Fires start in a variety of different ways. The ingredients of fires
are fuel, oxygen, and something to get the burning reaction going. This
last item is needed because while the oxidation reaction that is
burning releases energy, it needs a little activation energy to get it
going. Once going, the energy released by the burning fuel is enough to
supply the activation energy needed to ignite other fuel, and the fire
burns until all the fuel is consumed or all the oxygen is consumed. You
can put out many fires by throwing water on them -- this takes away the
heat needed to ignite unburnt fuel. Wet fuel (wood, for example) needs
to dry out first before it can burn, and evaporating water takes 540
calories per gram.
Once the fuel and the oxygen are there, and it's not too wet, then
just about any little addition of energy can get a fire going. Examples
1) a match -- this supplies heat energy to ignite fuel. Variations
on this theme are cigarette lighters and pilot lights in older gas
stoves. A forgotten cigarette can cause a house to burn down.
2) an electrical spark -- newer gas stoves ignite their fuel with
electrical sparks. Spark plugs in cars get the gasoline burning in the
engine's cylinders. Lightning strikes can cause forest fires and
3) Heat from focused sunlight. I'll confess I used to burn bugs
with a magnifying glass when I was small. You can ignite paper or wood
with this too.
4) Flint and steel. It's not the flint, but the steel, which makes
hot sparks when little bits of steel are forcibly pulled off of a
bigger chunk of steel. You also see this when grinding steel with a
grinding wheel. Or in movies, metal parts of cars scraping on the
ground during high-speed car chases make nice sparks. These little
glowing-hot bits of steel are enough to ignite flammable substances.
5) Bacterial metabolism heat. Grain that's stored in a big silo or
grain elevator has been known to get very hot because bacteria feed on
the dust and produce heat as a byproduct. The big pile of grain makes
an excellent insulator for heat, and the temperature can build up high
enough to cause the grain dust to burn and even explode. Grain-storage
silos often have cooling fans to help counteract this.
This effect probably was the cause of the disastrous explosion of
a truck carrying flour through the Mont Blanc Tunnel between France and
Italy in the late 1990's. The truck's cargo spontaneously exploded in
the tunnel, creating a larger fire as the gasoline and diesel fuel of
other cars which collided in the pile burned.
6) Meteors, when they strike the ground, can start fires too.
Can you think of other ways fires can be started? Or more importantly, stopped?
(republished on 07/13/06)