23 July 2014

Why you don’t get cooked by your microwave oven

By Dr Manas Kumar Haldar

When microwave ovens first came into the market in 1947 they were very expensive. But these days, almost every household has one of these “miracle device” because it is affordable. The oven is very handy and fast for warming up and cooking food that does not need constant stirring.

The method of heating in a microwave oven is very different from that in an ordinary oven. An electric oven has heating elements which you can see glowing. The heat from the oven travels from the outside to the inside of the food. It takes time to reach the inside and the outer parts may get overcooked. In a microwave oven, there is no heating element. Heat is generated inside the food by radio frequency (RF) radiation. The atoms and molecules in the food in the microwave oven start to jingle at certain radio frequencies. It is this jingling that produces heat. As the RF travels inside the food, the heating is more uniform. Many microwave ovens use a RF frequency of 2.45 GHz (one GHz is one followed by nine zeroes). This frequency is often used as it has been set worldwide for non-communication purpose. The RF power is generated by a vacuum tube called magnetron. Why a vacuum tube in the age of silicon microelectronics? This is because a magnetron can generate high RF power from electrical power very efficiently. However, due to heat losses and so on, the overall efficiency of the oven is around 60%.

You can stand in front of the microwave oven and see the food, say a chicken, being cooked. But have you wondered why you don’t get cooked by the RF while you stand near the microwave oven? Obviously, the RF must be blocked by the door of the oven, but light is not. Both light and RF are what is known as electromagnetic wave. So why are you not affected by the radiation?

This is where I’ll ask you to do a simple experiment. With the oven switched off, put your eyes close to the door. You will see a fine metal wire mesh. The mesh often consists of square holes. It is well known in electromagnetic theory that a mesh with holes which are much smaller than the wavelength of the waves can block the wave. Wavelength is the velocity of light divided by the frequency of the wave. So, for a frequency of 2.45 GHz used in a microwave oven, the wavelength is 12.2 cm. Now look at the door again, the holes are much smaller than this value. This blocks the RF.

On the other hand, the wavelength of visible light ranges from about 0.4 to 0.7 micrometers. A micrometer is 1/10,000 of a centimetre. You will note that the wavelength is much smaller than the size of the holes. So the holes cannot block light from the light bulb inside the microwave oven. Of course, it also helps that the wires are very thin; otherwise they will block some light. Now you understand why you can see the chicken cooking but you don’t get cooked. It is however advisable to stay away from the oven when it is switched on. This is because the blocking system may have deteriorated with age or the new microwave oven is faulty.

How do I know all these? It is because I studied electrical engineering. In the first lecture of RF electronics, I ask students to do the experiment I suggested above. There are, of course, several other interesting things about microwave ovens. But I will end this article with a word of caution. Just like the picture tube in a television set, the magnetron requires high voltage. Tinkering with the magnetron on your microwave oven is not a good idea.

Dr Manas Kumar Haldar is Associate Professor with the Faculty of Engineering, Computing and Science at Swinburne University of Technology Sarawak Campus. He is contactable atmhaldar@swinburne.edu.my