Why can the induction cooker heat up the hot pot, but people’s hands don’t get hot when they put them on it?

When you go to a hotpot restaurant to eat hotpot, there are basically two ways to heat the hotpot: the "copper pot hotpot" that uses charcoal fire directly, and the other is the induction cooker without open flames. Some netizens have discovered a phenomenon that when the induction cooker starts to heat the pot, it doesn't even get hot.

Is it true that the induction cooker will not be hot when it is heating? If your hands cannot feel the heat, how can it heat the pot? Today, let's talk about induction cookers.

Electromagnetic induction

The principle of the induction cooker is an important knowledge point in middle school physics - electromagnetic induction, which is the nightmare of many "physics scumbags". Although it is a headache to learn, it is extremely widely used in daily life, from various power stations to induction cookers for cooking.

Simply put, the electromagnetic induction phenomenon refers to the generation of an induced electric field around a changing magnetic field. If there is a conductor in this electric field, the electrons inside the conductor move in a directional manner, which can generate an induced current.

The internal structure of an induction cooker. Image source: Wikipedia

Inside the induction cooker, there is a large copper coil, which is used to generate a changing magnetic field. The frequency of this magnetic field is generally above 20kHz (that is, the number of changes per second exceeds 20,000 times). Such a rapidly changing magnetic field will generate an induced electric field in the area perpendicular to the surface of the induction cooker.

If a conductor, such as an iron pot, is placed in this induced electric field area, countless induced currents will be generated at the bottom of the iron pot. These induced currents are called eddy currents.

With eddy currents, the iron pan itself has a certain resistance. According to Joule's law, these currents will generate heat at the bottom of the iron pan, thereby achieving a heating effect.

Not all pots

Can be heated with an induction cooker

If you have an induction cooker at home, you will find that not all pots can be heated on an induction cooker. The pots that are usually used for induction cookers are iron pots and stainless steel pots. Clay pots, ceramic pots, aluminum pots, copper pots and the like cannot be used on an induction cooker.

It is easy to understand why casserole and ceramic pots cannot be heated by induction cookers, because as we have mentioned before, eddy currents can only be generated when a conductor is placed in a changing magnetic field . Clay pots and ceramic pots are obviously not conductors, so they cannot generate heat.

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Aluminum pots and copper pots are both conductors, so why can't they generate heat?

They can indeed generate induced current in a changing magnetic field. However, they are not ferromagnetic materials, and their magnetic permeability is too low to generate strong enough eddy currents . In addition, their magnetic permeability is too low, their skin depth is relatively large, and their own resistance is lower than that of iron, which also makes the thermal effect of current in aluminum and copper pots not as obvious as that in iron pots. Therefore, considering these factors, pots such as aluminum and copper pots are not suitable for induction cookers.

As we mentioned at the beginning, we don't feel any heat when we put our hands on the induction cooker. This is because our hands do not meet the requirements of being a conductor and ferromagnetic. No eddy currents will be generated in our hands, and of course they will not be heated.

This is the same reason why you can see on the Internet that when using an induction cooker to heat eggs, half of the egg in the iron pan is heated and turns white, while the half that is directly in contact with the induction cooker remains a clear and transparent egg white.

The egg in the iron pan is heated, while the part directly in contact with the induction cooker is still cold. Image source: Yuppiechef

However, I would like to remind you here that although the surface of the induction cooker is cool during the heating process, you must not touch the surface of the induction cooker with your hands after the pot is heated, because the hot iron pot will transfer heat to the surface of the induction cooker, and you will get burned if you touch it at this time.

Does the induction cooker have radiation?

When searching for information about induction cookers and electric ceramic stoves, you will find a saying that induction cookers have radiation, but electric ceramic stoves do not. This statement is not reliable. Both induction cooker radiation and electric ceramic stove radiation are non-ionizing radiation, and normal use will not cause harm to the human body, so there is no need to worry about it.

Electric furnace wire stove Image source: screenshot of e-commerce platform

But there is one group of people who really need to be careful when using induction cookers, and that is people with pacemakers.

A 2006 study on the relationship between induction cookers and pacemakers found that induction cookers generally do not affect pacemakers, but under specific conditions, such as being very close to the induction cooker, the height of the induction cooker is just below the chest, and the pacemaker happens to be a specific type of pacemaker, the induction cooker may affect the pacemaker.

The British Heart Foundation's official website also has questions and answers about pacemakers and induction cookers, and recommends that patients with pacemakers keep a distance of more than 60 cm (about 2 feet) from induction cookers.

Therefore, if there is a person with a pacemaker at home, you need to be careful and consider using a ceramic stove instead of an induction cooker. If there is no such situation, you can choose any one you like.

References

[1] Sweeney, Micah; Dols, Jeff; Fortenbery, Brian; Sharp, Frank (2014). Induction Cooking Technology Design and Assessment (PDF) (Report). ACEEE Summer Study on Energy Efficiency in Buildings.

[2] https://www.allaboutcircuits.com/technical-articles/lossy-transmission-lines-introduction-to-the-skin-effect/

[3] Irnich W, Bernstein A D. Do induction cooktops interfere with cardiac pacemakers?[J]. Europace, 2006, 8(5): 377-384.

[4] https://www.bhf.org.uk/informationsupport/heart-matters-magazine/medical/ask-the-experts/induction-hobs-and-pacemakers

Planning and production

Author: Science Scraps Popular Science Writer

Review丨Sun Mingxuan, Professor of Shanghai University of Engineering Science

Planning丨Fu Sijia

Editor: Fu Sijia

Proofread by Xu Lai and Lin Lin

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