A wobbly pan on an induction cooker, is it less effective? According to the Wikipedia, one of the limitations of the induction cooker is that the bottom surface of the pot should be flat.
Accordingly, I commented on a question on Seasoned Advise, but I'd like to know whether this is actually true (My username is BaffledCook there).
For a wok, it seems reasonable to believe the induction cooker will not be effective, but how about a wobbly pan? How wobbly should it be before it really affects efficiency?
 A: An induction cooker works with a flat coil setup similar to the one shown below. The diameter of this coil depends on the exact model (in the range of 30 cm). This implies that the field is also extending quite a bit into the air above the cooker. 

As the large field can be potentially hazardous (think metal ring on finger) there are relatively strict safety standards for induction cookers. The field of of the coil measured at a horizontal distance of 30 cm from the end of the coil and along the vertical axis (z=0 is the center of the coil) is shown below (the absolute values are not comparable with the field above the center but the profile is certainly comparable). The data is from this study: "B-Field Exposure From Induction Cooking Appliances" by C. Viellard and coworkers.

As you can see the field strength decreases by about an order of magnitude in a vertical distance of 200 mm. A very wobbly pan might be less effective but only if it is warped by more than a few cm. 
A: Induction will work whether the pan is flat or not. However Foucault's currents (eddy currents) locally induce important temperature gradients within the metal. Unflat pan generates a non uniform heating that in turns induces large deformations or even cracks in the steel. Furthermore the power transfered by microwaves to the pan is optimal if the field lines are perpendicular to the metal surface, and it is easier to achieve with a flat pan.
