Replyits not heat that cannot be destoyed.
the actual statement is...energy can not be created nor destroyed, its state can only be changed
Regular Guest
One law of thermodynamics is "heat cannot be created or destroyed", I think this means we just move it around.
How does fire work according to this principle?
How does resistance heating work according to this principle?
Professional Member
its not heat that cannot be destoyed.
the actual statement is...energy can not be created nor destroyed, its state can only be changed
Please, Please Please......keep the Factory Smoke in the Wires!!!!!
Is it Rum'Oclock yet???
Professional Member*
That isn't a law, it isn't even true. Try "energy cannot be created or destroyed [in a closed system]."
Regular Guest
If heat is a form of energy, then wouldn't the statement hold true?
Professional Member*
No.
Regular Guest
It is a law of "THERMO" dynamics. Can you think of a form of energy that does not involve heat? I was under the impression that every thing in the universe happens because of heat flowing from a hot place to a less hot place.
Professional Member
There is a lot of that going on naturally, but it's not accomplishing much. Most of the good stuff going on is due to conversion of one form of energy to another.Originally Posted by gregscott
Professional Member
that is not actually the first law of thermodynamics.
the first law encompasses three different principles.
the law of conservation of energy:
This states that energy can be neither created nor destroyed. However, energy can change forms, and energy can flow from one place to another. The total energy of an isolated system remains the same.
the flow of heat. which is the one that applies to fire and resistance heat!
and
performing work.
true knowledge exists in knowing that you know nothing.
Professional Member*
Fire fits very easily within this principle. Since combustion is a chemical reaction between 2 or more substances, the separation and recombination of various atoms in the molecules releases heat. Look at it as in a piece of firewood you have potential energy, under the right conditions (heat, oxygen) the carbon and other chemicals release their potential into heat.
As far a resistive heating element, this is even easier. the heating element has a high resistance value to the flow of electrons (amperage) in the circuit. As the electrons move through the resistor, they generate heat since there is such opposition to flow - almost like friction.
As far as the reference to a "closed system", since the universe is a closed system, that point is moot, no?
However, once you start throwing entropy into the mix, the first law becomes harder to understand as processes don't become entirely reversible. ie the Carnot cycle that we've all come to know and love.
Regular Guest
Best answer. Thank you. So, can you think of anything that does not require the transfer of heat? I cannot, I suspect that the reference in post #7 to conversion of one form of energy to another still requires the transfer of heat, and that the conversion is never 100% efficient because there has to be a transfer of heat; work was done to complete the conversion, energy was lost in the form of heat.
Regular Guest
Can you give me an example of one of these conversions?
Professional Member
I'm not a student of thermodynamics. I read some of the laws 30 years ago, didn't fully understand the implications then and don't fully understand them now. But a little of what I remember tells us all matter contains some amount of heat energy (unless it's at "absolute zero" temperature). So there is always the possibility for naturally occurring heat transfer to take place.
You also noted:
"..I suspect that the reference in post #7 to conversion of one form of energy to another still requires the transfer of heat, and that the conversion is never 100% efficient because there has to be a transfer of heat; work was done to complete the conversion, energy was lost in the form of heat."
Once you throw "work" into the equation, the discussion has to take a different direction. Work requires some input, of some form of energy, to produce some result. If you "input" enough heat energy to a piece of combustible material, in an oxygen containing environment, it will "combust", which is a chemical reaction. In that case, I suppose "heat transfer", from a direct heat energy "input", is the explanation for the result.
If you apply voltage to a resistive heat element, the element gets hot. There wasn't any direct transfer of "heat" energy to the element. The resulting heat was due to something else, explained by some other law of physics. The electrical energy was inputted to the element, some work was done and some heat energy was a by-product. And don't forget, there was a lot of "energy" input somewhere else, in some other form, to generate the electrical energy.
The Law of Conservation of Energy is telling us there was no "new" energy created. Some other forms of energy were "manipulated" and expended, in a way that resulted in some heat energy being produced.
Professional Member*
Yep. I got that one twisted around somehow. Should have been "isolated" system, and stated like this, "The energy content of an isolated system is constant." In this form there's nothing reduntant about the "isolated system". It defines the boundary conditions relevant to the first law. IOW, the energy content of an open system is not constant.
