# Why is the lid of the cookware kept on induction cooker not hot?

Induction cookware cooks food by inducing an electro magnetic field in the ferro-magnetic cookware. Since iron offers a lot of resistance to the current, the current is converted into heat in the cookware, which is then used to cook the food.

My question is, if this is the case, why is the top of the cookware not hot ? If the current flows evenly through the cookware, even the top of the cookware should have been hot ?

Note : This question is based on my observations while heating milk. The milk itself is hot, but the top of vessel is cool.

Another minor question:

Is all the current converted into heat ? Some vessels contain more iron than others. Is there any situation which will lead to electric shock on touching the metal ?

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## 1 Answer

Your first assumption is not really correct. The induction cooker creates an electromagnetic field with a frequency in the range of 10 kHz to 100 kHz. The coil used in the cooker looks as follows:

The coil and the frequency are choosen in such a way that the field does not extend more than a few centimeters along the axis of the setup. Now when you put a ferromagnetic material on top it will rapidly get magnetized and demagnetized with with field created by the coil. Every ferromagnetic material exhibits hysteresis, which leads to dissipation. This dissipation heats the pot. This is not connected to the resistivity of the material, as eddy currents are not the main cause of the heating (a copper pot works poorly on an induction cooker).

With the field strength rapidly decreasing with distance, it is clear why the top of the pot and the lid do not become hot. There is no even current flow through the cookware.

To the second part of your question: Almost all electric energy is converted into heat, the total efficiency of input energy versus the heating of the food inside the pot is 84%, so very high compared to other methods but not perfect.

An electric shock is unlikely but not impossible. The Swiss health board recommends non-metallic spoons to avoid any current going through your body (Best practices, in German). Modern cookers have metallic or graphite surface to provide a path to ground for the pot which minimizes this issue.

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