General concepts
When people talk about electromagnetic fields, there are unavoidable technical and physics terms used. In order to make it easier for the reader, a few general concepts are explained here.
Electric and magnetic fields
Electrical charges create an electric field. This means that around every power outlet, there is an electric field. When an electrical flow moves through the wires, therefore when electricity is used (for example when a lamp is on or a vacuum cleaner is running), the electrical charges move and in doing so, create a magnetic field (induction).
The strength of the electric and magnetic fields decrease sharply as the distance from the source increases. The more voltage in the wires, the greater the electric field. The strength of the magnetic field depends on the electric current. The power is dependent on both the voltage and the current and reflects the energy use per unit of time.
The strength of an electric field is measured in volts per metre (V/m).
The strength of a magnetic field is expressed in tesla (T). The magnetic fields in our environment are generally so weak that they are expressed in microtesla (1 microtesla = one millionth of a tesla, 1 µT = 0.000001 T).
Electrical voltage is measured in volts (V). That’s why it’s called voltage. For high-voltage one uses kilovolts (1 kV = 1,000 volts).
Electrical current is measured in amperes (A). This is the amount of electrical charge passing per unit of time.
The power is expressed in watts (W), that’s why it’s also sometimes called wattage. In practice the milliwatt is also used as a unit (1 milliwatt = one thousandth of a watt, 1 mW = 0.001 W).
At very low frequencies (50 Hz, for example) the electric and magnetic fields are considered separately. At high frequencies, the electric and magnetic fields are inseparable and are collectively designated electromagnetic wave or electromagnetic field. In an electromagnetic wave the electrical and magnetic components propagate perpendicular to each other in space.
The word ray is usually used for high frequency radiation: in this case there is a transfer of energy (energy flow) in space. For low frequencies, energy radiation is negligible. For that reason, people usually say fields, although this word is also sometimes used for high frequencies.
Fig. 1 : schematic representation of an electromagnetic wave.
Frequency and wavelength
The frequency of an electromagnetic wave is the number of peaks that pass in one second. One cycle per second is one hertz (Hz). One cycle per second is one hertz (Hz). Derivative units are used for high frequencies such as the kilohertz (1 kHz = 1,000 hertz), megahertz (1 mHz = one million hertz), gigahertz (1 GHz = 1,000 million hertz).
Derivative units:
milli (m), micro (μ): 1 mW = 1 milliwatt = 0.001 W or one thousandth of a watt; 1 μT = 1 microtesla = 0.000001 T
kilo (k), mega (M), giga (G): 1 kHz = 1 kilohertz = 1,000 Hz; 1 MHz = 1 megahertz = 1 million Hz; 1 GHz = 1 gigahertz = 1 billion hertz
The distance between two peaks is the wavelength (expressed in metres, millimetres, micrometres, etc.). The higher the frequency, the smaller the wavelength.
A wave’s phase
The phase expresses how much delay one wave has with respect to another wave. The phase is expressed in degrees or wavelengths (1 wavelength = 360°). If two waves are parallel, the phase difference is 0. These waves are then in phase. If this is not the case, then they are out of phase. If two waves have a phase difference equal to the half of the wavelength (180°), then they are in opposition, or in antiphase. Two waves in antiphase cancel each other out.
Waves that are in phase
Waves that are out of phase
Fig. 2, Wikipedia, User Kieff
Example: Electricity is generated in three phases
A generator in a power plant consists of three separate windings that are arranged spatially 120° from each other. Because the generator continually turns past these windings, and passes them one by one, the voltages generated are not at their maximums at the same time; the three voltages are 120° out of phase (see Fig. 2). This explains the name three-phase voltage.
The Electromagnetic spectrum
Radio waves, infrared light, visible light, ultraviolet light, x-rays, gamma rays, these are all electromagnetic waves. They only differ from one another in frequency: the faster the waves follow each other, the higher the frequency. The frequency determines the type of wave, its specific properties and the application. Our bodies react differently to waves at different frequencies.
An electromagnetic wave transports energy in small packets, called photons. The higher the frequency, the higher the photon energy.
The complete range of electromagnetic waves is called the electromagnetic spectrum. This spectrum includes both ionising and non-ionising radiation, depending on the frequency and therefore the photon energy.
Fig. 4, source: www.infogsm.be
Energy-rich photons are capable of knocking electrons away from atoms and molecules that they encounter. The atoms and molecules then become electrically charged: this is called ionisation.
Electromagnetic waves in which the photons’ energy is not great enough to cause ionisation fall under non-ionising radiation. Electromagnetic radiation from artificial sources – electricity, microwaves, mobile phones – are in this part of the spectrum.
The transition range is in ultraviolet light. Gamma rays, x-rays and part of the ultraviolet spectrum rays are ionising radiation. Non-ionising radiation includes ultraviolet light with a lower frequency, visible light, infrared radiation, radio waves and electromagnetic fields of intermediate and extremely low frequencies (IF and ELF fields).
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Published on 01/06/2011 – Page last updated on 01/06/2011