When using an induction stove to heat water or milk, the heat is primarily generated through electromagnetic induction. Induction stoves contain a coil of copper wire beneath the cooking surface. When an electric current passes through this coil, it produces a rapidly alternating magnetic field.
To understand why the upper portion of the vessel can remain relatively cool, we need to consider how induction heating works. When a ferromagnetic material, such as certain types of stainless steel or cast iron, is placed on the cooking surface of an induction stove, the magnetic field induces electric currents within the material. These electric currents, also known as eddy currents, generate heat due to the material's electrical resistance.
However, the heat transfer in an induction stove is different from that in traditional gas or electric stoves. In a gas or electric stove, heat is primarily transferred through conduction, where heat energy travels from the heat source to the pot via direct contact or through convection, where hot air or gas transfers heat to the pot. This results in more uniform heating of the vessel.
In the case of induction stoves, heat is primarily generated within the ferromagnetic material itself, thanks to the eddy currents induced by the magnetic field. The heat is produced directly in the base of the vessel, where the ferromagnetic material is in contact with the induction cooktop. This heat is then conducted from the base to the surrounding contents, such as water or milk, through conduction.
Since the heat is primarily generated within the vessel's base and transferred through conduction, the upper portion of the vessel may not receive as much heat as the bottom. The heat gradually spreads throughout the liquid due to natural convection currents, where the heated portion rises and cooler portions sink, but it may take some time for the upper portion of the liquid to reach the same temperature as the bottom.
If you want to ensure more even heating, you can stir the contents periodically or adjust the heat settings accordingly to promote better circulation and heat distribution.