High School Physics

# Microwave ovens working principle & physics behind its operation

In this post, we will briefly discuss the science behind the operating principle of the microwave oven. This marvelous invention somehow heats only the soft parts of the food and leaves the inorganic and hard materials, like ceramic and the surfaces of bone, at approximately the same temperature. A neat trick, indeed, but how is it done? Let’s see how microwave ovens work.

## How do Microwave ovens work?

• Microwave ovens take advantage of a property of water molecules called bipolarity. Water molecules are considered bipolar because each molecule has a positive and a negative end. In other words, more of the electrons in these molecules are at one end of the molecule than the other.
• Because microwaves are a high-frequency form of electromagnetic radiation, they supply an electric field that changes polarity billions of times a second.
• As this electric field passes a bipolar molecule, the positive side of the molecule experiences a force in one direction, and the negative side of the molecule is pushed or pulled in the other direction.
• When the field changes polarity, the directions of these forces are reversed. Instead of tearing apart, the molecules swing around and line up with the electric field.
• As the bipolar molecules swing around, they rub against one another, producing friction.
• This friction in turn increases the internal energy of the food.
• Thus, Energy is transferred to the food by radiation (the microwaves) as opposed to conduction from hot air, as in a conventional oven.

## Energy produced

Depending on the microwave oven’s power and design, this rotational motion can generate up to about 3 J of internal energy each second in 1 g of water. At this rate, a top-power microwave oven can boil a cup (250 mL) of water in 2 min using about 0.033 kW•h of electricity.

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