Re: [Phys-l] Food Liar's calorie chart

[John Denker <jsd@av8n.com>]

On 04/17/2009 03:58 PM, Philip Keller wrote:
> I would like to understand this. Is it correct to say that though our
> bodies "burn" food, producing CO2 and water, the mechanism of
> digestion and respiration never "allows" the energy from the food to
> become randomized the way that just combusting it would? 

"never" is too strong a word;  try this instead:

The key is that a heat engine combusts the fuel _before_ 
using it.  To say the same thing in more modern and precise 
terms: the combustion step creates huge amounts of entropy.  
Thereafter, the heat engine is severely constrained by having 
to deal with all that entropy.

>  And is that
> why the carnot limit does not apply?  

The Carnot formula does not apply in situations where your fuel
has a high energy per unit entropy, and you use it without (or
before) doing anything unduly dissipative, anything that creates
too much entropy.

In a steam engine it is not the piston/cylinder that locks you
into the Carnot constraint;  it is the firebox.

> And it is easy to mix up food energy with combustion energy.  Don't
> they measure food calories in a bomb calorimeter? 

They do, but they often get wrong or irrelevant numbers.

As an obvious example, a bomb calorimeter treats olestra (fancy 
indigestible fat) the same as regular digestible fat.

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

Also some general advice that may help:  Whenever you have two
concepts under the same name, it leads to endless trouble.  For
example, once upon a time, the notion of "phlogiston" was replaced 
by two concepts, namely oxygen and energy.  Kuhn had something to
say about this.

It is our turn now.  Trying to quantify "heat" is a losing proposition.
It is better to quantify two other things instead, namely energy and
entropy.  In many contexts we know what the term "heat" means, but in 
general it is better to talk about energy and/or entropy instead.

  http://www.av8n.com/physics/thermo-laws.htm#sec-heat-disambiguate