Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-l] energy is well defined



Hi, Jack, (I hope I don't have to greet you on an airplane)
I think we waste a lot of energy (!) trying to settle on firm, perfect definitions when really there are some deep philosophical issues underlying this and similar issues. I don't think we can get the perfect definition because we talk about so many types of energy. We talk about the concept of energy, the transfer of energy, mass energy, kinetic energy, potential energy (does that exist, or is it merely a balancing tool?).

I'm not defining energy in the fluid. Temperature is merely some standardized measurement that results from obtaining a thermal equilibrium between a thermometer and a substance. It is a statistical measurement of a population which is related to the average kinetic energy per molecule in a substance. That's different from the "energy" of the assemblage. Again, I wasn't trying to define energy. Kinetic energy of an object is pretty easy to define once you establish a reference frame so that you can measure a speed: (gamma-1)*mass*c-squared. But that doesn't define any other energy.

We have determined that there are properties of objects and systems that can transfer back and forth and seem to add and subtract nicely, and we determined (labelled them?) that they are forms of energy. Leaning on relativity and E=mc^2, we physicists have become comfortable viewing everything physical as one form of energy or another, and we look for the ways it's moved around.

I think trying to define "energy" with one all encompassing sentence is fruitless. What we really do is analysis of "energetic" systems, and we determine how a property relates to energy, and whether that property is important in the specific analysis we want to do. The simple example: for an object moving perpendicular to gravity, we ignore Earth's gravitational potential energy because it's not changing and the rest of the universe because the changes are so small.

Now, to the mammal: which energy are you talking about, thermal, mass, KE of blood flow, electrical PE of brain and nerves, electrical PE of cellular processes, KE of center of mass, gravitational PE and relative to what planet, star or galaxy? It depends on the problem one is trying to solve. I don't think there is an answer.

All the Best,
Bill
Jack Uretsky <jlu@hep.anl.gov> 2/19/2008 11:31 pm >>>
Hi Bill-
Can you explain how your explanation defindes "energy".
You seem to define "energy" of fluid in terms of "energy' of assemblage.
But even if I know how to define, the first, you don't tell me how it
relates to the second. Try this one for size: what is the energy of a
mammal at rest?

On Tue, 19 Feb 2008, Bill Nettles wrote:

Jeff,
On Mon, 18 Feb 2008, Jeff Weitz wrote:

Hi everyone,
Why define "energy"? Look at the linguistic knots we get stuck in.

I agree. What's this obsession with definitions? Has anyone given an all inclusive definition of "force?"

I do like Feynmann's "currency of the universe." He was always elegant with language. Before I read that I was going to try for "the fundamental substance of the physical universe...at least for now."

Ludwik said: "One can say that energy is a concept similar to work." This bothers me a little bit. Energy is a fundamental, work is a process. They are similar like dirt (energy) and a shovel (work). Rather than the defining energy in terms of "work," I think a better, more developed concept is that work is a process (the process?) that moves (transfers) energy from place to place and object to object. Define work in terms of what it does to localized energy "content" (the work-energy theorem). Then say that work is done on an object by a force when the object has a component of motion (displacement) parallel to the force.

Jack Uretsky <jlu@hep.anl.gov> 2/18/2008 11:10 pm >>>

Hi Jeff-
How do you describe what happens when I mix a kilogram of 79
degree water with a kilogram of 40 degree water in an insulated container?
Regards,
Jack

Jack: The kinetic energy of the molecules in the 79 deg. water is redistributed to more molecules which had less KE on the average. The molecules do work on each other during collisions, gaining or losing KE with the ultimate result being a temperature between 79 and 40 (59.5 if we ignore the work done to the inside surface of the container) with an average KE per molecule corresponding to that temp. Were you looking for more than that?

Bill Nettles
Union University--yes, tornados did a lot of work here, massive increase in entropy, too.
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l


--
"Trust me. I have a lot of experience at this."
General Custer's unremembered message to his men,
just before leading them into the Little Big Horn Valley



_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l