The electric current (outdated and current intensity ) is a physical quantity from the theory of electricity , which the electric current is measured. The current strength always relates to a suitably selected oriented surface , for example the cross-sectional area of a conductor ( convection current ) or the cross-section of a capacitor ( displacement current ). In the simplest case of a constant current flow, the current intensity is the amount of charge that has flowed through the cross section and is related to the period of time under consideration .
The current is a basic quantity of the international system of units (SI) and is specified in the unit of measurement amperes with the unit symbol A. Your formula symbol is that , to identify a time dependency, the lower case letter is also used for the instantaneous value .
In the case of sinusoidal alternating current, as it is most frequently used for practical electrical energy supply , the mean value of the current over time is zero - regardless of the peak value as the maximum instantaneous value of the current. The rms value of the current is constant for currents that are periodic over time and is also indicated with the symbol .
Relationships that can be used for definition
The following applies to a charge flow that is constant over time
with the amount of charge that passes through an oriented surface in the period of time .
If the current intensity changes over time , this relationship provides the mean value of the current intensity during the duration . Instead of a mean value, the instantaneous value is given here:
The area-related current strength is called current density and is the associated surface element. With it the current strength can be written as:
If the current density is evenly distributed over the cross-sectional area, which is fulfilled in the case of direct current through a homogeneous conductor, then this relationship is simplified to or to or , depending on the orientation of the surface, for a perpendicularly flowed surface.
Direction or sign
The direction of electric current is defined as the direction in which positive electric charge moves. In the case of negative charge carriers, for example electrons, the “positive” direction of current is correspondingly opposite to the direction of movement of these charge carriers.
In many representations of electrical circuits , counting arrows are used parallel or anti-parallel to the direction of movement. In principle, their direction is arbitrary. Since the current intensity is a scalar quantity , the arrows only define the sign of the current intensity for direct current: If the direction of the current corresponds to the direction of the arrow, the current intensity is positive. Arrows can also be useful for alternating current if they are to mark the direction of the energy flow.
To measure the current, the current to be measured must flow through the measuring device. It is therefore connected in series with the consumer .
Digital current measuring devices are usually voltage measuring devices in their basic structure that measure the voltage drop across a built-in or external measuring resistor ( shunt ). Analog current measuring devices use different effects of the electric current:
- chemical effect in the silver coulometer (no longer used)
- Thermal effect in the hot wire measuring mechanism (no longer used) and in bimetal sensors (usually not with a display but with a switching function such as in overcurrent protection devices and for temperature control in the iron)
- Magnetic effect in the moving coil measuring mechanism (for direct current ) and in the moving iron measuring mechanism ( also for the effective value of alternating current ). There are various other current sensors that are based on the magnetic effect.
"Natural" currents on Earth range from a pico amps through a sodium channel to over one hundred kilo amps in flashes . Examples from everyday life are the charging current of a cell phone battery ( around 2 A) and the current per pixel (around 1 µA ). With a microampere, around six trillion elementary charges flow through the conductor cross-section per second.
Of potential barriers for the charge carriers occurs at low current strengths shot noise on. Very small currents can be measured or specifically generated by counting the charge carriers. This makes use of the fact that the electrical charge and thus the voltage of a capacitor shows a quantization if a barrier is built into one of the supply lines through which the line electrons tunnel individually (in a conductor without a tunnel barrier they change and continuously).
- Wilhelm Weber, Heinrich Weber: Wilhelm Weber's works . Springer-Verlag, July 29, 2013, ISBN 978-3-662-24693-1 , p. 610.
- Ludwig Bergmann, Clemens Schaefer: Textbook of Experimental Physics, Volume II, Electricity . 4th edition. Walter de Gruyter & Co., Berlin 1961, p. 123 .
- DIN 5483-2 "Time-dependent quantities"
- IEC 60050, see DKE German Commission for Electrical, Electronic and Information Technologies in DIN and VDE: International Electrotechnical Dictionary entry 131-11-29
- DIN 40110-1: 1994 alternating current quantities , chap. 3.1.
- Leonhard Stiny: Active electronic components: design, structure, mode of operation, properties and practical use of discrete and integrated semiconductor components . Springer Fachmedien Wiesbaden, September 2, 2015, ISBN 978-3-658-09153-8 , p. 260.
- Ernst O. Goebel, Uwe Siegner: Quantum Metrology: Foundation of Units and Measurements . Wiley-VCH 2015, ISBN 9783527680924 , limited preview in Google Book Search.