Induction cooking: barefoot, with a metal spoon

[veek]

Anyone know anything about induction cookers? Induction cooking? What if the lady sticks a metal spoon to stir the pan and is bare-foot? What prevents the eddy current from breaking out of the loop given the new path your body provides?

 

[Studiot]

The lady would be cooking red herrings

Induction cooking only works with special saucepans. It does not affect most metal pans.
The lady would be quite safe, although it is possible the spoon might get hot if it was the wrong material. Barefoot has nothing to do with the price of bacon.

Oh and welcome to Physics Forums

 

[veek]

hi, glad to be here. That doesn't sound right: http://en.wikipedia.org/wiki/Induction_heating (special pans are metal/ferromagnetic pans)

What I don't follow is why the current won't leave the vessel and flow through you..

 

[veek]

Or perhaps you do get a bad shock if you try to cook barefoot, with a metal spoon.. (but that sounds rather dangerous - like cooking with a live wire) does anyone have such a cooker? you could test with a multimeter..

 

[Dale]

What I don't follow is why the current won't leave the vessel and flow through you..

How could it leave? There is no voltage to drive it and no loop to ground.

 

[veek]

I'm thinking in terms of Lenz's law and back e.m.f; that eddy current is setup because of a need to oppose a changing magnetic field - it would then have a resulting back e.m.f/voltage associated with it. Now your body is at 0V therefore there is potential difference between the base of the plate and your feet.. base-of-plate-with-backemf-eddy-current-->metal-spoon-->body-->ground
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I was hoping someone might mention skin-effect and it somehow restricting/holding/forcing the current to the bottom of the pan with a kind of dead-zone between the top-side of the base-of-pan and the actual area where the eddy current is flowing.. that might make sense, but I'm not sure..

 

[likephysics]

wow! I got an induction stove couple of weeks ago. I had the exact same question lingering in my mind for a week and was about to post the question.

I don't think the stove coil is small enough to induce currents in a spoon.
Even when I touch big sauce pans, I don't feel anything except for vibrations.
Stainless steel pans do work on induction stoves.
Anyway, If the current breaks the loop, then it wouldn't oppose the mag field, therefore breaking lenz law.
Any other explanations?

 

[Studiot]

Unlike a microwave, an induction cooker heats the container, not the food. But only certain containers.

We have some Stellar brand stainless steel saucepans, bought over 20 years ago and also some cheap aluminium ones.

None work on our induction hob.

We also have a couple of nearly new Stellar stainless saucepans, outwardly identical to the older ones.

These both work fine on the hob. The blurb that came with them states they have a layer in the baseplate for this.

 

[Dale]

Btw, I have an induction cooktop and it is really great. My wife seems to prefer cast iron cookware using it.

I'm thinking in terms of Lenz's law and back e.m.f; that eddy current is setup because of a need to oppose a changing magnetic field - it would then have a resulting back e.m.f/voltage associated with it.

Lenz's law really doesn't have anything to do with it other than to determine which direction the current flows inside the pan.

Regarding the back e.m.f., in order to get that you must have an open loop where current cannot flow, then you will get a voltage across the ends of the loop which opposes the induced current and stops it. But if you close the loop then the induced current flows and there is no back voltage to oppose it. This is the situation with the pan.

Now your body is at 0V therefore there is potential difference between the base of the plate and your feet.. base-of-plate-with-backemf-eddy-current-->metal-spoon-->body-->ground

There are two problems wrong with this. The first is that the pan (as described above) does not have a back e.m.f. voltage and is also at 0V so there is no potential difference. But the second problem is that what you describe here is not a circuit. So what is the other side of the circuit which would get the current back to the pan? Remember the pan is sitting on a glass cooktop and glass is a very good insulator. So all you would get if the pan were at some non-zero voltage is a brief static discharge which would be unnoticeable unless it were at several million volts.

There are only two mechanisms of inducing current through your barefoot wife in this scenario. The first is through induction, but the inductive coupling between the cooktop element and your wife is very low (by design), so that is not significant. The second mechanism is through Ohmic conduction, and that is zero since the pan is not at a non-zero voltage and there is no circuit. Your wife may safely cook using metal spoons, even barefoot, etc.

 

[veek]

I think, I've figured it out a bit: my understanding of back-e.m.f was wrong. The standard interpretation of back-emf is that it's a voltage.. similar to the stuff below..

http://electricalandelectronics.org/2009/09/04/back-or-counter-e-m-f/
"When the armature of a d.c. motor rotates under the influence of the driving torque, the armature conductors move through the magnetic field and hence e.m.f. is induced in them as in a generator The induced e.m.f. acts in opposite direction to the applied voltage V(Lenz’s law) and in known as back or counter e.m.f. Eb. The back e.m.f. Eb (= P Φ ZN/60 A) is always less than the applied voltage V, although this difference is small when the motor is running under normal conditions."

The wiki however says this:
http://en.wikipedia.org/wiki/Electromotive_force

"A sixth group applies the term to electromagnetic induction. These authors define e.m.f. as the spatial line integral of the electric-field strength taken over a complete loop. To them the term 'counter e.m.f.' means something."

"It is common in some fields, such as circuit theory, to refer to the voltage created by the emf as the emf. Some authors do not distinguish between the emf and the voltage it creates. Some use emf to refer to the open-circuit voltage and voltage to the potential difference when current is drawn."
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If you treat back e.m.f as a voltage, then you'll go wrong because you'll think that the pan is at X-volts and there is a return path through the ground. On the other hand, if you treat back e.m.f as a kind of force encouraging free electrons already in the plate to run in circles creating heat (the line integral of a field bit) then you get the right interpretation.. the pan has no additional charge buildup on it.. it's essentially a neutral body it's just that the emf is causing its electrons to move in a narrow region (skin effect) generating heat..

Only if the electric field direction somehow changes and takes a path through the lady, only then, will she fry.. but since the electric field here is closely linked to permiability and the path of the magnetic flux.. no chance of that happening.. unless the lady's hand/spoon is more permiable and even then the induced current will take the direction of the mag field.

So i think this would be correct..

 

[Studiot]

You do not get a high (=significant) electric field with low resistance paths.

An induction heater is a high current, low impedance (resistance) device. This generates very little electric field, but a high magnetic one.