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

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? And is that why the carnot limit does not apply? I don't know much about this. And it is easy to mix up food energy with combustion energy. Don't they measure food calories in a bomb calorimeter?
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From: phys-l-bounces@carnot.physics.buffalo.edu [phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of John Denker [jsd@av8n.com]
Sent: Friday, April 17, 2009 4:50 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Food Liar's calorie chart

On 04/17/2009 12:48 PM, Philip Keller asked:

Does the Carnot efficiency provide an upper limit on the efficiency
of the human engine?

No.

There are numerous widespread and deep-seated misconceptions
about this. For example, chemists habitually refer to "the
heat of reaction". There exist big detailed tables of "the
heat of reaction" denoted ΔH. But the thing being tabulated
is not "heat"; it is enthalpy. In thermodynamics, H is the
symbol for enthalpy, not "heat".

Carnot's efficiency formula applies to "heat", but not necessarily
to other forms of energy. In particular, it does *not* apply
to batteries or fuel cells. In some fuel cells, the efficiency
can be as high as 80 percent, far in excess of the Carnot limit.
http://americanhistory.si.edu/fuelcells/basics.htm

Muscle cells don't look like heat engines, so there's no good
reason to think the Carnot formula applies.

OTOH muscle cells are not designed for efficiency. In particular
human muscles at rest make a mockery of efficiency; they dissipate
energy even when they are not doing anything. (OTOH the adductor
muscle in a clam can hold the clamshell shut with almost zero
dissipation.)

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