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The diameter isn't really of huge consequence. The resistance is proportional to the mass, but the surface area increases less, so the real gain comes from a slower loss of heat, which then produces a slightly higher equilibrium temperature.jimmy101 wrote:Joanna: Shouldn't that be a shorter thinner lead? A fat conductor has lower resistance than a thinner one (assuming same material and same length). The fat conductor will draw more current and dissipate more power but it has a higher mass so the temperature of the conductor rises slower.
Not to "bicker and argue about who killed who ..." but the resistance is not proportional to the mass of an object. It is proportional to the cross sectional area of the object and the length of the object. Two objects of the same material and equal mass can have different resistances. A short fat wire will have a lower resistance than will a long skinny wire of the same mass.joannaardway wrote: The diameter isn't really of huge consequence. The resistance is proportional to the mass, but the surface area increases less, so the real gain comes from a slower loss of heat, which then produces a slightly higher equilibrium temperature.
With a 9V battery, the problem is the internal resistance. Try the steel wool and steel bolt idea with a low resistance car battery, and the same incandescence will arise from both, as long as the wires you are using are of a reasonable gauge.
I was referring to it solely in terms of diameter - I thought that was clear, apologies if it wasn't.jimmy101 wrote:Not to "bicker and argue about who killed who ..." but the resistance is not proportional to the mass of an object. It is proportional to the cross sectional area of the object and the length of the object.
I was talking in fairly general terms. If the supply has no internal resistance, the fatter bolt will reach a higher final temperature, because of it's lower mass to surface area ratio.The issue here is trying to get high enough temperature (not energy or heat) to ignite a gaseous fuel. A thin wire can be heated to incandescence by a cheapo 9V batter supplying 5A or so. A similar length of 1/4 bolt can be gotten just as hot but it is going to take a car battery putting out hundreds of amps to do it (actually even a car battery won't do it for a 1/4 bolt, it would probably take a couple thousand amps.) So, as an ignition system you want the thinnest wire you can get so that you can use the wimpiest (and cheapest, and safest) power supply possible.
Actually, I did take that into account with my maths. If you do a basic calculation taking into account heat generated, rate of heat loss, and increasing resistance with temperature for conductors of different diameters, then you get a nice distribution curve that shows the maximum temperature of each of the conductors.jimmy101 wrote:The resistance of a conductor increases with temperature, this is the most fundamental definition of what a conductor is.