ASK YOUR SCIENCE TEACHER

QUESTION: How does a toaster work and know when the toast is done?

ANSWER: A toaster, like so many things we use in everyday life, seems quite simple but really has some useful and incredible science principles behind it. Indeed, the wire in the toaster gets red-hot, but the wire cord leading to the toaster remains at pretty much room temperature. It must have something to do with the kind of wires used. The “secret” of the toaster is the nichrome wire that is wrapped around thin sheets of mica. You can see the red glowing nichrome wire when you peer down into the toaster. Nichrome wire is an alloy, a mixture of nickel and chromium. It’s the same kind of wire that is used in hair dryers, clothes dryers, electric stoves and electric space heaters.

Nichrome has two properties that make it a great producer of heat. The first is that nichrome has fairly high resistance to electrons. We know that everything is made up of atoms, the tiniest particles known. Within the atom, tiny negative electrons orbit a dense positive nucleus—much like planets orbiting the sun. In nichrome, some of these electrons are not behaving. Instead of orbiting, they migrate through the wire and bump into atoms, causing them to vibrate. That vibration, jostling and electrical resistance heat up the wire, throwing off infrared radiation. Infrared radiation dries out the bread and chars the surface of the bread. Just add peanut butter and jam!

The second unique property of nichrome wire is that it will not oxidize when heated. That means it can get quite hot without wasting away and burning through. Also, nichrome won’t rust easily, so you won’t have metallic-tasting bread.

How does the toaster know when the toast is done? The older method uses a bi-metallic strip. A bimetallic strip is used to convert a temperature change into mechanical movement. Two strips of metal, brass and nickel, are fused or sandwiched together, each having a different expansion rate. The different expansion rate forces the metal strip to bend one way when heated. Brass expands more than nickel, so brass is on the outer side of the curve. As the strip heats up, it bends and eventually releases a switch, and up pops the toast. The bi-metallic strip works much like a thermostat that turns our furnace on and off.

Newer toasters use a simple circuit with a microchip, charging capacitor and electromagnet. The darkness control (light or dark toast) is a variable resistor. The capacitor charges through the resistor, and then when the correct voltage is reached, the electromagnet is released and up pops the toast. That same kind of mechanism is used to regulate windshield wipers.

The toaster arose out of a need to make toast without a burning fire, and the first ones were made by General Electric in 1909. They had a wire-heating element on a porcelain base and the toaster needed to be unplugged or else the toast would burn. The pop-up toaster came later.

Scheckel

Sources: hotstuffworks Send comments and questions to: [email protected]. Larry Scheckel is a retired Tomah High School physics teacher.

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