Posted on 03 November 2011 at 13:36h

When I go camping I like to start fires with my "survival professional" flint. Actually what I do is light the gas fire with the flint then light the wood fire with the gas fire so a bit of a cheat really. I was wondering how the flint works to produce sparks so applying simple physics:

Work is done against friction when the flint moves across the blade, this work is converted to heat which melts the iron causing it to glow, simple. But how much work is done and is it enough to melt the little bits of iron? And another thing why do you have to have a special sort of iron?

I guess that the force needed to move the flint is in the region of 1N and the distance moved 1cm so W = 0.01J.

To make the iron glow it would have to be about 1000°C. It's specific heat capacity is 460 Jkg^-1K^-1 so about 2 x 10^-8 kg of iron would be made to glow assuming the iron got all the energy. Density of iron = 7000 kgm^-3 so the little bits of iron would have volume of 1.7 x 10^-11m³ if the bits a spherical they would have a diameter = 0.3mm. This is a bit big but shows that it's possible.

Unfortunately it's completely wrong.

What actually happens is the freshly ground iron filings spontaneously ignite as they oxidise in the air. So it's nothing to do with work done and friction. That's why you need the special flint. I was begiining to wonder how all the energy managed to get concentrated into the little scrapings of iron.

Survival topics