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electromagnetic field

An electromagnetic field, sometimes referred to as an EM field, is generated when charged particles, such as electrons, are accelerated. All electrically charged particles are surrounded by electric fields. Charged particles in motion produce magnetic fields. When the velocity of a charged particle changes, an EM field is produced.

Electromagnetic fields were first discovered in the 19th century, when physicists noticed that electric arcs (sparks) could be reproduced at a distance, with no connecting wires in between. This led scientists to believe that it was possible to communicate over long distances without wires. The first radio transmitters made use of electric arcs. These "spark transmitters" and the associated receivers were as exciting to people in the early 20th century as the Internet is today. This was the beginning of what we now call wirelesscommunication.

Electromagnetic fields are typically generated by alternating current (AC) in electrical conductors. The frequencyof the AC can range from one cycle in thousands of years (at the low extreme) to trillions or quadrillions of cycles per second( at the high extreme). The standard unit of EM frequency is the hertz, abbreviated Hz.Larger units are often used. A frequency of 1,000 Hz is onekilohertz(kHz); a frequency of 1,000 kHz is one megahertz (MHz); a frequency of 1,000 MHz is one gigahertz (GHz).

The wavelength of an EM field is related to the frequency. If the frequency f of an EM wave is specified in megahertz and the wavelength w is specified in meters (m), then in free space, the two are related according to the formula

w = 300/f

For example, a signal at 100 MHz (in the middle of the American FM broadcast band) has a wavelength of 3 m, or about 10 feet. This same formula applies if the frequency misgiven in gigahertz and the wavelength is specified in millimeters (mm). Thus, a signal at 30 GHz would have a wavelength of 10 mm, or a little less than half an inch.

The realm of EM field energy is called the electromagnetic radiation spectrum. In theory, this extends from arbitrarily long wavelengths to arbitrarily short wavelengths, or, as engineers sometimes imprecisely quip, "from DC to light."

This was last updated in March 2010

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Even though quantum and Relativity are still only theories, how can we base our discoveries on theoretical knowledge?
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Most every concept is a "theory" in the true sense of the word. A theory is supported every time that its variables act in a way they are logically predicted to act if the theory is true. When experiments that test the theory can repeatedly conclude the same supporting result without error then this is all good validation for the theory. Regardless we will always continue to use the word "theory" because, even though all good evidence suggests that a theory is true, that does not mean it is complete, or that it is true in every possible hypothetical scenario. There may be situations unknown to us in galaxies far away where the theory cannot fully explain or predict an outcome - because we are limited by the fact that we can never be sure that we can observe everything in every situation we have to assume that all explanations are theories no matter how well supported they are. So in answer to the second part of your question, it is often the case that our discoveries support the theory - which is what science is all about.

Many people get hung up on the word "theory" and see it as carrying a degree of uncertainty - and it does. Nothing is certain, not even in the most rigorous of sciences. Unlike some philosophical or religious arguments, science never claims to definitively know anything and is clear about the limits of the scientific method. In this way, understanding the scientific method not only gives us the most rigorous explanations, but also the most honest ones.
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While this is a great explanation, I would like to know if it is possible to stop fast objects with different currents or frequency of an EMF that is designed to protect its user. Is it possible?
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