Apple #279: Fire

The fires in Southern California, which seem to be lessening, got me thinking about fire.


San Diego on fire -- in 2003
(Photo from The San Diego Wildfires Education Project)


I'm not going to get into a lot of nitty-gritty details about the physics and chemistry of fire -- which, like water, is more complex than you might think. I'll just provide you with a few fast fire facts. For free. (Fire starts with f)

• You might think that fire is a gas or some object like that. But it's actually a chemical reaction -- one that's strong enough that you're able to see it happening.

• A fire needs three factors to exist
• Heat -- often the friction provided by striking a match
• Fuel -- anything that will burn: wood, paper, clothing, gasoline, oil, etc.
• Oxygen -- we need 21% oxygen in our air to breathe. Fire needs only 16%.

This is what's known as the fire triangle, or the three factors necessary for a fire to exist.
(Image from University of Oklahoma Police Department)

• Take away any of these three, and the fire will go out or won't start in the first place.
  

• If you want to build a fire in your fireplace, start with lighter-weight materials with lots of surface area. More surface area means more oxygen. Thin twigs or flat chips of wood, or even strips of newspaper are especially good items to start with. Once those are burning, you'll need to give the fire more material, so you'll want to add slightly larger pieces of fuel to match the fire's increasing heat and energy.

As everyone knows, toasting marshmallows is one of the best ways to make use of an outdoor fire.
(Photo sourced from Love to Know: Camping)

• Whether a fire is large or small, you can usually hear a noise, like a whooshing sound. Sometimes it's pretty loud, other times it's kind of faint. Sometimes there are also great big pops or explosions. These noises are all caused by the same thing: whatever material is burning has little pockets of air or other gases inside it. When the fire creates enough heat, those pockets of air will explode out of the material and make that whooshing or popping or gigantic ka-boom sound.

• Fire also often produces steam. Wood, a common fuel of fire, often has a lot of water trapped in it. As the fire heats the wood, the water trapped inside heats until it boils and turns into a gas, or steam.

• Metal rusting and something burning are actually the same kind of process. Both are kinds of oxidation, or when air combines with atoms in the material and the reaction releases energy. In burning, though, it all happens a lot faster, with greater energy, and at higher temperatures.

Actual combustion occurs at the outside of the flame, where it appears blue, specifically, at the very base of the flame where the arrow marked BLUE is pointing. Temperatures there can reach 2700 degrees F, even around a candle wick.
(Diagram from Gresham High School Sophomores' Fire Succession page)

• As fire burns, the air around it is doing all kinds of things. Air heated by the flame rises (which is why fires tend to burn what's above them). The motion of the hotter air traveling upward allows for cooler, more oxygen-rich air to flow in underneath it -- which just happens to be the spot where the fire needs more oxygen to keep burning. In this way, a forest fire can generate winds up to 100 miles per hour, or greater.

• Most wildfires, though, travel about 14 miles per hour. Which is still pretty speedy.

• This movement of air currents around the flame is also what makes the flame of your candle flicker. Of course, air traveling through the room for other reasons will have that effect, too. But even if there are no drafts in the room, your candle flame will flicker.
  

• On another planet or on the moon, where gravity would differ or where there would be no gravity, fires would burn very differently. Gravity is what makes cool air sink while hotter, lighter-weight air rises. Without gravity, the oxygen-rich air would not cycle down to the base of the flame and thus the fire would not self-perpetuate. The flame would not flicker, nor would it even have a pointy tip.

These four flames are each burning various combinations of methane, oxygen, and nitrogen, all at extremely low levels of gravity. The differing combinations of gas are what cause the flames to be different colors. Low gravity makes the flames round.
(Photo by Jason Taylor and Richard Axelbaum, posted at CSSCI)

• In movies, you see people swatting at fires with bath towels or blankets. This can work sometimes, if the fire is small enough, and if there aren't any flammable liquids like gasoline involved. But if there are accelerants or electrical equipment involved, the towel-swatting method will only make the fire spread.

If there are towels in that dryer, they won't do a thing to stop this fire.
(Photo from Globe and Mail)

• In general, how do you know when you can put the fire out, and when you should get the heck out of there? It's probably an instinctive response, but just in case you have time to think about it, here's when you should not stay and fight the fire, but get out as fast as you can:

• If the fire is spreading
  
• If you can't fight the fire with your back to an escape exit
• If the fire can or soon will block your only escape
• If the fire is too big for whatever fire-fighting tools you may have.
  

One of my favorite fire photos ever. This is from Joel Sternfeld's American Prospects. Nothing online does this -- or any of his photos -- justice. If you ever run across this book, take some time to flip through it.
(Photo sourced from Personism)



Sources
Robert Kunzig, "The Physics of . . . Fire," Discover, January 1, 2001
AllExperts, Physics, Fire, Why does fire make noise?
PBS Nova, On Fire by Rick Groleau
Tom Harris, How Fire Works, Howstuffworks
A Moment of Science, Indiana University, Fire
University of Oklahoma Police Notebook, Everything You Wanted to Know About Fire Safety