Friday, May 26, 2017

Light and Heat

Consider three items - a big 10-lb log of wood, a litre of gasoline, and a stick of dynamite. Which one would you throw on a fire?

What a stupid question.  Throwing a log on a fire is routine and harmless, throwing a bunch of gas on a fire is the stuff of crazy Youtube videos, and throwing a stick of dynamite on a fire is suicidal. everybody knows that. And yet, if you look at how much energy each of them produce - the total amount of light, heat, and so on that's released when they burn - the gas is 40x more energetic than the dynamite, and the log is twice as energetic as the gas.

The difference, as any child could tell you, is in how fast they burn up. The dynamite goes off in a tiny fraction of a second, the gas takes a few seconds, and the log will happily burn for hours. So yes, the log produces the most heat, but it spreads it out over such a long time that you'll sit around it for hours roasting marshmallows and chatting with your friends. That heat doesn't build up to damaging levels,your body sweats away excess heat if it needs to, you can move your seat back if it's getting too warm, you might walk away for a while and miss some of it, and so on. With the others, none of that happens - the energy is released quickly, and it'll blow you away or singe off your eyebrows if you're too close, and you won't have any time to react.

Now, imagine how clueless you would sound if you said "I don't trust logs as an energy source - fire is basically the same as an explosion, and they last such a long time, so it'd never be safe to go anywhere near a burning log". They're actually correct about fire and explosions being basically the same thing chemically, and they're also right about how long the log lasts, but their conclusion just doesn't follow. The danger of an explosion exists precisely because it's short, due to it taking a small amount of energy and lighting it all off in an instant. More total energy isn't much of a problem as long as you can react as it happens, particularly if it lasts long enough that you won't even be around for a big part of it.

If you've ever complained about how radioactivity lasts for millions of years after a nuclear accident, this is exactly how you sound to people who understand nuclear science.

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