Friday is here again, and with it the weekly "A Taste of Physics" column — number 26.
This week: how to melt metal without touching it, Faraday's law, and the connection to the induction cooktop in your kitchen.
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In the video you can see a piece of metal levitating inside a coil, heating up rapidly until it melts.
This short clip involves a large number of physical processes and principles — let's touch on the main ones to understand what's happening.
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The metal is ferromagnetic. This means that the atoms composing it realign their magnetic poles when placed in a magnetic field, effectively turning the metal itself into a magnet.
An alternating current runs through the coil surrounding the metal — an electric current that reverses direction 50–60 times per second.
This electric current generates a magnetic field at the center of the coil, which through magnetic induction turns the metal itself into a magnet and causes it to levitate.
The rapid change in the magnetic field produced by the alternating current induces an electric current in the metal — a phenomenon described by Faraday's law. The direction of this induced current also alternates in sync with the alternating current.
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These rapidly alternating electric currents form swirling vortices (eddy currents), and because the metal's molecules resist the flow of electric current, some of that energy is converted into heat (Joule heating).
An additional source of heating is the rapid flipping of the atomic magnetic poles within the metal in response to the alternating current's direction. This back-and-forth motion of the atoms generates heat.
As the electric currents and atomic motion intensify, the metal's internal temperature rises until it melts.
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Heating in this way offers many advantages, making it a particularly useful phenomenon.
Metals can be heated without any physical contact — a significant advantage when high precision is required, as in the jewelry industry.
Energy efficiency is exceptionally high, since the metal heats from within and no energy is wasted heating the surrounding air.
This method is also safer, because the coil itself does not heat up — only the metal at its center does.
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You encounter this phenomenon whenever you place a pot on an induction cooktop.
The alternating current in the cooktop induces a changing magnetic field in the base of the pot, causing it to heat up.
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One final point to close with:
Why does the metal fall out of the coil when it melts, rather than continuing to levitate inside it?
The reason is that magnets lose their magnetic properties above a certain temperature, known as the Curie point.
When the metal becomes hot enough to exceed this temperature, it ceases to function as a magnet and falls from the coil.
Shabbat Shalom 😊
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