Electric current

The electric current, the movement of charge carriers. In the generalized sense, electrical current also refers to the displacement current, which move no charge carriers but changes the electrical flow. Then, the electric current is the whole of the electric phenomena, the cause of a magnetic field are understood.

Flowing charge carriers are typically electrons in a metal or in a vacuum, or ions, for example in an electrolyte, or a gas discharge lamp. The effects of the current include magnetic, thermal and chemical effects and luminous phenomena in gases.

In the technical language is often referred to as " current " whose strength, so the physical size of current with the formula sign and the unit ampere, in the vernacular, the transmission of electrical energy is usually meant or merely the opportunity to do so in the form of an energized installation.

• 3.1 DC
• 3.2 AC
• 3.3 mixed flow

• 5.1 Power Consumption
• 5.2 Effects of electric current on the human

History

Already Thales of Miletus is said to have discovered in the 6th century BC that amber attracts light bodies when it is first rubbed with cloths. One explanation he could not find though, the word electricity ( from the Greek " elektron " for " amber " ) but has still not back to this ancient discovery.

The technical use of the electric current began in the mid-19th century with the telegraph and electroplating. Initially ranged from the performance of batteries for both applications. 1866 was Werner von Siemens, the dynamo-electric principle and used it in the development of the first electric generator, which he could market it as blasting machine for the ignition of explosive charges. From 1880, these generators were developed more and more to large machines in order to meet the electricity needs of the ever-growing electricity grids. Primarily, these networks were used to provide electric power for lighting with arc and incandescent lamps in the public and the first private households. Another application of the electric current was in its use in lighthouses since the arc lamp has a much higher light intensity than the candles or kerosene lamps used previously. As a result, the first power plants that were initially driven with simple water turbines and steam engines emerged. Since the beginning of the 20th century, powerful steam turbines are available that dominate up to the present than combustion engines in power generation.

In the last years of the 19th century fell under the so-called current war, the decision between direct current and alternating current system in favor of the alternating current.

Physical relationships

For more quantitative information on electric current is used, the physical size of current.

Emergence of the current flow

Electrical current can have different causes:

• Redox reactions in batteries,
• Coulomb in electric fields, for example in capacitors,
• Lorentz forces in magnetic fields, such as generators,
• Entrainment of charge carriers through a flow (convection ), eg at influence or in the ionosphere,
• Diffusion of charge carriers in the differences in the concentration, such as at the boundaries of semi-conductors, with or without the presence of fields, referred to as a diffusion current.
• Change of the displacement flux and the field energy in non-conductors and the resulting displacement current.

Connected with the electrical voltage

For example, when a potential difference between the poles of a battery, it is called an electrical voltage. Then, due to the existing electric field forces are exerted on the carrier. Carriers that are exposed to this electric field can thus experience an acceleration when it is movable. This happens for example when a light bulb is between the poles, which is connected via metal wires to the poles. The drift velocity of the charge carriers in this directed movement is created in the interplay with scattering processes. The current density can be calculated by multiplying the drift velocity of the charge carrier density.

In many semiconductor materials of the drift current is proportional to the voltage at a constant temperature. If he accepts a stream of starch, the empirical fact is expressed as Ohm's law:

Wherein the proportionality factor is referred to as electric resistance of the current and voltage in this case is independent.

In a circuit with a voltage source whose fixed voltage and the resistance, determine what amperage. On the other hand builds using a current source whose fixed current across the resistor voltage to the concrete. In practice, however, tension springs occur much more frequently than current sources, such as in power supplies, which is why the actual value of the electric current by the consumer ( more precisely: its resistance ) depends.

Current conduction in metals

Metals in a portion of the electrons, known as the conduction electrons, not bound in each case to a specific atom, but ' includes ' all the atoms in common, see metallic bond. By the Drude model, the conductivity of metals is proportional to the number of the conduction electrons, and their mobility. More realistic is the band model.

