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Conservation of mass for burning log

by alegoull
Tags: burning, conservation, mass
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alegoull
#1
Mar3-05, 12:28 PM
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Suppose I burned a log. If I collected all the products of the burning process (the smoke particles, the ashes, etc.) would they have the same exact mass as my original log? Or would they have less mass because they are at a lower energy state then the original log (Energy-mass equivalence)? Thanks.
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James R
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Mar3-05, 04:51 PM
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They'd have a little less mass, since some of the mass has been released as heat, light etc.
Andrew Mason
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Mar3-05, 06:20 PM
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Quote Quote by alegoull
Suppose I burned a log. If I collected all the products of the burning process (the smoke particles, the ashes, etc.) would they have the same exact mass as my original log? Or would they have less mass because they are at a lower energy state then the original log (Energy-mass equivalence)? Thanks.
You would have to also capture all of the CO2 and H2O produced by combustion and then you would have to subtract all the O2 used in the combustion process. The mass of the electromagnetic radiation and thermal energy lost by conduction/convection would be so tiny as to be unmeasurable (ie. E/c2).

AM

HallsofIvy
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Mar3-05, 08:19 PM
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Conservation of mass for burning log

Quote Quote by James R
They'd have a little less mass, since some of the mass has been released as heat, light etc.

That happens only in nuclear processes. Burning a log does not convert mass into energy. This experiment has, in fact, been done many times- the total mass- solid ash, water, and gases, less, as Andrew Mason reminded me, the atmospheric oxygen trapped in carbon dioxide and perhaps carbon monoxide- remains the same.
Andrew Mason
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Mar3-05, 10:04 PM
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Quote Quote by HallsofIvy
That happens only in nuclear processes. Burning a log does not convert mass into energy.
I don't think that can be right. The release of a photon carries away some mass. So loss of any amount of energy, whether chemical, nuclear or electromagnetic - even gravitational - must result in loss of mass. But it is so tiny as to be immeasurable.

AM
BillyDee
#6
Mar3-05, 11:24 PM
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Quote Quote by Andrew Mason
I don't think that can be right. The release of a photon carries away some mass. So loss of any amount of energy, whether chemical, nuclear or electromagnetic - even gravitational - must result in loss of mass. But it is so tiny as to be immeasurable.

AM
I think Halls is right. I'm a complete physics newb (well, not complete, but I'm quite the novice still) so I could be wrong with this thought. When a log is being burnt, isn't there energy being put into the log from the burning reaction? Couldn't the energy from burning excite atoms in the log and cause light emission from electrons jumping to a higher energy level and coming back to ground state? Therefor you wouldn't lose mass from the log because the extra energy is from the burning reaction and not from the log?

(Btw, first post! Hey everybody! )
Andrew Mason
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Mar3-05, 11:29 PM
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Quote Quote by BillyDee
I think Halls is right. I'm a complete physics newb (well, not complete, but I'm quite the novice still) so I could be wrong with this thought. When a log is being burnt, isn't there energy being put into the log from the burning reaction? Couldn't the energy from burning excite atoms in the log and cause light emission from electrons jumping to a higher energy level and coming back to ground state? Therefor you wouldn't lose mass from the log because the extra energy is from the burning reaction and not from the log?

(Btw, first post! Hey everybody! )
Welcome to PF.

Have a look at this, for example:
http://www.ccmr.cornell.edu/education/ask/?quid=590
"As an example, the energy released in chemical reactions when an average person shovels snow for one hour amounts to a mass loss (by E=mc2) of only 10 billionths of a gram!"
AM
pervect
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Mar5-05, 06:04 PM
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There's a derivation / disucssion of the "gravitational mass" (not my wording) of an electromagnetically bound system in http://lanl.arxiv.org/abs/gr-qc/9909014 by Steve Carlip which dots all the i's and crosses all the t's. (The argument is presented in detail only for the weak field case and in the case where the internal velocities of the matter are nonrelativistic, but another paper is referenced to support the argument in general).

The end result is that the total "gravitational mass" of a system is the sum of the rest masses, mc^2, plus the total energy of the system E. Use is made of the virial theorem to derive this result. E is divided into two parts, potential energy U and kinetic energy T, and the non-relativistic virial theorem states that T = -U/2.-


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