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What are static and low frequency electric and magnetic fields?
- Electric and magnetic fields are force fields.
- Constant fields (static or stationary) that do not change in time and alternating fields varying with time are distinguished. Slowly varying fields are referred to as "low-frequency fields".
- Static and low-frequency electric fields emanate from electric charges. They exert forces on other electric charges.
- Magnetic fields are caused by moving electric charges. They occur e.g. around current-carrying electric conductors. In permanent magnets, electric currents at an atomic level cause the field. Magnetic fields exert forces on magnets, magnetisable material or moving electric charges.
Electric and magnetic fields are force fields. The direction and strength of a field can be visualised as field lines: The density of the lines symbolises the field strength. As opposed to electric field lines - which start and end at electric charges -, magnetic field lines are closed.
In an electric field, the arrow indicates the direction of a force exerted on a positive elementary charge. If magnetisable material or permanent magnets are moved into a magnetic field, they try to line up along the field lines.
The electric field
Electric charges exert forces on each other. Like charges repel, opposite charges - i.e. positive and negative charges – attract each other.
An electric (force) field builds up between oppositely charged bodies, the strength of which is expressed in the unit volt per metre (V/m). The field strength mainly depends on the charges of the bodies involved (potential difference) and on the distance between these bodies.
The Earth’s fair weather field between the ionosphere (electrically well-conducting atmospheric layer) and the ground is a natural electric field.
Static and time-varying electric fields
Fields with forces constant in time are referred to as "static or stationary electric fields". The term "DC electric field" is also common. However, in everyday life, we mostly deal with technically generated fields that change in strength and direction in a specific routine. Such fields can be referred to as "alternating electric fields" (AC fields).
For example, household plugs and sockets have an alternating voltage at a frequency of 50 Hz. This means that the electric field building up around the wall socket varies in time in such a way that it reaches again its maximum strength in the same direction 50 times per second. The voltage applied to the plugs and sockets changes its polarity and the electric field changes its direction twice as often, i.e. 100 times per second, respectively.
Static or low-frequency electric field forces acting on a conductive body cause movements of electric charges on the body surface. This process is referred to as "(electrostatic) induction". The surface is charged, whilst the interior of a conductive body is practically field-free.
Electric fields can be shielded very well using electrically conductive materials (especially metals) because of the electrostatic inductive effect. The mode of action involved in the so-called "Faraday cage" is also based on the principle of electrostatic induction.
The magnetic field
Magnetic fields are caused by moving electric charges (electric current flow). In permanent magnets, electric currents at an atomic level cause the field. Just like the electric field, the magnetic field is a force field. It is able to exert forces on magnets, magnetisable material or on moving electric charges.
The strength of a field is expressed in the unit Ampere per metre (A/m). The so called "magnetic flux density" is often used as well, expressed in the unit Tesla (T). These units can be easily converted: In air or in biological tissue, 80 amperes per metre correspond to 100 microteslas, i.e. 0.0001 Tesla.
Magnetic material, mostly metal, which aligns itself as compass needles with the Earth's magnetic poles, has been used for about 1,000 years to provide evidence of magnetic fields.
Static and time-varying magnetic fields
Fields the forces of which remain constant in time, are referred to as static or stationary (DC) fields also in the case of magnetic fields. An example of a naturally occurring magnetic field is the Earth's magnetic field. It can be described as a "static" field, because significant changes in the field's strength occur only over very long periods of time (several thousand years).
When electric charges are conducted through wires, i.e. when current flows, a magnetic field builds up around the wires - the higher the amperage, the stronger the magnetic field.
When current continuously changes its direction, as is the case with the technically generated 50 Hz alternating current, the magnetic field is changed at the same rate as well. This is the case with an alternating magnetic field of the same frequency. Electrical direct currents, in contrast, are surrounded by magnetic DC fields.
Alternating magnetic fields induce electric voltages and fields in electrically conductive bodies. The strength of the induction effect depends upon
- the magnetic field strength acting on the body,
- the body's alignment with the magnetic field lines,
- the body's shape,
- its size and
Alternating magnetic fields induce electric voltages and fields in the human body, too.
In contrast to electric fields, magnetic fields are not easily shielded. House walls, as well as organic tissues and the human body, are penetrated by magnetic fields. Only special metallic shields can have an appreciable effect in the case of magnetic fields.
State of 2018.03.01