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## Laws of Thermodynamics?

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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its not heat that cannot be destoyed.

the actual statement is...energy can not be created nor destroyed, its state can only be changed

3. That isn't a law, it isn't even true. Try "energy cannot be created or destroyed [in a closed system]."

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If heat is a form of energy, then wouldn't the statement hold true?

5. No.

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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.

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Originally Posted by gregscott
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.

8. Originally Posted by gregscott
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?
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.

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Originally Posted by syndicated
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.
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.

10. Originally Posted by gregscott
So, can you think of anything that does not require the transfer of heat?
A form of energy that doesn't require the transfer of heat?
Potential Energy...

11. Originally Posted by syndicated
As far as the reference to a "closed system", since the universe is a closed system, that point is moot, no?
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.

12. ## 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?

13. 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?

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