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Starting a fire and controlling it was one of the key discoveries made by humankind. It affected many aspects of human history such as diet, agriculture, technology, and rituals. Fire and its flames have been a source of inspiration and metaphors for generations of artists and thinkers, as well as an object of research by scientists. But what is a flame? In a wood fire, there is a reaction between the wood and the oxygen in the air leading to the conversion of wood carbon to carbon dioxide (CO2) with the emission of heat and light. As the fire heats the air, it makes it less dense. So the heated air rises, and air from the periphery of the fire rushes in to take its place, feeding more oxygen to the fire and creating a current that blows elements of the fire upwards (a process called buoyant convection). Part of these elements are the small shapeshifting clouds of gases made up of hot carbon dioxide, water, oxygen, nitrogen, hydrogen, and other molecules that we call flames. Wood fire flames have a lot of very hot partially burnt or unburnt carbon compounds (soot particles). Some of these compounds are still in the process of burning up and give off light. However, most of the light in the flames comes from the very hot soot particles briefly producing a light of their own (called black body radiation) causing flames to have their characteristic yellow and orange color. Charcoal or propane fires, on the contrary, tend to emit blue flames because all the carbon present is burnt completely, and there is not soot. As the flames rise and dissipate, the soot cools down and ceases to give of light. In the video below you can see the flames of a wood fire filmed in slow motion.
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All of us have seen fire burning. Fire emits light of its own, and the colors we associate with fire are colors such as red, yellow, orange, and blue. Additionally, when a regular light beam is directed at a regular fire, it will go right through it, so fire is not an entity that casts a shadow. In the video below, James Orgill from the YouTube Channel The Action Lab manipulates the lighting and the material burning in the fire to produce black fire, a fire that absorbs light and casts a shadow! Gav and Dan, The Slow Mo Guys, are interested in visualizing how a liquid on fire spreads on a flat surface. To achieve this, they set a bucket of gasoline on fire, throw the lit gas at a glass windowpane, and film it in slow motion from the other side. You can see the results in the video below! In the video below, Dianna Cowern (Physics Girl) shows the interesting thing that happens when the flame of a candle is exposed to falling. While I understand the need to make the science accessible and fun in a short video, Dianna describes the effects of freefall on the candle flame to be a consequence of “taking away gravity”, and she also says that, “Falling towards the Earth pulled down by gravity, is just like floating out in space with no gravity at all”. Neither of these things is true. The gravity that we are talking about, is that produced by the Earth. To “take away gravity” or to “have no gravity”, the Earth would have to disappear! What really happens during freefall that gives the candle flame that shape is that the forces that oppose gravity are diminished or nullified. Because this eliminates buoyancy, hot air can no longer rise. This is counterintuitive, and visualizing it is more complicated and difficult to explain in a video, but if you are interested, I wrote a post about it here. Here is an old trick that you can perform with a candle. Take a lit candle, snuff it out by blowing on it, and bring a lit match in contact with the smoke trail that arises from the candle. As shown in the first part of the video below, the fire from the match seems to “jump” very fast to the wick of the candle lighting it again. In the second part you can see how this happens when the video is slowed down to 240 frames per second. The reason this happens is that a candle fire doesn’t just melt the wax; it also turns it into gas. When the fire is extinguished, the smoke trail that emerges from the candle carries a significant amount of wax vapor which is flammable. When this smoke trail catches fire, the rate at which it burns is faster than the rate at which it rises from the candle. As a result of this, a flame will “travel” down the smoke trail to the candle’s wick and reignite it. Pretty cool, eh? |
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