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
Besides 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 radiowaves are used to transmit information TV antennas, radio stations and
mobile phone base stations.
The basics of wavelength and frequency
What 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 ionising radiation?
Wavelength and frequency
determine another important characteristic of electromagnetic fields:
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 metre (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, the electric fields from power lines outside the house are reduced by walls, buildings, and trees. 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 field.
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.
How 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 frequency fields?
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 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
Mobile 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.
