Getting to know the radiofrequencies of electromagnetic radiation around us:

France Info 105.5MHz, SFR mobile 3G 900MHz and 2100 MHz, all microwave ovens 2450MHz

 

 

How do we use the 2450MHz radiofrequency of microwave ovens

 

What happens when we heat water in a microwave oven?

In short: Microwave electromagnetic radiation exerts electric forces that change direction 2.450.000.000 times per second and rotate water molecules because these have electric poles!

(wikipedia image)

Image from an excellent resource by colorado.edu/physics 

 

We are using electromagnetic radiation with a frequency of 2450MHz; this is a radiofrequency, a radiowave that belongs to the subcategory of microwaves.

 

What does this number mean? Electromagnetic radiation consists of an electric and a magnetic field. These fields will change direction 2.450.000.000 times per second.

 

This frequency has been reserved for microwaves. In this way, we do not have interference with other equipment such as mobile phones. For instance SFR 3G mobiles use 2100MHz and 900MHz. France Info is at 105.5MHz.

 

You don’t want your phone interfere with your microwave!

 

When the sun radiation hits the water in a pond, it gets hot. Radiation carries energy. How much energy does a microwave oven provide? Most ovens provide 1000 Watts. They have a magnetron that produces 1000 Watts. What happens when you adjust the rotary selector? Do you change the Watts that the magnetron provides? You actually change the time that the magnetron is switched on and off. You are creating pulses of radiation.

 

If you set the selector half way, then the magnetron will be switched on for 50% of the time you set. The defrost button actually does a program of switch on/off with off spaces initially long and then shorter (you don’t want to burn the surface and leave the core frozen, this is why you give time to allow for heat propagation). For more information page 3 of this article.

 

The microwave radiation corresponds to an electric and a magnetic field inside the chamber. What is the strength of the electric field? Answer: 2 kV/m [Ref].

 

In practice, how much energy hits a surface? This is expressed using “power density”. The power density of a microwave oven is approximately 2.8 W/cm3 [Ref].

 

Why does water get hot inside such an electric field and magnetic field that changes polarity 2.450.000.000 times?

 

Because the water molecule has electric poles! Because water is a polar molecule. As the polarity changes it will try to follow the change and align itself with the provided polarity. As a result, a molecular rotation will occur. It is like turning a car steering wheel.

 

Picture

Image source

 

 

 

Dipole rotation Image source

 

 

While doing that, neighboring molecules (which in a liquid are close to each other) will bump shoulders and friction will occur, or in other words we will have small collisions during which the kinetic energy of the molecules will be dissipated as heat.

 

Image from an excellent resource by colorado.edu/physics 

 

Notice that the air in the microwave will not get hot! Molecules in the aerial phase are far from each other and will not “bump shoulders” and as a result we will not have friction!

 

 

 

Additional references

 

 

1. Scientific american article

 

 

 

2. Quora.com

 

3. Wikipedia Joule heating vs Dielectric heating

 

Some notes

Joule heating | e.g. toaster electric element/resistance: current accelerates charges particles which collide with others; collisions increase thermal motion

 

Dielectric heating | e.g. microwave oven: water is a dipole, which means it has a (+) electric pole and a (-) one. As it operates in 2.45GHz, the electric field changes direction/polarity 2,400,000,000 times a second. 

 
 

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