Electric fields, Magnetic fields, Electric and magnetic alternating fields, science basics!

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science basics every T.I. should know.

Electric fields, Magnetic fields, Electric and magnetic alternating fields, science basics!

Electric fields

An electric field originates from an electrical potential difference between two charge carriers, which is called electric voltage U and the unit is volt (V). Electric voltage can occur without current flow. The unit of the electric field strength is volt per meter (V/m).

The intensity of the electric field increases with higher electric voltages and it decreases with growing distance from the source.

If the intensity of the electric field is stable in magnitude and direction, it is a homogenous field as it occurs e.g. in a plate capacitor.

Homogenous electric field between the plates of a capacitor

Conversely, in an inhomogenous electric field, magnitude and direction depend on the location (e.g. in a two-core power cable; generally with a dipole).

Inhomogenous electric field of a dipole (e.g. two-core power cable, here shown in cross section)

Electric fields are strongly influenced by their environment.

Every conductive object can change the electric field. This can be explained by electrostatic induction, when under the influence of an electric field a transfer of charge distribution occurs in a conductive object. In a closed and conductive cage (“Faraday cage”) the electric field inside is practically zero. Also buildings can act as a shield, so that the electric field strength inside a building is negligibly small compared to the external electric field strength. Conversely, an electric field inside a conductive object (e.g. microwave oven) can be shielded almost completely to the outside world.

Full insulation by a Faraday cage: rearrangement of charges in the metal of the cage by moving of the positive and negative charges to opposite sides. This leads to a cancellation of the field inside the cage, because the resulting opposing field inside the cage superimposes the external field and exactly compensates it.

When a conductive object is exposed to a time-varying field, the constant charge exchange creates an alternating current in the object with the unit Ampere (A). The rate of charge transfer per unit area is called electric current density with the unit ampere per square meter (A/m²).

Magnetic fields

A magnetic field occurs when an electric charge is moved, i.e. current flow exists. The strength of the magnetic field is measured in ampere per meter (A/m) and is called magnetic field strength H. In contrast to the electric field strength E, the magnetic field strength does not represent the total power of the magnetic field, because this power depends not only on the current but also on the material that is permeated by the magnetic field. Thus, the strength of the magnetic field in matter is described by the magnetic flux density B with the unit Tesla (T). The term magnetic induction is often used instead of magnetic flux density. The magnetic flux density B is directly linked to the magnetic field strength H through the material constant µ, which stands for magnetic permeability.

B = µ × H

In simple terms, the magnetic permeability µ is a measure for the permeability of different materials to magnetic fields.

The magnetic field strength increases with amperage and decreases with growing distance from the source. The figure shows an example of a magnetic field with its magnetic field lines around a straight current carrying conductor.

Magnetic field lines of a current carrying conductor

In contrast to an electric field, a magnetic field can easily permeate through most materials. A shielding is only possible with high effort using expensive special materials. How much the magnetic field strength decreases with the distance from the source, depends also on the kind of the respective electric circuit (see figure below).

Decrease of the magnetic field strength with increasing distance from different sources

Electric and magnetic alternating fields

Electric and magnetic fields that do not change in time are called static fields. In alternating electric fields the polarity (+/-) of the field varies over time. If a current flows, the moving electric charges produce an alternating magnetic field. Inversely, alternating magnetic fields can generate eddy fields and eddy currents. This phenomenon is called induction, and the correspondent fields and currents are called induced electric fields or currents. Alternating fields are described according to their shape (e.g. sinusoidal) and time course (frequency f). The frequency of alternating fields is specified by the unit Hertz (Hz), i.e. the number of oscillations per second (1 Hz = 1/s). In public power supply, sinusoidal alternating fields with a frequency of 50 or 60 Hertz are used, i.e. the polarity of the electric field changes 100 times per second (see figure below).

Time course of a sinusoidal wave