Cooking Using Electricity  Print page

In any kitchen in Hong Kong, you will find various cooking appliances. Most utilize electricity for energy. Electricity is clean, convenient and efficient. One can access it by just plugging an electrical appliance into an electric socket. Technological advancements have made way for appliances such as microwave ovens, induction cookers, electronically controlled rice cookers and stewing mugs. Not only do these appliances make cooking more convenient, but they are also designed to save energy.

The Electrical and Mechanical Services Department (EMSD) of HKSAR government has an energy efficiency labelling scheme to inform users that the labelled products have met certain energy efficiency requirements [1]. Many household electrical appliances have been designed and labelled complying with these requirements, including electrical cooking appliances like rice cookers. The label for rice cookers is a recognition type label, informing users that the labelled rice cookers met the energy efficiency and performance requirements [2].

Fig. 1   Electronically controlled rice cookers and induction cookers are two kinds of common electrical cooking appliances available in the department store. Fig. 2   These electronically controlled stewing mugs are ideal appliances for preparing Chinese medicine and stewing food.

The following gives a brief description of these various electrical cooking appliances and their operating principles. You will see some completely different and very innovative ways to convert electrical energy for cooking purposes.


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.


Induction cooker

Fig. 5   Induction cookers like the one in the picture are commonly used in Hong Kong.
Fig. 6    When there is a change in the magnetic flux through the coil, a current will be induced. The induced current will produce a magnetic field so as to oppose the motion of the magnet.
Fig. 7   An experiment to show the electromagnetic induction between a solenoid and a coil.

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  Watch videoWatch videoDownload video: 6.5mb
 

Why does an induction cooker have these heating characteristics? This is to do with the principle of electromagnetic induction by which an induction cooker operates. Fig. 6 illustrates this principle. If you push the bar magnet rapidly towards the coil, this changes the magnetic flux through the coil, an induced e.m.f. (electromotive force) will cause a current to flow in the coil. Lenz's law tells us that the induced current will flow in the direction so as to oppose the change that produces it. In Fig. 6, when the North pole of the bar magnet is being pushed towards the coil, the induced current will flow in an anti-clockwise direction to produce a magnetic field (North pole) which opposes the motion of the magnet (to repel the incoming magnet's North pole). Note that the induced current will exist only when there is a change in the magnetic flux. When the bar magnet stops moving, there is no change in flux and the induced current will disappear.

In an induction cooker, the change in magnetic field is produced by a high frequency AC current flowing in a solenoid. The experiment shown in Fig. 7. illustrates this process. An AC current produces a changing magnetic field in a solenoid. The field strength is enhanced by the presence of a soft iron core inside the solenoid. When a coil is placed around the iron core, a constantly changing magnetic flux is produced, and an AC current is induced in the coil. Due to its electrical resistance, the coil is heated by the induced current. We can detect the current using a multimeter and measure the increase in temperature using a thermocouple. See the video below.

Electromagnetic induction  Watch videoWatch videoDownload video: 4.6mb

The animation below will give you a clear picture of the flow of the induced currents.

Flash animation: Electromagnetic induction
 
Fig. 8   A dismantled induction cooker. Note the large solenoid at the centre. (Beware: students should not remove the cover of electrical appliances as this may cause malfunction or even electrocution)

A dismantled induction cooker shown in Fig. 8 reveals a similar structure to that of the experiment above. During operation, an AC current is passed through the solenoid to produce a rapidly changing magnetic field. The change in magnetic flux produces induced currents, known as eddy currents, around the base of the steel cookware. Since the electrical resistance of steel is low, large eddy currents are induced and they generate a great deal of heat in the cookware. The video below demonstrates that an induction cooker can produce a large current and strong heating effect to a conducting coil.

Working principle of an induction cooker  Watch videoWatch videoDownload video: 5.6mb

As energy is transferred to the steel cookware by electromagnetic induction and not by heating, the induction cooker itself is not heated to a high temperature during operation. This makes the induction cooker a very energy efficient cooking appliance. In the following activity, we will do an experiment to compare the efficiency of an induction cooker and a hotplate, and from this you may learn more about the factors affecting the energy efficiency of the cooking appliances.

Activity: Comparing an induction cooker with a hotplate


Conventional ovens

A conventional oven uses infrared radiation. Heat is transferred from a red hot heating element to food through an electromagnetic wave easily absorbed by objects. On absorbing infrared radiation, food surfaces are heated to a high temperature producing a crispy effect of roast chicken skin or bread crust, for example. However, food is usually a bad conductor of heat, and it may take quite a long time for the heat on the surface of the food to reach the interior. Cooking time is thus dependent on the speed of heat conduction of the food itself. In addition, if the oven's power is not carefully controlled, food can be burnt on the outside but remain uncooked inside. This is the major disadvantage of the conventional oven and cooking methods such as frying and baking.


Microwave ovens

Fig. 9   A household microwave oven

Microwave ovens are very popular in Hong Kong because they can cook or reheat food very quickly. They can heat food without heating up the food container.

A microwave oven generates microwaves inside its cavity to cook food. Microwaves are electromagnetic waves with wavelengths between about 1 mm to 100 cm. Domestic microwave ovens use microwaves of frequency 2.45 × 109 Hz (of wavelength 12 cm). Unlike infrared radiation, microwaves can penetrate much deeper into food (e.g. about 1 cm into beef). The energy of the microwaves spread out to fill a greater volume inside the food so the entire food item is more evenly cooked. This significantly speeds up the cooking process.

But why are microwaves so effective in heating up food? This is closely related to the molecular structure of water (Fig. 10), a major component of food. Water molecules are formed from two hydrogen atoms and one oxygen atom sharing their outermost electrons. The oxygen atom exerts a stronger attractive force on the shared electrons, pulling them closer to the oxygen side of the water molecule. The oxygen side therefore has slightly more negative charge and the hydrogen side has slightly more positive charge. Water molecules are said to be 'polar' because of this small separation of charge. When microwaves pass through water, their energy is absorbed by the polar water molecules, and these molecules oscillate violently (Fig.11). As the water molecules collide with each other, this energy of oscillation is changed into the random kinetic energy of the molecules, resulting in a rapid increase in temperature.

Fig. 10   A water molecule is said to be polar because its positive and negative charges are slightly separated. Fig. 11   When microwaves pass through water, the oscillating electric field causes the water molecules to oscillate up and down.

The much shortened cooking time means a high energy efficiency.


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