Homework Help: Question about induction cooker

1. Jan 2, 2017

Clara Chung

1. The problem statement, all variables and given/known data
My book said that when an aluminium sheet of thickness 0.5 mm is placed on a induction cooker, it will float and be heat up.

2. Relevant equations

3. The attempt at a solution
I don't think there is a net repulsive force from the eddy current. When the B field from the induction cooker decrease in magnitude, the eddy current tends to produce a B field in the same direction as the decreasing B field in order to resist change. Therefore, the B field from the eddy current and the induction cooker are in the same direction and attract each other. So half of the cycle is attractive and half is repulsive.
Why am I wrong?

2. Jan 2, 2017

cnh1995

The solution that I have thought is very complicated to explain. It involves skin effect, transformer phasor diagram and geometry of the B field at the site of eddy current. I am waiting for someone to come up with a simpler explanation.

Last edited: Jan 2, 2017
3. Jan 2, 2017

Staff: Mentor

Anyone have an induction cooker they can try this, using aluminium foil? Take care in case it becomes heated.

If the claim is true, an aluminium saucepan might feel slightly lighter with the cooker powered on.

4. Jan 2, 2017

Staff: Mentor

I just tried it, and it does indeed "float." Actually, the induction cooker works in bursts, so the aluminum foil kept jumping. I didn't try it for long, but I did feel a slight heating.

Actually, my stove doesn't work with aluminum pans. The material has to be magnetic for it to work. (In the case of the aluminum foil, it worked for a few seconds before the stove stopped because it didn't detect any pan.)

5. Jan 2, 2017

haruspex

Is that right? If I place two magnets next to each other, N next to N and S next to S, the fields are in the same direction and repel.
Aluminium cookware is not suitable for induction cooktops because it does not get hot if sufficiently thick (0.5mm is enough).

6. Jan 2, 2017

Staff: Mentor

For it to work properly for cooking, yes, but I expected with Al there may still be a detectable ∆w, albeit slight. Miniscule, even.
I see, thank you. So if the steel pot didn't cover the whole pad there would be room to sit a disc of Al beside it to observe it levitating longer?

7. Jan 2, 2017

TSny

This would be true if the B field and the eddy current are exactly 90 degrees out of phase. However, due to self-inductance of the aluminum sheet, the phase difference is not exactly 90 degrees. The result is that during a cycle, there is a greater time of repulsion compared to attraction. So, the force does not average to zero, but averages to a net repulsive force.

This is the same effect as the "hovering ring" demonstration, starting at time 3:40 in this video.