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.
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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?
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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.
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An infrared camera shows heat propagating through the steel wool.

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