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What are Electromagnetic Fields?
What are electromagnetic fields?
Definitions:
Electric fields are created by differences in voltage:
The higher the voltage, the stronger will be the resultant field.
Magnetic fields are created when electric current flows:
The greater the current, the stronger the magnetic field.
An electric field will exist even when there is no current flowing. If current does flow, the strength
of the magnetic field will vary with power consumption but the electric field strength will be
constant. (Extract from Electromagnetic fields published by the WHO Regional Office for Europe
in 1999 (Local authorities, health and environment briefing pamphlet series; 32).
Natural Sources of Electromagnetic Fields (EMFs)
Electromagnetic fields are present everywhere in our environment 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. And various kinds of higher frequency radiowaves are used to transmit information –
whether via TV antennas, radio stations or 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.
A simple analogy should help to illustrate the concept: Tie a long rope to a door handle and keep
hold of the free end. Moving it up and then down slowly will generate a single big wave; more
rapid motion will generate a whole series of small waves. The length of the rope remains
constant, therefore, the more waves you generate (higher frequency) the smaller will be the
distance between them (shorter 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 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.
What is the difference between DC (direct current) and AC ( alternating current) ?
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 magnetic 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.
In contrast, time-varying electromagnetic fields are produced by alternating currents (AC).
Alternating currents reverse their direction at regular intervals. In most European countries
electricity changes direction with a frequency of 50 cycles per second or 50 Hertz. Equally, the
associated electromagnetic field changes its orientation 50 times every second. North American
electricity has a frequency of 60 Hertz.
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.
At radio frequencies, electric and magnetic fields are closely interrelated and we typically
measure their levels as power densities in watts per square metre (W/m2).
Key Points:
The electromagnetic spectrum encompasses both natural and human-made sources of
electromagnetic fields.
Frequency and wavelength characterise an electromagnetic field. In an electromagnetic wave,
these two characteristics are directly related to each other: the higher the frequency the shorter
the wavelength.
Ionizing radiation such as X-ray and gamma-rays consists of photons which carry sufficient
energy to break molecular bonds. Photons of electromagnetic waves at power and radio
frequencies have much lower energy that do not have this ability.
Electric fields exist whenever charge is present and are measured in volts per metre (V/m).
Magnetic fields arise from current flow. Their flux densities are measured in microtesla (µT) or
millitesla (mT).
At radio and microwave frequencies, electric and magnetic fields are considered together as the
two components of an electromagnetic wave. Power density, measured in watts per square metre
(W/m2), describes the intensity of these fields.
Low frequency and high frequency electromagnetic waves affect the human body in different
ways.
Electrical power supplies and appliances are the most common sources of low frequency electric
and magnetic fields in our living environment. Everyday sources of radio frequency electro -
magnetic fields are telecommunications, broadcasting antennas and microwave ovens.
Source: World Health Organization 2006.
Definitions:
Electric fields are created by differences in voltage:
The higher the voltage, the stronger will be the resultant field.
Magnetic fields are created when electric current flows:
The greater the current, the stronger the magnetic field.
An electric field will exist even when there is no current flowing. If current does flow, the strength
of the magnetic field will vary with power consumption but the electric field strength will be
constant. (Extract from Electromagnetic fields published by the WHO Regional Office for Europe
in 1999 (Local authorities, health and environment briefing pamphlet series; 32).
Natural Sources of Electromagnetic Fields (EMFs)
Electromagnetic fields are present everywhere in our environment 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. And various kinds of higher frequency radiowaves are used to transmit information –
whether via TV antennas, radio stations or 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.
A simple analogy should help to illustrate the concept: Tie a long rope to a door handle and keep
hold of the free end. Moving it up and then down slowly will generate a single big wave; more
rapid motion will generate a whole series of small waves. The length of the rope remains
constant, therefore, the more waves you generate (higher frequency) the smaller will be the
distance between them (shorter 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 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.
What is the difference between DC (direct current) and AC ( alternating current) ?
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 magnetic 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.
In contrast, time-varying electromagnetic fields are produced by alternating currents (AC).
Alternating currents reverse their direction at regular intervals. In most European countries
electricity changes direction with a frequency of 50 cycles per second or 50 Hertz. Equally, the
associated electromagnetic field changes its orientation 50 times every second. North American
electricity has a frequency of 60 Hertz.
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.
At radio frequencies, electric and magnetic fields are closely interrelated and we typically
measure their levels as power densities in watts per square metre (W/m2).
Key Points:
The electromagnetic spectrum encompasses both natural and human-made sources of
electromagnetic fields.
Frequency and wavelength characterise an electromagnetic field. In an electromagnetic wave,
these two characteristics are directly related to each other: the higher the frequency the shorter
the wavelength.
Ionizing radiation such as X-ray and gamma-rays consists of photons which carry sufficient
energy to break molecular bonds. Photons of electromagnetic waves at power and radio
frequencies have much lower energy that do not have this ability.
Electric fields exist whenever charge is present and are measured in volts per metre (V/m).
Magnetic fields arise from current flow. Their flux densities are measured in microtesla (µT) or
millitesla (mT).
At radio and microwave frequencies, electric and magnetic fields are considered together as the
two components of an electromagnetic wave. Power density, measured in watts per square metre
(W/m2), describes the intensity of these fields.
Low frequency and high frequency electromagnetic waves affect the human body in different
ways.
Electrical power supplies and appliances are the most common sources of low frequency electric
and magnetic fields in our living environment. Everyday sources of radio frequency electro -
magnetic fields are telecommunications, broadcasting antennas and microwave ovens.
Source: World Health Organization 2006.
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