by B. Leslie Baird

Created on: February 25, 2013

Microwave ovens make reheating foods fast and easy. They are great for quickly defrosting frozen items and work well for cooking. While you do not have to know how a microwave works to use one successfully, some background information can be helpful. The knowledge may also help you understand why some foods will cook better than others, and why metal objects do not belong in a microwave.

Physics behind microwave action

The heating process occurs because of the way water molecules react when exposed to electromagnetic waves. Microwaves are part of the electromagnetic band and have alternating magnetic and electric fields. The wave frequency is about 2,450 megahertz or 2.45 gigahertz. A molecule of water has a negative charge on one end and a positive charge on the other. When a charge encounters an electric field it will experience a force that can be towards or away from the field. The force either attracts or repels the molecule and creates motion in the process.

Because the electromagnetic wave alternates, the force on the molecule of water alternates. The molecule will rotate and change from clockwise rotation to counterclockwise rotation as the wave alternates. This rapid movement of the water molecule creates heat. The heat is then dissipated to surrounding molecules, in this case the food in the microwave oven.

How microwave ovens work

When you turn on your microwave oven it sends microwaves through the food inside the oven. This energy causes the water, or fat, molecules in the food to become excited. Because this energy passes through the food, it heats fairly uniformly. If an item is extremely thick, the waves may not penetrate the center as well. Regular cooking warms food from the outside, working the heat to the interior of the item.

Microwaves work to excite water molecules. The higher the water content the more quickly the item will heat up. If a food item has a very low moisture content, the heating process will not work as effectively. A microwave will pass easily through paper, glass and plastic. Food items in any of these containers will heat equally well.

Why does metal spark in a microwave?

Pointed parts of a metal can build up voltage. The voltage can accumulate until it releases in a spark or arc. Points do not need to be visible to allow the build up of voltage. Metal also reflects the microwave and this reflection can send damaging waves into sensitive parts causing failure.

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