How Do Microwave Ovens Work?

Heating Water Molecules in Food with Electromagnetic Waves

Nov 11, 2007

How do microwave ovens cook food? How is the energy of the microwaves converted to heat energy in our meals? How does microwave heating work?

What Are Microwaves?

Microwaves, like other radio waves, are a form of electromagnetic waves. Electromagnetic waves are wavelike oscillations of electric and magnetic fields. Electric fields are what makes electric charges attract or repel. Positive or negative electric charges produce electric fields which in turn act on other charges. In a similar way, magnetic fields cause magnetic forces.

These fields are perpendicular to each other and continually oscillate between maximum positive and maximum negative (pointing in the opposite direction). The microwaves used to heat food in microwave ovens have a wavelength of 12.2 cm and oscillate at a frequency of 2.45 gigahertz. Giga means billion, so the electric and magnetic fields oscillate fast enough to make 2.45 billion complete cycles each second.

How Do Microwaves Heat Food?

Microwave heating works by heating the water in foods. The frequency used in microwave ovens optimally heats water molecules. The microwave energy is converted into heat energy of the water molecules, which in turn heat the other molecules in the food. That is why dry foods such as rice and pasta will not cook in a microwave unless they are in some water.

How does microwave heating affect water? A water molecule consists of three atoms: Two hydrogen atoms and one oxygen atom. These atoms share their electrons in such a way that the positive charges are on one side of the water molecule and the negative charges are on the other side. Chemists call this a polar molecule. Water molecules are polar molecules.

Like charges repel each other and unlike charges attract. This happens because the charges each create their own electric fields and each electric field exerts a force on the other particle. Because microwaves are electromagnetic waves they produce constantly changing electric fields in the food inside the microwave oven. The water molecules in the food react to the electric field by rotating to minimize the force on both the positive and negative ends of the molecule. When the electric field changes from positive to negative, as it does with an electromagnetic wave, the water molecules rotate again and orient themselves in the opposite direction.

In ice the molecules can not rotate as easily, so microwaves do not heat ice as effectively as they heat liquid water. The heated liquid water must heat the frozen water by normal conduction. So frozen foods can heat unevenly.

The 2.45 gigahertz frequency works in microwave ovens because the time it takes for the electromagnetic wave to change the electric field from positive to negative is just the right amount of time for the water molecules to rotate. Hence the water molecules can rotate at the fastest possible rate. In addition, this frequency is not used for communications, so microwave ovens won't interfere with cell phones, televisions, and so forth.

Why do Rotating Water Molecules Heat Food?

The answer has to do with the nature of heat energy and temperature. Heat energy is the random kinetic energy (energy of motion) of the individual atoms and molecules. When something is hotter, the atoms and molecules are randomly moving around more rapidly. As the temperature decreases, the atoms and molecules move more slowly.

As the water molecules rotate, they bump other molecules causing them to begin moving randomly. The process is like frictional heating. Microwave energy converts to heat energy by causing the molecules in food to increase the speed of their random motions.

Your meal gets nice and hot.