At 05:02 PM 4/22/2012, [email protected] wrote:
Here's a little conundrum that has troubled me for some time.
Take a cup of gasoline and place it in open sunlight. It will slowly
evaporate.
Bring a flame near it and it will suddenly ignite.
Why don't the UV rays from sunlight cause ignition?
I don't know the specific energies involved, but my sense of this is
as follows:
First of all, yes, it's obvious: gasoline does not ignite just
because an individual molecule is oxidized, as it will be if, say, a
cosmic ray with hign energy hits it. You can have a mixture of
hydrogen and oxygen, an "explosive mixture" and it will just sit
there, even if you expose it to, say, a few high-energy photons or
energetic particles.
There is obviously a "hump" to get over to allow the oxidation
reaction to occur. Under conditions well below ignition temperature,
if a single reaction occurs, it cannot raise the temperature of the
local environment enough, the products are immediately cooled.
However, at -- or very close to -- the ignition temperature, only a
little extra heat is needed. The closer to ignition temperature, the
less the needed heat, until, at ignition temperature, the oxidation
starts happening en masse, it all heats up and a wave of ignition
passes through the material.
Badly explained, perhaps. But the basic idea is that at low
temperatures, a little puff of heat doesn't do anything. A single
reaction is just that, it has no observable effect.
UV light probably isn't energetic enough, by the way. I don't think
it would be absorbed by the gasoline. But I don't know.
The gasoline, by the way, doesn't ignite just because you bring a
flame near it, not directly. Rather, the vapors will ignite from
contact with the flame, and the ignition, from that mixture of
gasoline and air, can ignite the gasoline.
