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

[John Denker <jsd@av8n.com>]

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.)