You can do this experiment at home with just a few things. 9 Volt battery, steel wool, non-flammable surface, and some adult supervision.
After fluffing up the steel wool a bit place it on your non-flammable surface, then simply touch the 9 volt battery to it. It’s that simple! But what’s really happening? When you touch the 9 volt battery to it, the steel wool makes a completes the circuit of the battery.
Steel wool is made mostly of iron which is extremely conductive. The surge of electrical current from the battery heats up the iron which causes it to react with the oxygen in the air surrounding it. Electrical current is the flow of charged particles which collide with the iron atoms, slowing them down. The slowing down of the charged particles is also known as electrical resistance.
The collisions create heat. The hotter something gets, the more electrical resistance it will have. It heats up so much, that it creates a spark. The release of heat from the spark causes a new piece of iron to heat up, react, spark, release of heat… and creates a chain reaction that can continue even when you remove the battery from the steel wool.
This reaction creates iron oxide, FeO2. The hotter the steel wool gets, the more likely the oxidation reaction will happen. It can burn, unlike a nail, screw, knife, or forks because of the increased surface area ratio. There is more oxygen available in a given area for it to react with, and there can be more of a heat build up in a smaller area.
To start a fire, you need three ingredients: fuel, oxygen, and heat. Friction is a good source of heat (think striking a match, or rubbing two sticks together). Heat can also come from flowing electricity! Have you ever noticed that electronic devices get warm after they’ve been running for a while? This is the result of resistance.
Resistance acts like friction between electrons and the medium they move through. It keeps electricity from moving freely and infinitely, and generates heat as a byproduct. Some substances have lower resistance than others (these are called conductors), but all everyday materials have some resistance.
We can use household supplies to explore this property! On a fireproof surface, hold the business end of a 9v battery up to a pad of steel wool. When a strand of wire touches both terminals, it completes a circuit. Electrons flow through the loop created by the battery and the steel. The wire resists the electrons’ movement and warms up dramatically. We have our heat source, and there’s plenty of oxygen around– but what about fuel?
Steel is mostly made of iron. We know that iron interacts with oxygen. When water is involved, it rusts. When heat is applied, it has the potential to burn. Iron doesn’t catch fire under normal circumstances because its inner molecules are shielded from the surrounding oxygen, but the tiny filaments in steel wool have an enormous surface area per volume. So much iron is exposed to the air that a combustion reaction can start!
So, we have fuel. We have oxygen. We have a heat source. Electrical resistance can start a fire just as well as friction, and with less effort on our part! The spark is easy to create, and it travels quickly along the tiny wires.
An infrared camera shows heat propagating through the steel wool.
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