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Re: [Phys-L] defining energy



Oops. I cut that short. Intended to hit delete. It seems I hit send.
I will adress some of the issues John had with what ive already sent. It may not clean it up completely, but it will get closer.

When we talk about what it costs you, we say it might be a trade of ine for another (trade moving fast for increasing temperature or trading temperature for light etc). We can also trade with things that are outside of what we might initially consider to be out arrangement or system.
That is closer to our first discussion.
Covering area under f d curve and energies of our system come quickly.


.:. Sent from a touchscreen .:.
Paul Lulai




-------- Original message --------
From: John Denker <jsd@av8n.com>
Date: 10/29/2013 4:43 PM (GMT-06:00)
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] defining energy


On 10/29/2013 11:25 AM, Paul Lulai wrote:
For the first general shot at it, i go with... If you want to speed
something up, slow it down, bend it, break it, change its
temperature, make it glow, change its mass, change its direction, it
is going to cost you. What it costs, is energy.


That's not the route I would have taken.

For starters: As previously mentioned, there is already a
terrible problem because of conflicting definitions of
energy:
-- The physics energy, versus
-- the "DoE" energy aka "available" energy or "useful"
energy ... which is an important concept, but emphatically
not the same as the physics energy.

As a related point: The "cost" of energy is *not* the same
as the amount of energy. Physics deals with the amount of
energy. Questions about the price / cost / value of energy
belong to microeconomics, which is very much uglier than physics.

Therefore, it seems to me that introducing energy in terms of
the "cost" of this-or-that is doing the devil's work for him,
reinforcing some pernicious misconceptions.

=======

At a more nit-picky level, several of the given examples are
not even reliably true. For a coil spring, bending it does
not require energy; rather, straightening out the bends
requires the input of energy.

Perhaps more importantly, it's not at all clear that this
"costs" energy in any technical sense, because you can
get back virtually all of the energy you put into the
spring. In microeconomic terms, lending money is not the
same as spending money, not even close. Ditto for energy.
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