Re: How come when you rub steel wool to 9 volt battery it starts on fire?
Date: Sat Mar 14 00:42:47 1998
Posted By: William Beaty, Electrical Engineer / Physics explainer / K-6 science textbook content provider
Area of science: Chemistry
The steel wool catches fire for two reasons:
- If you touch a thin wire across a 9v battery's terminals, you have made an electric heater.
- Steel is a flammable material!
WARNING TO READERS: DON'T CONNECT A WIRE ACROSS A 9-VOLT BATTERY TERMINALS. IT MAKES THE BATTERY GET HOT AND GO DEAD. IT ALSO MAKES THE WIRE HOT, AND CAN CAUSE BURNS AND MAYBE FIRES. IT CAN MAKE LITHIUM BATTERIES BOIL INSIDE AND BURST
When you touch a thin wire across a 9-volt battery, charges inside the wire flow fast. The flowing charges heat the wire because of a type of electrical friction. It can get white hot and catch on fire. The long answer is below.
All metals are made of atoms, and atoms are made of positive particles surrounded with negative particles. The negative particles of the atoms in metals do not stick to single atoms. Instead they can wander around inside the metal. All the metal's electrons together act like a liquid. This liquid is visible: it makes metals opaque, and it gives them their silvery color. This silvery electric liquid could move if we could push it along. But we certainly can't make it move with our fingers! If the wire's electric liquid moves, we call that motion an Electric Current.
Imagine that you have a complete circle of wire. Suppose that there was a way to push the "electron liquid" along. You could push a little part of the liquid along, and this would make all the liquid move inside the wire like the moving rim of a wheel. A circle of wire is like a tube which contains a movable belt. The word for this is Electric Circuit.
Electric charge pump
There is a way to push the electric liquid along. A battery acts as an "electron pump". If you connect a thin wire across the terminals of a 9v battery, then you have created a metal circle. (The insides of the battery act like part of the circle.) The battery sucks electrons from one end of the wire and spits them out into the other end. Charges get pushed through the inside of the battery. It causes the "electric fluid" to flow along. We cannot see anything flowing. After all, there are no bubbles or dirt moving in the silvery "electric liquid"
If you rub hands together very hard, they will grow very hot. This happens with electric fluid in wires too. The battery push hard on the electron-liquid in order to force it to move ahead through the thin wire. The electron-liquid rubs on the metal as it flows. The metal gets hot. This is how all electric heaters work and all incandescent light bulbs work. Electric current inside wires always makes the wire warm. If the electric current is fast enough, the wire will get so hot that it glows red or even white hot.
Steel can burn. It can be used as fuel. But there is a problem. It will only catch fire if it is first heated white hot. It takes a lot of energy to heat a big piece of steel white hot. Normal fires are not hot enough to do this.
It is easier to heat up tiny bits of steel until they catch fire. This happens often when a power grinder is grinding some steel. We see sparks fly. These are not electric sparks. Instead they are tiny bits of steel which have become white hot and are burning. Have you ever used flint and steel to start a fire? Same thing. Tiny bits of metal get heated white hot, and they catch fire.
When the steel wool touches the 9-volt battery terminals, the battery forcefully pumps the electron-liquid within the steel. There is an electric flow inside the thin steel wires, and they heat up. They grow white hot and catch fire, and the steel wool burns. This is an excellent way to start a campfire if you have no matches. But remember, it drains energy from your 9-volt battery, and you can only start a few fires before the battery stops working. Better learn fire-starting with a "friction bow" or "fire drill." There might not be any batteries for sale in the middle of the woods!
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