Re: [Phys-l] energy is well defined.

[John Denker <jsd@av8n.com>]

Referring probably to
  http://www.av8n.com/physics/thermo-laws.htm#sec-workability

On 02/17/2008 10:44 PM, Ken Fox wrote:
>  2 hot potatoes can do more
> work as long as we keep the cold one as well, I think.

Well, yes, you can do that if you want, but then it becomes
a different experiment.  Both experiments are OK, but we
need to keep track of which is which.

I just now added the words "and nothing else" to clarify the
description of my two-potato heat engine.


> I am still looking for the well defined statement of energy, suitable for my
> neophytes, 

The best I can do is:
  http://www.av8n.com/physics/thermo-laws.htm#sec-energy-def

It's less than 175 words.  It's not super-simple or super-concise,
but it has served me well all these years.  It's an operational 
definition, optimized more for practicality than elegance.

I just now re-arranged this part of the document, putting the
definition _per se_ into its own section, and moving the /remarks/
about the definition into separate sections.



===================

On 02/18/2008 01:11 AM, Leon de Oliveira suggested:

> > Energy - a conserved substance like quantity that can be transferred 
> > from one body to another or transformed from one form to another. [1]

That's not wrong as far as it goes, but it's incomplete
without the other half of the definition:

> > We also talk about the forms 
> > objects store energy in. A moving object stores Kinetic energy.  [2]

IMHO that needs to be included in the definition.  Otherwise
lots of quantities other than energy that would be inadvertently
swept up by part [1]... other quantities such as charge and 
(especially) momentum.  Energy is conserved, but it's not the
only conserved quantity.  Just because XXX is conserved doesn't
mean XXX is energy.

Taking these two parts [1] and [2] together gives a valid inductive 
definition.  Induction requires /two/ things: 
 -- there needs to be at least one initial example
 -- then you turn the crank on the induction step to bring in more
  examples, possibly innumerably more examples.

> > to get the students thinking about it 
> > as a conserved quantity.

Yup.