Why does glass not conduct heat

health : Why can you cook on glass?

At first glance, the matter seems pretty transparent. You turn on the ceramic hob and after a short time you can see a heating coil lying under the glass. She glows. Red. But how does the heat reach the pot?

The traditional electric stove has cast iron plates. The metal conducts heat well and transfers it to the pan or kettle. Glass, on the other hand, is an extremely poor conductor of heat. Anyone with a ceramic hob can check that: the glass next to the hob stays cold. Although the surface consists of a continuous sheet of glass, the heat hardly spreads in it.

The fact that the pot gets hot even faster has to do with another property of the glass: it is transparent to heat radiation. The more the heating coil glows, the more infrared radiation it emits.

This passes through the glass largely unhindered, hits the pot and heats it up directly. The energy consumption of the ceramic hob is so low because you don't have to heat the hotplates first and little heat is lost.

None of this would be feasible with conventional glass. Because the glass itself also gets hot, of course, not least because of the direct contact with the pot. Ordinary glass expands when heated and contracts when cold. Since only individual areas are heated on the stove, the glass plate would break quickly with the resulting temperature differences.

Ceran hobs consist of a glass ceramic. "Finely distributed crystals are embedded in the glass material, which are so small that it remains clear and transparent," says the physicist Lutz Klippe, product manager at Schott in Mainz, which has been developing the hob since 1972. Unlike glass, the crystals do not expand when exposed to heat. "They're shrinking."

Some atomic particles in the crystal move to positions where they take up less space. When they cool down, they jump back in place. If the crystals are mixed in the right proportion with glass, the result is a material that hardly reacts to temperature changes of up to 700 degrees Celsius.

Astronomers also benefit from this. The mirrors of a telescope should also not contract and distort the image if the temperature drops drastically at night. The special glass didn't just earn three stars in the kitchen.

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