I had an electric kettle that broke. It would not turn off after the water boiled, which means that the mechanism to switch off the kettle when the water boiled had stopped working. It led me to break inside the electric kettle to figure out what was wrong and repair it if I could. What I saw was a complete surprise.
I was expecting to see a temperature sensor that would detect the temperature of water and complex electronics that would interpret and send a signal to switch off the electric kettle once the boiling point of 100 °C was reached. But none of those fancy electronics were present inside. What I saw was a piece of a coin-sized thin metal, and that was it. The rest was utter ingenuity.
Instead of electronics engineering, electric kettles rely on simple physics.
To boil water using an electric kettle, we just put in some water, plug in the power, press the switch, and then wait until the water boils and the kettle turns itself off by reversing the switch. Amazing. But how does it do this without temperature sensors to detect the water’s boiling point? It turns out that it does not detect the water’s temperature. What it detects is the actual boiling of water. You might ask, how does it detect the boiling without detecting the boiling point? That’s a great question with a great answer — physics!
When water boils, my common sense tells me that the temperature is the only physical indicator that it reaches its boiling point. However, there is another obvious physical characteristic of water related to boiling that I never considered. That is, vaporization! Liquid water turns into a gaseous state of steam when it reaches boiling point, and this is the mechanism that the electric kettle’s simple design takes advantage of.
The electric kettle’s design
An electric kettle has 3 main actors in its spectacular display of brilliant engineering.
- The heating element – This is the heat source connected to the kettle’s base plate for heating the water. This is directly powered by AC power on wall outlets.
- The bimetallic strip – This is responsible for the kettle’s automatic switching off. It is composed of two different metals that expand at different lengths when subject to heat. An example is the combination of steel and brass. Steel has a low coefficient of thermal expansion, meaning it expands less when heated. Brass, on the contrary, has a higher coefficient of thermal expansion, meaning it expands more when subject to heat. Because those two metals are bonded together in parallel, the different rates of expansion of both metals would cause them to curl or bend.
- The hollow path for the steam – This is the space where the steam gets through to reach the bimetallic strip.
The electric kettle’s working
This is how electric kettles work:
- The electric kettle is plugged into the power supply.
- The power switch is pressed to turn on the heating element. After a while, the water boils.
- The boiling water lets off steam. This steam is a hot gas so it fills all open space in the kettle. It passes through the hollow path leading to the bimetallic trip.
- The bimetallic strip is heated by the steam and it bends. This bending action triggers the switch to revert to its “off” position, cutting the power supply, and turning off the kettle.
- When the kettle turns off, the water stops boiling and the steam cools down to water droplets. The absence of steam causes the bimetallic strip to cool down and straighten again to its original form.
- With the kettle turned off and the bimetallic strip back to normal form, the kettle is ready to be used again!
The electric kettle does not detect the water’s temperature. It detects the presence of steam!
I just can’t help but admire this engineering design with a brilliant application of physics. This is the reason I enjoy learning and writing.
Btw, I am glad that electric kettles are simply designed like this, and not with temperature sensors to detect the water’s boiling point. Find out why here.
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