[IP] more on Brian Greene: That Famous Equation and You
Begin forwarded message:
From: Brian Sniffen <bts@xxxxxxxxxxxx>
Date: September 30, 2005 2:22:46 PM EDT
To: dave@xxxxxxxxxx
Cc: brett@xxxxxxxxxx
Subject: Re: [IP] Brian Greene: That Famous Equation and You
Alas, the above are common misconceptions that bear correcting. When
one burns gasoline, one doesn't convert any of its mass into energy.
One is merely allowing the chemical bonds in the gasoline (and the
air used to burn it) to rearrange themselves so that the resulting
atoms
and molecules have less chemical potential energy.
This certainly does involve a change in mass. A mole of O2 has a
different mass from two moles of O. You need to look at very large
amounts of matter to see this effect, but the chemical binding energy
is mass, nonetheless.
You can read about atomic and chemical binding energies a bit here:
http://www.einstein-online.info/en/spotlights/binding_energy/
It's the equivalent of allowing an anvil to descend from the top of
a building to the bottom, and using a rope attached to the anvil to
turn a shaft or do other work as it falls. The anvil has precisely
the same mass at the bottom as it did at the top; it merely has less
gravitational potential energy because it is closer to the center of
the Earth. A similar process occurs on a molecular level when a
battery is discharged or gasoline is burned.
I *think*, but am less certain, that a dropping anvil also exhibits a
change in mass. You'd need to drop an awfully big anvil an awfully
long way to be able to measure it, though. That anvil's falling
deeper into a gravity well, so the potential energy has to go
*somewhere*. It goes into rest mass.
-Brian
--
Brian Sniffen bts@xxxxxxxxxxxx
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