Comments on: A Pi Day Physics Lesson on the Induction Cooktop — With Dancing!

By: Bobby Rhett

Bobby Rhett — Tue, 01 Apr 2014 03:33:04 +0000

On an experimental level, the
On an experimental level, the watch thing is a non-issue. I was actually more curious about the field interfering with Wi-Fi or cell phone signals, but I tried it with digital and mechanical watches too. I left a cheaper digital watch on the cooktop (adjacent a pan smaller than the element, because the range won't run without a pan, assumedly by measure of resistance in the coil). No effects noted. A metal watch on my wrist I couldn't get to heat to the touch. Off my wrist, I got the watch band on the element (alongside a pan as before) to heat, but that took some work.

So far, with my cooktop, the most interesting experience has been leaving a potato masher flat on the bottom of a pan and seeing it heat up to the handle. So, just like electric or gas, all-metal utensils might heat up if left flat on the bottom of the heating pan.

No real answer yet on the cellular and Wi-Fi interference. I am waiting for access to good EMF and RF meters to test that, alongside the microwave. But I have not observed any difference on Speedtest.com with my desktop Wi-Fi, with either the microwave or the induction cooktop.

By: Allison Bailes

Allison Bailes — Sat, 15 Mar 2014 17:34:08 +0000

Dale S.:
Dale S.: Good point, but in calculating the circumference of the universe, the uncertainty of the other variables would prevent ever knowing the answer to within the width of a hydrogen atom, no matter how many digits of pi we used. It's fun to think about though.

Tom: Thanks for the extra clarification. The field strength actually falls off even faster than you say. Rather than just decreasing linearly with distance, it falls off with the square of the distance. So when you double the distance, the field is a quarter of its original strength. What this means, as you said, is that you'd really never have to worry about your watch heating up.

John C.: Ah, the question of how much of the heating is due to the induced eddy currents versus heating from the magnetic hysteresis in ferromagnetic materials. I probably should have gone into that part of the process, and now you've caused me to spend an hour searching for the answer. As is true of most online information, I find conflicting answers to the question. Clearly the hysteresis plays a role, though, because an induction cooktop is much more effective with ferromagnetic cookware. But the induced eddy currents certainly contribute. Do you have a good reference on this topic?

By: John C.

John C. — Sat, 15 Mar 2014 15:06:44 +0000

Nice article and videos ab3,
Nice article and videos ab3, but you are missing a key point.

The induction heating in cooktops is NOT primarily due to induced eddy currents in conductors, but rather, due to the fact that flipping spins in ferromagnetic materials is often a lossy process.

That is, if you put a piece of iron in an AC magnetic field, a certain amount of heat is generated every time you flip the magnetic polarity of the iron back and forth. Do it at high frequency, and you can create a lot of heat.

This is why induction cooktops only work with pots and pans made of iron, or containing iron plates in their base.

By: Tom

Tom — Sat, 15 Mar 2014 09:04:36 +0000

It isn't really a problem if
It isn't really a problem if you wear a watch.

The induction effect falls off with field strength, and thus it falls off with distance. Your range hood only heats up a negligible amount, and likewise for your watch.

What's more, the induction effect is also restricted to the area directly above the cooktop. If you try using a pot that's larger than the coil, the water will start boiling from the area directly above the coil.

As you're cooking, your hands will be moving in and out of the induction "cylinder", so to speak. That will further reduce the heating effect.

By: Dale Sherman

Dale Sherman — Fri, 14 Mar 2014 23:58:58 +0000

Pi calculated to 39 decimal
Pi calculated to 39 decimal places is enough to calculate the circumference of the universe to within the width of a hydrogen atom.
NASA keeps space stations and GPS satellites in precise location with only 15 or 16 digits of Pi. 3.14 serves the rest of us quite sufficiently.
And Happy Birthday to Albert Einstein!

By: Allison Bailes

Allison Bailes — Fri, 14 Mar 2014 18:47:52 +0000

Carl C.:
Carl C.: Never is a strong word. I assume you didn't take it out to very many decimal places.

Armando: You rock!

james f.: I would take you up on that challenge, but I'm still trying to perfect my Turbo Thermo-Encabulator Max.

stephen d.: That hadn't occurred to me before, but yes, induction cooktops would be safer, too, not only from someone burning themselves but also from someone accidentally leaving a burner on and starting a fire or filling the house with gas.

Charles: Yeah, if you were wearing a watch made of an appropriate metal and you put it right there on the cooktop, it would get hot.

Mike M.: Yes, I'm totally irrational. My imaginary friends are always telling me that. ;~)

David F.: That was just a test to see if anyone was paying attention. ;~) Actually, I think there was a disconnect between my brain and my fingers because I never forget those 4 digits.

By: David Fay

David Fay — Fri, 14 Mar 2014 18:33:02 +0000

"Today is Pi Day, named
"Today is Pi Day, named for that special number, 3.14159265358797323846264338327950288..."

Oops. That should be 89793, not 87973.

By: Mike MacFarland

Mike MacFarland — Fri, 14 Mar 2014 18:19:04 +0000

Allison, your posts are
Allison, your posts are sometimes so irrational. ;-)

"The first follower is what transforms a lone nut into a leader." Sure brings the critical role of Facebook fans to light, eh?

By: Charles

Charles — Fri, 14 Mar 2014 18:17:11 +0000

So if I was wearing a metal
So if I was wearing a metal watch – or my wife a metal bracelet, and our wrists were in close proximity to the “burner”, our wrists would heat up by induction? I won’t even consider what that would do to an LED watch – I was just thinking of the old fashioned mechanical watch my father used to wear.

By: james ferguson

james ferguson — Fri, 14 Mar 2014 18:14:52 +0000

The thing I always loved
The thing I always loved about Maxwell's equations is they apply to any "irrotational flow" of which water is a fine approximate example.
So you can build anything with water that you can build with electronics (it just gets a bit bigger and messes up the kitchen a lot).

Examples :
Voltage = Potential Difference = Water tower

Current = Rate of Flow = Current

Induction = angular momentum = whirlpool (or stirred coffee)

Capacitance = Energy Store = Diaphragm

So Allison - my challenge to you is to build a mobile phone using conventional kitchen utensils ;)