Energy
Energy and Thermal Physics

Getting the measure of energy

Stories from Physics for 11-14 14-16 IOP RESOURCES

Energy is an abstract concept and it can be hard to get a feel for the quantity of energy transferred in different events. The table below gives the orders of magnitudes of energy transferred by a range of events in the universe:

EventEnergy transferred (J)
Big Bang10 68
Typical supernova10 44
Krakatoa eruption10 18
Burning 1 L of petrol3 x 10 7
Daily food intake of a human adult2 x 10 7
Kinetic energy of a cricket ball hit for six10 3
Work done by a human heart per beat0.5
Work done in turning the page in a book10 -3
Work done in discharge of a single neuron10 -10
Typical energy of an electron in an atom10 -18
Energy needed to break one bond in DNA10 -20

When comparing fuels, it can be more useful to consider the amount of energy stored per unit mass (the energy density) rather than the absolute magnitude of energy transfer. A comparison of the energy densities of different fuels highlights the potency of uranium:

FuelEnergy density (MJ/kg)
Uranium (in breeder reactor)80,620,000
Hydrogen (compressed at 70 MPa)142
Liquefied petroleum gas46
Jet fuel43
Fat (animal/vegetable)37
Coal24
Carbohydrates (including sugars)17
Protein17
Wood16
TNT4.6
Lithium battery (non-rechargeable)1.8

Excluding nuclear blasts, perhaps the most energetic human-created explosion was a test carried out by American scientists in 1987, codenamed Misty Picture. In order to mimic the effect of a low yield nuclear weapon, the scientists detonated 4,685 tonnes of ammonium nitrate and fuel oil explosives at White Sands Missile Range in New Mexico. The explosion is estimated to have had a yield of 33.4 TJ (33.4 x 10 12 J).

References

Energy
appears in the relation ΔEΔt>ℏ/2 ΔQ=mcΔθ E=hf E ∝ A^2
has the special case Photon Energy
is used in analyses relating to Emission/Absorption Spectra Phase Change
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