What are Electromagnetic Fields?
Natural sources of electromagnetic fields:
Electromagnetic fields are present everywhere but
are invisible to the human eye.
Electric fields are produced by the local build-up of electric charges in the atmosphere
associated with thunderstorms.
The earth's magnetic field causes a compass needle to orient in a North-South direction
and is used by birds and fish for navigation.
Human-made sources of electromagnetic fields
natural sources the electromagnetic spectrum also includes fields generated by human-made sources:
X-rays are employed
to diagnose a broken limb after a sport accident.
The electricity that comes out of every power socket has associated
low frequency electromagnetic fields.
Various kinds of higher frequency radio waves are used to transmit information
TV antennas, radio stations and mobile phone base stations.
The basics of wavelength and frequency
makes the various forms of electromagnetic fields so different?
One of the main characteristics which defines an
electromagnetic field (EMF) is its frequency or its corresponding wavelength.
Fields of different frequencies
interact with the body in different ways. One can imagine electromagnetic waves as series of very regular waves that travel
at an enormous speed, the speed of light. The frequency simply describes the number of oscillations or cycles per second,
while the term wavelength describes the distance between one wave and the next. Hence wavelength and frequency are inseparably
intertwined: the higher the frequency the shorter the wavelength.
What is the difference between non-ionizing
electromagnetic fields and ionizing radiation?
Wavelength and frequency determine another important characteristic of
Electromagnetic waves are carried by particles called quanta. Quanta of higher frequency (shorter
wavelength) waves carry more energy than lower frequency (longer wavelength) fields. Some electromagnetic waves carry so much
energy per quantum that they have the ability to break bonds between molecules.
In the electromagnetic spectrum, gamma
rays given off by radioactive materials, cosmic rays and X-rays carry this property and are called 'ionizing radiation'. Fields
whose quanta are insufficient to break molecular bonds are called 'non-ionizing radiation'.
Man-made sources of electromagnetic
fields that form a major part of industrialized life - electricity, microwaves and radiofrequency fields are found at the
relatively long wavelength and low frequency end of the electromagnetic spectrum and their quanta are thought to be unable
to break chemical bonds.
Electromagnetic Fields at Low Frequencies
Electric fields exist whenever a positive
or negative electrical charge is present. They exert forces on other charges within the field.
The strength of the
electric field is measured in volts per meter (V/m). Any electrical wire that is charged will produce an associated electric
field. This field exists even when there is no current flowing. The higher the voltage, the stronger the electric field at
a given distance from the wire.
Electric fields are strongest close to a charge or charged conductor, and their strength
rapidly diminishes with distance from it. Conductors such as metal shield them very effectively. Other materials, such
as building materials and trees, provide some shielding capability.
Therefore, walls, buildings, and trees reduce
the electric fields from power lines outside the house. When power lines are buried in the ground, the electric fields at
the surface are hardly detectable.
Magnetic fields arise from the motion of electric charges. The strength
of the magnetic field is measured in amperes per meter (A/m); more commonly in electromagnetic field research, scientists
specify a related quantity, the flux density (in microtesla, µT) instead. In contrast to electric fields, a magnetic field
is only produced once a device is switched on and current flows. The higher the current, the greater the strength of the magnetic
Like electric fields, magnetic fields are strongest close to their origin and rapidly decrease at greater
distances from the source. Magnetic fields are not blocked by common materials such as the walls of buildings.
do static fields differ from time-varying fields?
A static field does not vary over time. A direct current (DC) is an
electric current flowing in one direction only. In any battery-powered appliance the current flows from the battery to the
appliance and then back to the battery. It will create a static electric field. The earth's magnetic field is also a static
field. So is the magnetic field around a bar magnet which can be visualized by observing the pattern that is formed when iron
filings are sprinkled around it.
Time-varying electromagnetic fields are produced by alternating currents (AC). Alternating
currents reverse their direction at regular intervals.
What are the main sources of low, intermediate and high
The time-varying electromagnetic fields produced by electrical appliances are an example of extremely
low frequency (ELF) fields. ELF fields generally have frequencies up to 300 Hz.
Other technologies produce
intermediate frequency (IF) fields with frequencies from 300 Hz to 10 MHz and radiofrequency (RF) fields with frequencies
of 10 MHz to 300 GHz. The effects of electromagnetic fields on the human body depend not only on their field level but also
on their frequency and energy. Our electricity power supply and all appliances using electricity are the main sources of ELF
fields; computer screens, anti-theft devices and security systems are the main sources of IF fields; and radio, television,
radar and cellular telephone antennas, and microwave ovens are the main sources of RF fields. These fields induce currents
within the human body, which if sufficient can produce a range of effects such as heating and electrical shock, depending
on their amplitude and frequency range. (However, to produce such effects, the fields outside the body would have to be very
strong, far stronger than present in normal environments.)
Electromagnetic Fields at High Frequencies
telephones, television and radio transmitters and radar produce RF fields. These fields are used to transmit information over
long distances and form the basis of telecommunications as well as radio and television broadcasting all over the world. Microwaves
are RF fields at high frequencies in the GHz range. In microwaves ovens, we use them to quickly heat food.