perpetual motion [quick question]

[Tom Mattson]

Back to the OP...

 

or is 'perpetual motion machine' more than just something that perpetually moves untill it breaks (i'm taking 'because it'll one day be destroyed' as a lame answre btw)

 

Yes, it something more than that. A perpetual motion machine of the first kind produces more energy than it consumes, which violates the first law of thermodynamics. A perpetual motion machine of the second kind spontaneously converts thermal energy into work, which violates the second law. The Earth does neither of these.

[Dak]

I don't think that hurts...

 

damnit, i allways get those two confused

 

You'd have to get down to the atomic level and below to even get close. There is no "friction" in individual nuclear reactions — that's as close as you can come to having an elastic collision. Once you add in other forms of energy (e.g. photons), it's no longer a reversible system.

 

hmm... other forms of energy as in other ways that the travelling thing can shed energy?

 

Yes, it something more than that. A perpetual motion machine of the first kind produces more energy than it consumes, which violates the first law of thermodynamics. A perpetual motion machine of the second kind spontaneously converts thermal energy into work, which violates the second law. The Earth does neither of these.

 

so... it'd have to either be 100% efficient (not possible), or, to compensate for it's ineficiency and sustain perpetual motion, it'd have to break one of the first two laws of thermodynamics? which also isn't possible?

[swansont]

hmm... other forms of energy as in other ways that the travelling thing can shed energy?

 

Right. Basically, once you have somewhere else for the energy to go, it's not reversible (more states = more entropy).

[Dak]

so... lets see if i've got this right...

 

1/ traveling things have energy

2/ if they loose this energy, they will slow down

3/ to avoid loosing their energy, a traveling thing would have to break the 2nd law of thermodynamics, i.e., the energy would have to avoid being spread out (?)

4/ the only way around the above involves breaking the 1st law of thermodynamics

5/ hence, all traveling things will shed energy and thus slow down.

[swansont]

so... lets see if i've got this right...

 

1/ traveling things have energy

2/ if they loose this energy, they will slow down

3/ to avoid loosing their energy, a traveling thing would have to break the 2nd law of thermodynamics, i.e., the energy would have to avoid being spread out (?)

4/ the only way around the above involves breaking the 1st law of thermodynamics

5/ hence, all traveling things will shed energy and thus slow down.

 

Other than spelling entropy (lose vs loose), correct at a basic level.

[Dak]

cool, basic understanding of something to do with physics is cool for me

 

thanks

 

(grumble grumble, stoopid 'lose', never get that one right...)

[Tom Mattson]

so... it'd have to either be 100% efficient (not possible),

 

It's more specific than that. It would have to convert thermal energy into work with 100% efficiency. In that case you would have a violation of the 2nd law.

 

The other type of perpetual motion machine (PPM) would violate the first law. So if, say, the Earth were speeding up with no expenditure of energy, that would be such a PPM.

[thedarkshade]

It's more specific than that. It would have to convert thermal energy into work with 100% efficiency. In that case you would have a violation of the 2nd law.

Correct. You measure the efficiency by: [math]E=1 - \frac{T_C}{T_H}[/math]! You'll need a big temperature change in order to get 100% efficiency, which I have not heard of so far.

 

 

The other type of perpetual motion machine (PPM) would violate the first law. So if, say, the Earth were speeding up with no expenditure of energy, that would be such a PPM.

Yup! Conservation of energy!;-)

[Dak]

OK, couple of questions before i carry on googling:

 

1/ would googling 'carnot engines' and looking into what hot/cold reservoirs are help me understand this:

 

It's more specific than that. It would have to convert thermal energy into work with 100% efficiency. In that case you would have a violation of the 2nd law.

 

2/ by any chance does the 2nd law demand that Th and Tc approach one-another over time?

[thedarkshade]

2/ by any chance does the 2nd law demand that Th and Tc approach one-another over time?

The 2nd law says that you cannot extract energy without a temperature difference. If there is no temperature difference then you get zero efficiency.

 

example:

[math]E= 1- \frac{90C}{90C} =1 - 1 = 0 \times 100= 0[/math]

 

The 2nd law does not demand Th and Tc approach one another, it does not demand anything just explains. It tells that the higher the difference in temperature, the higher the energy extraction. If Th approaches Tc then you get less and less efficiency.

[s pepperchin]

The Earth itself is not a perpetual motion machine for one simple reason, it is always being bombarded with energy from the sun. A perpetual motion machine has zero change in its total energy. This is not true for the Earth.

[thedarkshade]

The Earth itself is not a perpetual motion machine for one simple reason, it is always being bombarded with energy from the sun.

Making possible our existence and the existence of every living thing!

 

edit: and it is necessary to be "bombarded" by energy all the time. Energy flows! That means you once use it and convert it into useless energy (heat), and you need more and more all the time!

[swansont]

The 2nd law says that you cannot extract energy without a temperature difference. If there is no temperature difference then you get zero efficiency.

 

example:

[math]E= 1- \frac{90C}{90C} =1 - 1 = 0 \times 100= 0[/math]

 

The 2nd law does not demand Th and Tc approach one another, it does not demand anything just explains. It tells that the higher the difference in temperature, the higher the energy extraction. If Th approaches Tc then you get less and less efficiency.

 

But another way of saying that is that all "engines" have to reject heat, meaning you drive up the temperature of T_c, so the second law does actually demand that they approach each other over time in an isolated system.

[SteveW]

The earth's motion is determined by all the forces acting on it. The sun applies gravitational force on the earth and so do other planets and all of these forces determine the net external force acting on the earth.

[thedarkshade]

The earth's motion is determined by all the forces acting on it. The sun applies gravitational force on the earth and so do other planets and all of these forces determine the net external force acting on the earth.

Maintaining a constancy of [math]30\frac{km}{s}[/math] !

[ydoaPs]

Maintaining a constancy of [math]30\frac{km}{s}[/math] !

Relative to what?

[thedarkshade]

Relative to what?

huh?

 

edit: The gravitational pull between the Earth and Sun causes the Earth to travel around, or "orbit", the Sun at a velocity of 29.8 km/sec.

[swansont]

huh?

 

edit: The gravitational pull between the Earth and Sun causes the Earth to travel around, or "orbit", the Sun at a velocity of 29.8 km/sec.

 

The force is perpendicular to the motion, so how exactly is the speed a result of the gravitational force? And what does this have to do with perpetual motion?