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Hotplate

Electric hotplates, like rice cookers, use resistive elements for heating. When an electric current I passes through a resistor R, electrical energy is converted to internal energy (heat) at a rate of

P = I 2 R

Usually a long coil of resistive element is used in a hotplate. When the element is heated by an electric current, it transfers the heat to a metal surface on which the cookware is placed. The large metal surface enhances the rate of heat conduction such that the cookware and the food inside can be heated reasonably fast. In newer designs, the interface between the heating element and the cookware is a transparent layer. In this case the heat is transferred from the heating element to the cookware mainly by infrared radiation. Electric hotplates do not need the supply of liquid/gas fuel and do not produce any naked flame. This makes them safer and more convenient to use. However, keeping the heating element at a high temperature during cooking means that much heat is lost to the surroundings through conduction, convection and radiation. This makes hotplates not very energy efficient.

Fig. 3   A conventional hotplate has a metal surface covering the heating element to facilitate heat transfer. Note that the surface is in a dark colour to facilitate heat transfer by radiation. Fig. 4   A newer design of hotplate has a transparent layer covering the red hot heating element. Heat is transferred to the cookware mainly by radiation.
 
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Induction cooker

Fig. 5   Induction cookers like the one in the picture are commonly used in Hong Kong.

Using induction cookers to prepare hotpots is already very common in Hong Kong. Strangely enough, the surface of an induction cooker is not made of metal and will not heat up when the induction cooker is switched on. However, when a steel frying pan is placed on it, the pan gets hot very quickly. On the other hand, non-metallic cookware cannot be heated at all. The following video shows the operation of a domestic induction cooker.

Cooking with a domestic induction cooker
 
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