It Takes 2.99 Gigajoules To Vaporize a Human Body 272
Have you ever wondered how much energy is needed to power a phaser set to kill? A trio of researchers at the University of Leicester did, so they ran some tests and found out it would take roughly 2.99 GJ to vaporize an average-sized adult human body. Quoting:
"First, consider the true vaporization – the complete separation of all atoms within a molecule – of water. With a simple molecular structure containing an oxygen atom bonded to two hydrogen atoms, it takes serious energy to break these bonds. In fact, it takes 460 kilojoules of energy to break just one mole of oxygen-hydrogen bonds — around the same energy that a 2,000-pound car going 70 miles per hour on the highway has in potential. And that's just 18 grams of water! So as you can see, it would take a gargantuan amount of energy to separate all the atoms in even a small glass of water — especially if that glass of water is your analog for a person. The human body is a bit more complicated than a glass of water, but it still vaporizes like one. And thanks to our spies spread across scientific organizations, we now have the energy required to turn a human into an atomic soup, to break all the atomic bonds in a body. According to the captured study, it takes around three gigajoules of death-ray to entirely vaporize a person — enough to completely melt 5,000 pounds of steel or simulate a lightning bolt."
Awful calculation - the real answer is almost zero (Score:4, Informative)
I'm going to do some rough calculations - the paper's computation is also pretty rough - just to get the right order of magnitude.
First of all, to vaporize water, you don't even need to boil it. Spill some water on the floor and it vaporizes pretty darn quick just from the ambient environment - it changes from liquid water at room temperature to water vapor at room temperature. The only heat that needs to be added is the "Enthalpy of vaporization" which is 2260 kJ/Kg. For the 78kg human described in the paper, if it were all water, that would only be 176 Megajoules. Given that a human is normally at about 37C and room temperature about 25C, you can also take away 4kJ/Kg*78Kg*(37-25) = 4 Megajoules that the water vapor releases as you cool it from 37C to 25C. The net result is that with 172 Megajoules, you can turn a human body's mass of water to vapor.
However, as the paper suggests, the body isn't all water - it's about 85% water and 15% "dried pork." That means 172MJ*0.7 for the water, 146MJ, and the 11.7Kg of pork releases about 4KCal/g when oxidized (4 dietary Calories/g), 1 Kcal=4.2KJ, so burning the "dried pork" releases 196MJ. Assuming the "dried pork" gets fully oxidized (i.e burned) into CO2, the result is a gas. So overall, vaporizing a human body (in the sense of turning all the body into a gas) can release more energy than you started with - about 50MJ.
The paper estimates the energy required to break every molecular bond. However, all those bonds are going to reassemble into something else, whether into H2, O2, or H2O, or including the "dried pork," CO2, releasing much of the energy back.
Vaporize or ionize? (Score:5, Informative)
Hang on a moment... TFA isn't talking about vaporizing - turning water to steam. It's talking about ionizing, which is clearly going to require a much bigger quantity of energy.
For actual vaporization, making a very rough calculation - 60kg person, 2,270 kJ/kg latent heat of vaporization of water = 136 MJ,
Sure there's specific heat to add in there too, but the vaporization of water is the dominant term, so it's at least out by an order of magnitude.
Lesson learned - don't try and be 'all sciency' and use the wrong jargon!
Re:well done (Score:5, Informative)
Leicester is correctly pronounced "lay-ses-ter".
No it isn't. It's pronounced "les-ter".
Source: I've been there. Also, this [wikipedia.org].