Ion conductor

In ionic conductors, one speaks of conductors of the second class, it comes in direct current, in contrast to metals, as a rule, to a material modification of the electrical conductor. This effect is utilized in the electrolysis. Since the current transport is bound to a material transport from moving, electrically charged atoms ( ions) are eligible as ionic conductors mainly ionized gases and electrically conductive liquids. One calls this ion conductor electrolytes or plasma. Solids can also be ionic conductors in special cases. If such a material handling ( for example, in a gas discharge ) is undesirable, it can be largely prevented by alternating current.

Since no electronic current management, were chemical processes can change the nature of the conductor so that the electrical conductivity can gradually change.

Technical types of current

Direct current

As a DC ( Direct Current English, abbreviated as DC ) of said electric current is referred to, the direction and strength does not change with time, that is constant over time.

Virtually all electronic devices in the home such as radio and television receivers, computers, or even the controls of today's washing machines need for their power supply DC. But also in power engineering direct currents are used, for example, in fused-salt electrolysis for aluminum extraction, for good variable speed DC motors (now being replaced by power converters and induction motors), as an intermediate circuit in power converters in transmission equipment and in automotive electrical systems.

DC can be obtained by rectifying from AC. These are therefore used wherever direct current is needed, but is only the AC power distribution grid is available. Rare because much more expensive, we used direct DC power sources, such as galvanic cells, and photovoltaic cells. Curious special cases of no technical importance are electric machines that can produce direct current by means of unipolar induction directly without rectifier.

Alternating current

In AC ( alternating current English, abbreviated AC) there is a periodic change in the current direction. Each period consists of consecutive time periods with positive and negative instantaneous values ​​that add up to a mean current is zero. Crucial to the success of the alternating current was that the voltage using transformers can be changed very easily. All public power supply systems are operated with AC voltage - in Europe and many other countries with the mains frequency 50 Hz, in other parts of the world 60 Hz, see Overview of country plug types, voltages and frequencies.

A special form of alternating current is the three-phase alternating current ( colloquially Stark, rotary or electric power ) as for electrical energy distribution of high power is used in public distribution systems. This type of power makes particularly simply structured and robust electric motors.

Mixed flow

A combination of AC and DC current is called mixed. It does not necessarily lead to a change in direction of the mixed stream, but the time constant DC current component is changed periodically by the additionally applied alternating current in its strength (pulsating DC ). This mixed flow occurs, for example in rectifiers and smoothing with smoothing capacitors or chokes in power supplies. The while remaining (mostly unwanted ) alternating component is called Ripple which is coupled to a voltage ripple.

Impressed current

From an impressed current is when the current is independent of the value of the load resistor in a wide range. It may be direct current or alternating current at any frequency and waveform.

So-called laboratory power supplies have both an adjustable limiting the output voltage and an adjustable limiting the output current and thus have a rectangular characteristic. Which is obtained of the two limits, depends on the magnitude of the load. For example, if the limits are set at 30 V and 1.0 A, then (until neutral) achieved with a load resistance of 30 Ω, the voltage limit. A change in the resistance within the specified range, then only the current changes accordingly. The away unchanged residual stress is known as an impressed voltage. With a load resistance of less than 30 Ω ( until a short circuit ) the current limit is reached. A change in the resistance within the specified range, then only the voltage to match sets to values ​​below 30 V, while the change in load unchanged despite current flowing is an impressed current changes.

Electric current in everyday life

Electric phase current with a suitable power means the transport of electrical energy. In industrialized countries, the entire life of respect and transformation of this energy is penetrated.

Power consumption

The colloquial term " use current " is similar to the term " energy consumption ", technically not correct. Due to the conservation of charge and there exist no significant charges in technical devices, flows exactly the current that flows into a device, back out again.

Effect of the electric current to the human

Alternating electric currents in the mains frequency range is from 0.5 mA for the human body and noticeably at higher currents above 10 mA, which affect more than 2 s, dangerous. DC currents from 2 mA are noticeable and from 25 mA, which affect more than 2 s, dangerous. We also speak of an electric shock.

The following table shows the dangers of AC 50-60 Hz to IEC 60479-1 again:

In electric power systems and in the area of higher voltages, such as in high-voltage systems and in the field of electrical overhead lines in the web, even electrical accidents due to the arc effect occur. The power accident with arcing is almost invariably associated with burns and in addition it arise in the burn usually toxic combustion products.