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.
Photosynthesis: Stored Sunlight
In another of his wonderful talks, physicist Richard Feynman tells us that burning a log is kind of like the photosynthesis process in reverse.
Put simply, a tree takes in energy from sunlight, carbon dioxide from the air, and water from the ground to produce carbohydrate molecules made of carbon, hydrogen, and oxygen. The tree also releases oxygen molecules into the air in the process.
Burning a log, on the other hand, breaks up these carbohydrate molecules, which then combine with oxygen in the air to produce carbon dioxide and water. This process gives off energy in the form of heat and light — reducing the remaining log and its byproducts’ weight by E=mc2.
Converting Mass to Energy
So, to answer the reader’s question: Energy from sunlight is stored in the making of a tree, and released in the energy of heat and light given off in burning it — all governed by E=mc2.
For a more detailed and fun explanation of the chemistry of fire, check out the video What is a Flame.
FireWoodResource. Firewood BTU Ratings Charts for Common Tree Species. Accessed December 18, 2012.
Ames, Ben. What is a Flame. Alan Alda Flame Contest Winner, Center of Communicating Science, Vimeo.com. Accessed December 18, 2012.
Pössel, Markus. From E=mc² to the atomic bomb. Einstein Online Vol.4 (2010), 1004. Accessed December 18, 2012.
Wolfson, Richard. Simply Einstein, Relativity Demystified, Norton, New York (2003), p. 155.
Carter, J. Photosynthesis. (1996). University of Cincinnati. Accessed December 18, 2012.