Electromagnetic Field Properties
- What are electromagnetic fields (EMFs)? What are electromagnetic waves?
- The electromagnetic fields are a term for combining electric and magnetic fields. Electric voltage generates an electric field; electricity flowing generates a magnetic field.
Invisible EMF exist in many places around people. Home electric appliances, power equipment, and any other equipment that uses electricity generate EMF. TVs, radios, and mobile phones use EMF (radio waves) for telecommunications.
EMF also exists in nature. An electric field arises when a thundercloud is formed in the air. Static electricity is a phenomenon generated by an electric field. A magnetic field exists around a magnet. Compasses indicate north because of a magnetic field of the Earth (geomagnetism). Birds and fish find directions by the Earth magnetic field.
The electromagnetic waves describe the propagation of the electromagnetic fields.
An EMF generated from a transmission line or home electric appliance may be called an electromagnetic wave. However, since the wavelength of the commonly used EMF ranges from 5,000 to 6,000 km, the wave property can almost be disregarded in ordinary living space. Accordingly, it is more appropriate from a physics viewpoint that such an EMF be called an electromagnetic field rather than an electromagnetic wave.
- What is an electric field?
- It is a space (place) in which electricity exists. An electric field arises around any electrical charge or when a voltage difference occurs. For example, an electric field arises around the electric cord of a home electric appliance that is plugged in, even if the appliance is not turned on.
In general, any electric field decreases rapidly with distance.
- What is a magnetic field?
- It is a space (place) in which a magnetic field exist. A magnetic field appears when electricity flows. A home electric appliance generates a magnetic field only when the appliance is turned on. (However, a product requiring standby mode electricity generates a magnetic field even if the appliance is not turned on.)
In general, any magnetic field rapidly decreases with distance.
- I would like to know the types of electromagnetic fields and their actions.
- The entire spectrum of electromagnetic fields (EMFs) are divided into ionization radiation and nonionizing radiation. All the extremely low frequency (ELF) electromagnetic fields generated from transmission lines and home electric appliances, the intermediate frequency (IF) EMF generated from items such as induction heating (IH) stoves, and high-frequency EMF (radio waves) being used for television broadcasting and mobile phone communications are nonionizing radiation. X-rays and gamma rays have sufficient photon energy to cause ionization (generating positive and/or negative charged atoms and particles). Hence, X-rays and gamma rays are called ionization radiation.
Unlike ionization radiation, nonionizing radiation has no capacity to separate electrons from atoms (ionization) when they strike matter. If a person or any other living creature is exposed to nonionizing radiation, their DNA is not adversely affected.
As listed in the following table, EMF energy becomes smaller as the wavelength is longer.
＜Comparison of Ionizing and Nonionizing Radiation as Particle Energy＞
- Where EMF may be seen in nature?
- Very simple examples are the geomagnetic field which is created by our planet or a thunderbolt discharges. The intensity of geomagnetism varies from place to place, measuring approximately 30μT at the equator and 70μT at the North and South Poles. Geomagnetism refers to a static magnetic field whose direction is always fixed, unlike magnetic fields generated from power facilities whose directions change. The electric current discharged from a thunderbolt ranges from tens to hundreds of kilo amperes (kA) and the vicinity instantaneously forms a very large magnetic field.
An electric field is generated by electrically charged particles, the electric current is unidirectional flow of electrons. An example of the electric field is the crackling sound and feeling of static electricity that you may experience on a dry winter day when you take off your sweater.
- What are the units for electric and magnetic fields?
- The intensity of the electric field is expressed in volts/meter (V/m), while the intensity of the magnetic field is indicated in amperes/meter (A/m). This reflects to the fact that for electric field you need voltage difference, while for magnetic field you need a current.
In general, the intensity of the magnetic field is meant to be the magnetic flux density (number of magnetic force lines passing through a unit area), where tesla (T) and gauss (G) indicate the strength. The relationship between tesla and gauss is as follows:
1 G＝100 μT(1 gauss = 100 micro tesla)
1 mG＝0.1 μT(1 mill gauss = 0.1 micro tesla)
The intensity of the magnetic field in the SI unit system is indicated in tesla (T).
- When there are multiple sources of magnetic fields is the total strength of magnetic fields the sum of strength of each source?
- Because a magnetic field is vector quantity, the total strength is not obtained by simple summation. When a source is small, human exposure is not to the whole body but locally. Therefore, the space distribution has to be also considered.
- What is SAR (Specific Absorption Rate)?
- SAR (Specific Absorption Rate) is the energy amount per unit time absorbed by body tissue per unit mass when a living body is exposed to the electromagnetic fields. The unit is watt per kilogram (W/kg).
In case the whole body is exposed to a high-frequency electromagnetic field, thermal action have a harmful impact on the human body when the whole-body average SAR is approximately 4 watt/kilogram (W/kg) or more. Therefore, the standard for the whole-body average SAR is defined as 0.4 Watt/kilogram (W/kg) in working environment, considering safety factor of ten times, by the Radio Waves Protection Guidelines in Japan. In general environment, it is defined as 0.08 Watt/kilogram (W/kg) further adding five-time safety factor.
In case the energy of radio waves is absorbed intensively in a certain part of the body (for example, when using a mobile phone, the radio wave is absorbed intensively by the head), the standard for local SAR in general environment is defined as 2 Watt/kilogram (W/kg).
- What is the unit of high-frequency electromagnetic fields?
- The units for the strength of high-frequency electromagnetic fields are volt/meter (V/m) for electric field intensity, ampere/meter (A/m) for magnetic fields intensity or milliwatt/square centimeter (mW/cm2) for power density. As the following equation holds, if you know any value of the three, you can calculate the other two.
Power density (V/m) = Electric field intensity (V/m)2 ÷ 377 = 377 × Magnetic fields intensity(A/m)2
When a living body is exposed to a very strong electromagnetic field with frequency of approximately more than 100 kilohertz (kHz), part of the energy is absorbed by the body, is turned into heat and raises body temperature. This is called heat effect. As an indicator of heat generation inside the body, the amount of energy absorbed per unit time by body tissue per unit mass is represented as SAR (Specific Absorption Rate). The unit is Watt/kilogram (W/kg).