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 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


On Mon, 18 Feb 2008, Jeff Weitz wrote:

Hi everyone,

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

How about defining "kinetic energy" as the work done to accelerate an object
from rest to a given speed?

Then, how about defining "change in potential energy" as the negative of the
work done by a conservative force on an object that moves from one point to
another?

"Potential energy" as an absolute value remains undefined since the
reference point is arbitrary. But, in practice, we speak of "potential
energy" as the change in PE from a specified reference. The two definitions
insure that the sum of KE and PE remains constant as long as there are no
non-conservative forces acting, which helps to motivate the odd-looking
definition of (delta)PE.

An advantage of these definitions is that it puts the work in the past, not
in the future, as in "ability to do work," which, sadly, still appears, for
example, in the edition of Zumdahl used in our AP Chem course.

Also, since the definitions are in terms of "work done" it's easy to see how
PE can change into KE and back. "Work" becomes a unifying idea, and in new
situations, students can ask what force is doing work in order to see
whether total mechanical energy is conserved.

Finally, although I realize this has been quite the bone of contention
lately, might we think of "internal energy" as a composite of KE and PE?
After all, we do link average KE to the temperature of an ideal gas in our
intro courses. And there's that interesting model of a metal as vibrating
atoms attached by springs, which does approximate the molar heat capacity...

Best,
Jeff Weitz
Horace Mann School
Bronx, NY


_______________________________________________
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