Question

The condition under which a microwave oven heats up a food item containing water molecules most efficiently is :

• A. Infra-red waves produce heating in a microwave oven
• B. The frequency of the microwaves must match the resonant frequency of the water molecules
• C. The frequency of the microwaves has no relation with natural frequency of water molecules
• D. Microwaves are heat waves, so always produce heating

Answer 1

The Correct Option is B

The efficiency with which a microwave oven heats food largely depends on how well the microwaves can interact with the water molecules in the food. Here is a detailed explanation and reasoning for the correct answer:

Correct Answer: The frequency of the microwaves must match the resonant frequency of the water molecules.

Here's why this is the correct choice:

1. Basic Principle of Microwave Heating:
   • Microwave ovens operate by emitting microwaves, which are a form of electromagnetic radiation, usually at a frequency of about 2.45 GHz.
   • This frequency is chosen because it is close to the natural resonant frequency of water molecules.
2. Resonance Effect:
   • When microwaves hit water molecules, which have a dipole nature, they cause these molecules to rotate rapidly.
   • The rapid oscillation increases the kinetic energy of the molecules, thereby heating the substance containing these water molecules.
3. Explanation of Options:
   • Infra-red waves produce heating in a microwave oven: This is incorrect because microwave ovens use microwaves, not infrared waves, for heating. Infrared radiation is used in other heating appliances like toasters.
   • The frequency of the microwaves must match the resonant frequency of the water molecules: This is correct for efficient heating, as explained above.
   • The frequency of the microwaves has no relation with natural frequency of water molecules: Incorrect, as aligning these frequencies leads to effective energy absorption and heating.
   • Microwaves are heat waves, so always produce heating: Incorrect because microwaves need to be at specific frequencies to effectively heat materials like water.

Conclusion:

For a microwave to heat efficiently, its frequency must match the resonant frequency of the water molecules in the food, causing vibrational motion that leads to heating.