Grades 6–8 · Everyday Life
How does a kettle know when to switch itself off?
An ordinary automatic kettle does not need a computer to notice boiling. In many designs, a small passage carries steam from above the water to a temperature-sensitive control. The steam transfers heat to that control.
In the diagramSteam heats a tiny temperature switchJoined metals expand differently; heat trips the mechanismBoiling waterSteam pathTwo bonded layersContacts separatedFree mismatch: 0.096 mmFree-expansion example; actual snap shape is not solved.
The control contains bimetal: two different metal layers firmly joined together. Because their thermal expansion rates differ, heating makes the joined piece change shape. A shaped bimetal disc can snap through suddenly. Its movement trips a mechanism that separates electrical contacts, interrupting current to the heater.
The diagram enlarges the two-layer idea rather than calculating the snap of a real disc. For separate hypothetical 100 mm layers with an expansion-coefficient difference of 12 millionths per kelvin, an 80 K rise would create a 0.096 mm free-length difference. Bonding prevents free sliding, so this mismatch produces bending and stress instead.
The boiling control responds to heat delivered by steam, not steam pressure pushing a switch. A separate overheat protection system commonly guards against an empty kettle. Actual controls have carefully engineered shapes and operating conditions; the example is not a switch calibration.
About this explanation
Starting knowledge: Understand gravity, heat and a simple electrical switch.
Sources
- Strix — Kettle controls: AIM admission document
- DigiKey — Disc Bimetallic Thermostats