the dam will no longer be able to generate energy.

3. Why is a hydroelectric dam considered a renewable energy source if it's not perpetual motion?

A renewable energy source is one that can be replenished, either naturally or through human efforts. A hydroelectric dam is considered renewable because the water used to generate electricity can be replenished through natural processes such as rain or snowfall. However, this does not mean that the dam can produce energy indefinitely without any external sources.

4. Can't a hydroelectric dam be designed to operate perpetually?

No, a hydroelectric dam cannot be designed to operate perpetually. Even if the dam is built in a location with a constant and reliable water source, it will still require maintenance and repairs. Furthermore, the materials used to build the dam will eventually degrade, making it impossible for the dam to function indefinitely.

5. What about the Earth's water cycle? Doesn't that make a hydroelectric dam a perpetual motion machine?

The Earth's water cycle does not make a hydroelectric dam a perpetual motion machine. The water cycle is a continuous process, but it still relies on external forces such as the sun's energy to drive it. A hydroelectric dam only harnesses a small part of this cycle, and it cannot continue to operate without these external forces.

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