The Physics
Opus in profectus

Chemical Potential Energy

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Calorimetry and so on.

Q = mcT

Burn a sample of the substance in oxygen, use the heat given off to warm a bit of water and watch the temperature rise.

When comparing substances, it's often more instructive to speak of their specific energy or specific work or gravimetric energy density or volumetric energy density (energy per mass) symbol e or w.

e =  E

or energy density (energy per volume) symbol u or η (eta) or ε (epsilon).

u =  E

rather than their energy.

Specific energy of selected substances when combined with oxygen*
simple compounds specific energy
carbon (to CO2) 32.8
carbon (to CO) 22.7
ethanol 29.7
hydrogen 142
methane 55.5
methanol 22.7
nitromethane 11.6
octane 47.9
propane 50.3
sulfur 9.16
foods specific energy
carbohydrates 17.2
fats 38.9
proteins 17.2
explosives* specific energy
ANFO 6.3
C4 6.3
composition b  
composition c  
dynamite 7.5
gunpowder 3.0
lead azide  
nitroglycerine 6.4
PETN 5.8
RDX 6.3
silver azide  
silver nitride 0.93
sodium azide  
torpex 6.3
TNT 4.184
fossil fuels specific energy
coal, anthracite >32.6
coal, bituminous 24.4–32.6
coal, sub-bituminous 19.3–24.4
coal, lignite 14.7–19.3
diesel 45.3
gasoline, automotive 45.8
gasoline, aviation 43.1
jet a 43.3
jet b 43.5
kerosene 46.3
oil, crude (petroleum) 41.9
oil, heating 42.5
biomass fuels energy density
alfalfa straw 18.4
charcoal 29.0
coconut husks 9.8
coconut shells 17.9
coffee husks 16.0
cotton hulls 19.4
cotton stalks 17.4
dung, air dry 12.0
groundnut shells 19.7
maize cobs 18.9
maize stalks 18.2
peat 14.6
rice hulls 15.5
rice straw 15.2
sugar cane bagasse 19.0
sorghum bagasse 18.9
soybean stalks 19.4
wheat straw 18.9
wood, green 10.9
wood, air dry 15.5
wood, oven dry 20.0
* Explosives undergo decomposition and do not release energy by combining with oxygen.


A word about fuel based units.

Fuel-based energy units
fuel unit energy (MJ) source
cubic meter natural gas
(15 °C, 1 atm)
37.1–38.7 Energy Information Administration
37.3 Carbon Dioxide Information Analysis Center
38.3 ConEdison (New York City)
37.9 Niagara Mohawk (New York State)
38.4 PSE&G (New Jersey)
ton of oil equivalent
(1,000 kg)
41,868 International Energy Agency
42,622 Statistics Norway
44,760 Carbon Dioxide Information Analysis Center
ton of coal equivalent
(1,000 kg)
29,308 International Energy Agency
29,310 Center for Energy Efficiency
29,290 Carbon Dioxide Information Analysis Center
ton of tnt 4,184 exact value by definition


Respiration (cellular respiration, that is, not pulmonary respiration or breathing as it is better known) is chemically identical to combustion, but it takes place at a much slower rate.

C6H12O6 + O2 → CO2 + H2O + energy

or in English…

sugar combined with oxygen turns into carbon dioxide and water and releases energy

The energy content of foods is no longer measured directly much anymore. Instead, the food item in question is analyzed to determine the amount of protein, fat, and carbohydrate present in one serving. These numbers are then multiplied by the respective energy densities. In the US, where kilocalories are used instead of megajoules, the rule is 4-9-4: 4 kcal/g for protein, 9 kcal/g for fat, and 4 kcal/g for carbohydrate.

Energy density of macronutrients
foods kcal/kg MJ/kg
carbohydrates 4 17.2
fats 9 38.9
proteins 4 17.2

Adjustments are needed for indigestible fiber and specific energy factors/coefficient of digestibility.

Atwater system. Nutrition Labeling and Education Act of 1990.

Basal metabolic rate. Revised Harris-Benedict equations.

Qmen[kJ/day] =  0370 + 56.05 m[kg] + 20.08 h[cm] − 23.75 t[yr]
Qwomen[kJ/day] =  1872 + 38.69 m[kg] + 12.96 h[cm] − 18.12 t[yr]