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?
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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?
its not heat that cannot be destoyed.
the actual statement is...energy can not be created nor destroyed, its state can only be changed
That isn't a law, it isn't even true. Try "energy cannot be created or destroyed [in a closed system]."
If heat is a form of energy, then wouldn't the statement hold true?
No.
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.
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.Quote:
Originally Posted by gregscott
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.
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.Quote:
Originally Posted by gregscott
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.
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.Quote:
Originally Posted by syndicated
Can you give me an example of one of these conversions?Quote:
Originally Posted by Saturatedpsi
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.Quote:
Originally Posted by gregscott
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.
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.Quote:
Originally Posted by syndicated
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.