When stated in the form that I used initially "energy can neither be created nor destroyed", then boundary conditions are immaterial, since this is a general statement about energy. OTOH, it isn't necessarily true in this form, since we do not know it to be true.
AOP Committee/Professional Member*
energy conversion
Take for example your car's brakes. When your car is in motion, there is kinetic energy that must be dissipated to stop. When you apply the brakes, pads contact the rotors or drums creating friction, which causes them to heat up quickly. Air flowing over the brakes cools the brakes thus transferring the heat by convection and the car stops. Now, think about going down a mountain. You keep riding your brakes until they overheat. What happens? You can't stop and make the headlines.
Back in the '60s, there were a growing number of motor vehicle accidents from brake failures. The cause was traced to hub caps restricting the flow of cooling air and wheels without adequate air slots. Spoked wheels and those with big spaces can move a lot of air thus often improving braking. If you coat the brakes and wheels with mud, you may not be able to stop hard because the mud insulates the brakes keeping them hot instead of allowing cooling. I've had this happen many times when 4 wheeling.
Now, think about your rotors and drums. It used to be we turned them down on a lathe to extend their life. However, the amount of allowance you have is so marginal it usually is not worth it. Nowadays, the rotor is ususally replaced out of hand. Why? Because you need a minimum mass of metal to absorb heat from the friction of the brakes without distorting the metal, which would break contact causing failure. It takes a critical minimum mass of metal to stop a car.
Does this help?
Keep the fire inside the fireplace.
Professional Member*
I'll throw a loop for you guys. Ask yourself if energy can not be created then how did it become to be? Did the initial energy of the universe just pop out of nothing?
AOP Committee/Professional Member*
energy is the potential between atoms created by God caused by the polar charge of electrons or the lack thereof. This results in bonds between atoms that, when broken, releases energy to be used to form new compounds. When you burn a molecule of methane, the bonds between the CH4 and oxyen reform into carbon dioxide and water with the evolution of heat and light.
Keep the fire inside the fireplace.
Professional Member*
The Law of Conservation of Matter and Energy states it can not be created or destroyed............no the mass of the universe is finite and the energy in it is also finite. The Law just proves we can manipulate it.
A tree on the forest floor releases the same amount of carbon dioxide if it is left to rot or if it is burned in a fire, the Law in action.
Professional Member*
Maybe energy doesn't exist at all? That would resolve the paradox wouldn't it. If it never was, then it didn't have to be created.
A perception of a thing is not neccesarily a property of the thing. Take color for instance. If we say that some object is red, then this only means that it reflects red light and absorbs all other visible frequencies of em waves. But...we are hardwired by evolution such that we instinctively think of the color of an object as an intrinsic element of the object itself--something it "has". In reality, color is just something we percieve, and doesn't belong to the object at all, being light of some wavelength emitted from a source external to the object. To complicate matters, the "color" of an object will also change with relative motion to the object (due to the Doppler Effect.) It's no different with energy.
Suppose you're stationed by the railroad track and you're watching a train pass by. You have good info that the mass of the train is X kg, and you measure its speed at Y m/s. You calculate it's KE as
kg(m/s)^2 or XY^2
which in this case turns out to be a rather large number. A passenger on the train however measures the speed of the train as exactly 0 m/s. He calculates that the KE of the train is zero. It has no kinetic energy from his frame of reference. It should be obvious then that the KE that you attributed to the train is not an intrinsic property of the train, but rather just a perception, like color, relating to your particular frame of reference.
The bottom line is that our mode of thinking is tailored by evolution toward survival, and along with that came an aptitude for engineering, but not so much an ability to understand the true nature of the building blocks that we stack together. Anything beyond the engineering value of our physics is pure philosophy, which should be avoided for the sake of sanity (unless you're only speculating for personal amusement, or maybe for profit). Physics beyond the math is also called metaphysics.
I apologize for getting carried away with this. I've had this discussion with real physicists many times. The ideas above are not mine, just ideas that I was finally forced to accept.
What does this have to do with hvac anyway.
Last edited by hvacrmedic; 09-09-2012 at 01:02 AM.
Professional Member
Thank you.
Professional Member
My brain hurts! Excellent explanations though.
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