Energy
Energy and Thermal Physics

# Getting the measure of energy

Stories from Physics for 11-14 14-16

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:

 Event Energy transferred (J) Big Bang 10 68 Typical supernova 10 44 Krakatoa eruption 10 18 Burning 1 L of petrol 3 x 10 7 Daily food intake of a human adult 2 x 10 7 Kinetic energy of a cricket ball hit for six 10 3 Work done by a human heart per beat 0.5 Work done in turning the page in a book 10 -3 Work done in discharge of a single neuron 10 -10 Typical energy of an electron in an atom 10 -18 Energy needed to break one bond in DNA 10 -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:

 Fuel Energy density (MJ/kg) Uranium (in breeder reactor) 80,620,000 Hydrogen (compressed at 70 MPa) 142 Liquefied petroleum gas 46 Jet fuel 43 Fat (animal/vegetable) 37 Coal 24 Carbohydrates (including sugars) 17 Protein 17 Wood 16 TNT 4.6 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