Is There a Connection Between a Burning Log and E=mc2?


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Burning a Log. Heat breaks the bonds of carbohydrates (CnH2nOn) in the wood. The atoms then combine with oxygen from the air to produce CO2, CO, H2O, and electromagnetic radiation (heat and light).
Image Credit: Robert Craft

A Decoded Science reader asked the question:

When I burn a log in the fireplace, am I converting mass to energy? Some say that only chemical bonds are being broken, but so what? E=mc2, doesn’t it?

A common misconception is that Einstein’s famous equation applies only to nuclear processes such as in the core of stars or in atom bombs and atomic energy.

In fact, as physicist Richard Wolfson points out, “E = mc2 applies to all processes that release or absorb energy.” (In Einstein’s famous formula, E is rest energy, m is rest mass, and c squared is the speed of light multiplied by itself.)

Imagine you weigh a log to great accuracy. Then you burn it. Say as the log burns, you somehow capture all the smoke, gases, etc. released in the process.

Once the fire is out, you weigh the remaining log, ash and all captured smoke, gases etc. In principle, if you could do this with sufficient care and accuracy, you would find that the weight of the remaining log, ash, smoke, and captured gases after burning is less than the original weight of the unburnt log (by a miniscule amount). This missing weight is found in the energy of the heat and light produced by the fire per E=mc2.

According to FirewoodResource, a cord of white oak wood weighing 3757 pounds produces 24 million BTU’s of energy when burned. Per E=mc2, this released energy results in a material weight loss of 0.0000006 pounds.

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