Microwave oven: The spectrum of electromagnetic radiation contains a part known as microwaves. These waves have frequency and energy smaller than visible light and wavelength larger than it. What is the principle of a microwave oven and how does it work? Our objective is to cook food or warm it up. All food items such as fruit, vegetables, meat, cereals, etc., contain water as a constituent. Now, what does it mean when we say that a certain object has become warmer? When the temperature of a body rises, the energy of the random motion of atoms and molecules increases and the molecules travel or vibrate or rotate with higher energies. The frequency of rotation of water molecules is about 2.45 gigahertz (GHz). If water receives microwaves of this frequency, its molecules absorb this radiation, which is equivalent to heating up water. These molecules share this energy with neighbouring food molecules, heating up the food. One should use porcelain vessels and non-metal containers in a microwave oven because of the danger of getting a shock from accumulated electric charges. Metals may also melt from heating. The porcelain container remains unaffected and cool, because its large molecules vibrate and rotate with much smaller frequencies, and thus cannot absorb microwaves. Hence, they do not get eaten up. Thus, the basic principle of a microwave oven is to generate microwave radiation of appropriate frequency in the working space of the oven where we keep food. This way energy is not wasted in heating up the vessel. In the conventional heating method, the vessel on the burner gets heated first and then the food inside gets heated because of transfer of energy from the vessel. In the microwave oven, on the other hand, energy is directly delivered to water molecules which is shared by the entire food.
(i) As compared to visible light microwave has frequency and energy
(a) Frequency is less but energy is more
(b) less than visible light
(c) more than visible light
(d) equal to visible light
(ii) When the temperature of a body rises
(a) the energy of the random motion of atoms and molecules decreases.
(b) the energy of the random motion of atoms and molecules remains same.
(c) the energy of the random motion of atoms and molecules increases
(d) the random motion of atoms and molecules becomes streamlined.
(iii) he frequency of rotation of water molecules is about
(a) 2.45 THz
(b) 2.45 kHz
(c) 2.45 MHz
(d) 2.45 GHz
OR
In the microwave oven
(a) Energy is directly delivered to the food grains.
(b) The vessel gets heated first and then the water molecules collect heat from the body of the vessel
(c) Energy is directly delivered to water molecules which is shared by the entire food
(d) The vessel gets heated first, and then the food grains inside
(iv) Why should one use porcelain vessels and non-metal containers in a microwave oven?
(a) Because it will prevent the food items to become hot
(b) Because it will get too much hot
(c) Because of the danger of getting a shock from accumulated electric charges
(d) Because it may crack due to high frequency
