A microwave oven. Over 90% of North American homes have a microwave oven. These devices of course generate heat, but they also generate loads of controversy. However, there is no controversy about how they heat. Basically microwaves cause water molecules to go into a vibrant dance, flipping back and forth some 2.45 billion times a second. That’s an astounding number. If you want a feel for its magnitude, well, a human lives for about 2.45 billion seconds. So imagine a water molecule oscillating as many times a second as you have seconds in your life. The agitated water molecules bump into whatever molecules happen to be nearby causing these to increase their random motion, and presto, we have heat. Indeed, heat is nothing more than a reflection of the increased motion of molecules.
An intriguing question is why microwaves cause water molecules to engage in such vigorous gymnastics. It all has to do with molecular structure. Just like everything else in life. If there is one thing about chemistry that most everyone knows, it’s that the formula for water is H2O. They may even know that the oxygen and hydrogen atoms are held together by the sharing of negatively charged particles called electrons. Since electrons have a greater affinity for oxygen than hydrogen, the oxygen end of the molecule bears a negative charge while the hydrogen end is positively charged. Molecules that have such positive and negative regions are said to be polar and it is this property that allows them to interact with electromagnetic radiation.
Electromagnetic radiation is not an easy concept to grasp, because you can’t actually physically grasp it. But we certainly experience it. Light, radio waves and x-rays are all different forms of electromagnetic radiation. What they have in common is that they all carry energy and travel through space at an incredible speed, about 300,000 kilometers per second. That’s roughly going around the world eight times per second. While all such radiation travels at the same speed, different types obviously have different effects. Exposure to x-rays is not the same as exposure to visible light. That’s because the amount of energy carried by the different types of radiation can differ dramatically.
It is helpful to think of radiation in terms of waves, just like the ones we see in water. What distinguishes one form of radiation from another is the distance between successive crests, in other words, the wavelength. Since all waves travel at the same speed, more waves of shorter wavelength than longer ones pass a given point per second. The number of waves passing a point per second is referred to as the frequency. So, the longer the wavelength, the lower the frequency of the radiation. Since each wave carries a certain amount of energy, the more waves that impact per second, the more energy is transferred.
Microwaves are long, around 12.2 cm, therefore have a low frequency and transmit little energy. They can make water molecules spin about, but that is all. They do not have enough energy to break chemical bonds. Waves have an electrical field associated with them which can be conceptualized if we think of waves being composed of positively charged crests and negatively charged troughs. Then as the waves pass through water, the oscillating positive and negative fields tug at the positive and negative parts of the water molecule causing them to rotate. The microwave energy is converted to thermal energy and the food cooks. As long as it contains water, or some other mobile polar molecules.
