Principle: Dielectric heating
Each water molecule is made up of one oxygen atom which is slightly negative and two hydrogen atoms which are slightly positive. Thus the molecule is an electric dipole or a polar molecule. The net charge on it is zero.
When there is no electric field surrounding it, it sets itself in a random orientation. But when placed in a electric field, the dipole will turn towards the field.
If the electric field is oscillating, as in an electromagnetic wave, these molecules turn continuously by aligning with it. This is called a dipole rotation or dipole polarization. As the field alternates, the molecules reverse direction.
Rotating molecules push, pull and collide with other molecules distributing energy to adjacent molecules and atoms in the material. Once distributed, this rotational energy appears as heat.
Temperature is related to the average kinetic energy. Thus dipole rotation is a mechanism by which energy in the form of microwaves can raise the temperature of an object. This principle is used in microwave oven for heating food materials.
Construction and working of Microwave Oven:
Microwave oven is a kitchen appliance used to heat and cook food. Microwaves are centimetre waves. In in the case off microwave ovens, the commonly used microwave frequency is 2.5 gigahertz. Electromagnetic wave in this frequency range are absorbed by water, fats and sugars. When they are absorbed, they are converted directly into atomic motion and their motion converted into heat. The microwaves are not absorbed by plastics, glass or ceramics. This metal reflects micro waves.
Components of microwave oven and their function:
1. Inside the strong metal box, there is a microwave generator called magnetron. The magnetron takes electricity from the power outlet and converts into high powered electromagnet waves of wavelength 12.0 cm.
2. The magnetron blasts these waves into the food compartment through a channel called wave guide.
3. The food it is kept own a turntable, spinning slowly round so that the microwaves cook it evenly.
4. The microwaves bounce back and forth off the metal walls of the food compartment. When the microwaves reach the food itself they penetrate inside the food. As they travel through it, they make the molecules inside it vibrate more quickly.
5. The vibrating molecules have heat and the food becomes hot. Thus the microwave pass on their energy to the molecules in the food, rapidly heating it up.
In conventional heating, the object's temperature rises by spreading heat energy from the surface to inside. On the other hand, by microwave heating the object will generate heat on their own by the penetration of the microwave. Hence rapid heating is possible by microwave since microwave heats object's internally.
Advantages:
Microwave cooking can be more energy efficient than conventional cooking because foods cook faster and the energy heats only the foods, not the oven compartment. Microwave cooking does not reduce the nutritional value of foods as conventional cooking. Foods cooked in a microwave oven may keep more of its vitamins and minerals, because microwave ovens can cook more quickly and without adding water. Foods high in water content, like vegetables can be cooked more quickly than other foods.
Glass, paper, ceramic or plastic containers cannot be heated by microwaves.
Additional advantage:
1. Cooking time is short
2. No physical change of foods
3. Melting process is easy
4. Sterilization effect exists
5. There is no flame, then treatment is easy.
6. The whole heating process is different because we are exciting atoms rather than conducting heat.
Disadvantages:
a) Metal container cannot be used.
b) Heat force control is difficult.
c) Closed container is dangerous because it could burst.
d) Surface heating is impossible.
e) The air in the oven is at room temperature, so there is no way to form a crust.
Precautions on how to use a microwave oven:
1. Use suitable utensils for cooking.
2. Set the correct wattage and time before cooking.
3. Follow the food package heating instructions.
4. Do not leave the food unattended while heating.
5. Do not cook eggs with shell.
6. Do not put sealed cans or bottles in the microwave oven.
7. Do not fry food.
8. Do not use metal utensils.
9. Do not use or operate the microwave oven if there are no food items.
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