Pro-Human Extremist

Extremism in the defense of humanity is no vice

Archive for July 2012

Why does water put out fires?

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We learn about our world so early—as little children. We find out what gives us pleasure and what causes pain. Even the most loving parents speak sharply to warn their children away from dangers. In this way, we all developed an emotionally charged idea of danger long before we had the mental tools to understand why dangerous things are dangerous.

Fire is one of those dangerous things we all learned about early, along with other hot things like stoves and ovens. Watch out—you’ll burn yourself! Keep away from that, it’s hot! From a respectable distance, fire is warm and cozy and so lovely to look at, but if you get too close it’s painful and destructive.

Most people’s understanding of fire pretty much ends there. We know what sort of things will or will not burn, and we have a sense of how fire behaves, and that’s all we really need to know about it. The chemistry and thermodynamics of fire have been thoroughly described by scientists, and most people once learned something about that in science classes, but relatively few people walk around with a very clear idea of it in their head. That’s fine for practical purposes, but if we don’t ourselves understand at least the basics of how fire actually works, then in effect fire is for us a kind of magic—producing familiar effects by mysterious means.

The science behind fire is discussed in the Wikipedia article on the subject. Basically, fire occurs whenever the temperature is high enough to oxidize some fuel. When wood burns, organic molecules that make up the wood react with oxygen in the air to produce carbon dioxide and water. Because the products of this reaction are held together by stronger chemical bonds than the reactants were, the reaction releases energy. That release of energy speeds up the motion of nearby molecules in the air, which further raises the temperature, because temperature is a measure of how rapidly molecules are moving.

Fire is called a chain reaction because the energy released through the oxidation of organic molecules helps keep the temperature high enough to permit the oxidation of other nearby organic molecules. If the release of energy stopped, the temperature would rapidly drop as heat flowed away. But if enough energy is released in each second, the temperature will stay high enough to keep the oxidation going, and so the wood will keep burning. That’s the chain reaction.

So how do you put out a fire? You can deprive it of fuel or of oxygen. Or you can get rid of the heat fast enough that the chain reaction stops, and that’s what water does.

Two things happen when water finds itself in a fire: the water’s temperature rises to boiling point, and then the water evaporates from liquid to gas. Both of these physical changes absorb energy—particularly the evaporation, which absorbs about six times as much energy as is needed to raise the temperature of water all the way from freezing point to boiling point. Any energy absorbed in evaporating water means less energy is left to help keep up the temperature of the fire. Toss in enough water, and the temperature falls below the threshold needed to sustain the chain reaction, and the fire goes out.

Water turns out to be an excellent liquid for lowering the temperature of a fire. It doesn’t combust like alcohol or gasoline, which is kind of important. But even in comparison to other non-combustible liquids, water has both a high specific heat and a high heat of vaporization. The specific heat is the amount of energy that must be absorbed to raise the temperature of the water, and the heat of vaporization is the amount of heat that must be absorbed to evaporate the water. Thus, water does a really good job of absorbing energy as its temperature is raised and as it evaporates, which makes it very efficient in putting out fires by lowering the temperature enough to interrupt the chain reaction.

© Joel Benington, 2012.

Written by Joel Benington

July 17, 2012 at 3:14 pm

Posted in science

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Why do things look black and white in moonlight?

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Most of what we know about the world around us comes from seeing. Vision is such a useful sense because photons emitted by the sun stream down to earth all day long and reflect off objects or are absorbed by them. Materials that reflect more photons appear lighter to our eyes, while materials that absorb more photons appear darker. Because photons travel at the speed of light, the stream of photons reflecting off an object gives us practically instantaneous information about it, even if it is far away from us. The only other sense that gives us information about far distant objects is hearing.

opsin protein

The structure of an opsin protein. The corkscrew shapes represent alpha helix domains in the protein structure. The bound retinal molecule is just visible in front of the orange alpha helix and behind the two green ones.

We can see because the retinas in the back of our eyes contain cells called photoreceptors, which can detect the presence of photons. Photoreceptor cells can sense photons because they contain molecules of retinal that change shape when they absorb a photon. The retinal molecules are bound to proteins called opsins, which change shape when the retinal changes shape. This triggers a cascade of molecular events in the photoreceptor cell that alters the release of neurotransmitter molecules by the photoreceptor, thus sending a neural signal to other cells in the retina.

Our opsins have evolved so photoreceptor cells are most sensitive to photons in the range of wavelengths emitted by the sun. What we call visible light has wavelengths from 400 to 700 nanometers (a nanometer is one billionth of a meter), because when retinal is bound to opsins, it doesn’t readily absorb photons with wavelengths below 400 nanometers or above 700 nanometers. Photons with wavelengths above 700 nanometers are  in the infrared range, and they’re bouncing off objects all around us, but we can’t see them because they have no effect on the retinal molecules in our photoreceptors.

The different colors that we see are simply photons of different wavelengths within the 400-700 nanometer range of visible light. The colors in a rainbow from blue to green to yellow to orange to red correspond to photons having a range of wavelengths, from shorter to longer.

We can distinguish these different colors because the photoreceptors called cones come in three types containing three different opsin proteins. Those are called L, M, and S opsins because they interact with retinal so it preferentially absorbs photons having long wavelengths, medium wavelengths, and short wavelengths. Red light is absorbed best by cones containing L opsins, while green light is absorbed best by cones containing M opsins and blue light by cones containing S opsins. The visual circuits in our brains compare the neural activity triggered by these three different kinds of cones to distinguish between slightly different colors, like tangerine vs. pumpkin.

The blue, green. and red lines show the range of wavelengths of light that are absorbed by S, M, and L opsins. The dashed line shows the range of wavelengths absorbed by the rhodopsin proteins in our rods.

So why do we see these colors only during the day? Because cone photoreceptors aren’t sensitive to very dim light. The density of photons at night is so low that it has virtually no effect on any of the three different types of cones. Another type of photoreceptors called “rods” are the only ones responsive to dim light, and there’s only one type of opsin in rods, so there’s no way to compare the wavelengths of different photons. We can see, but only based on different intensities of dim light, so everything looks just like different shades of grey.

Of course all of that only applies to dim light at night—as bright as moonlight, for example. We can still see something like a neon sign at night in color, as long as our eyes receive a high enough density of photons to produce a response in our cone photoreceptors, so different wavelengths of light can be distinguished.

© Joel Benington, 2012.

All images come from Wikimedia Commons, and are used under a  Creative Commons Attribution-ShareAlike 3.0 Unported license.

Written by Joel Benington

July 5, 2012 at 11:28 pm

Posted in biology, science

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