Electricity (physics)

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In physics, a stream is the transport of a quantity-like quantity . Its measures are the current strength and the current density .

A stream is a special flow that is characterized by the fact that a quantifiable amount is transported. A similar analogy applies to current density and flux density . The specialization in electricity and flow is not common in all languages, for example “heat flux” denotes the heat flow in English . In quantum physics , electricity also means the transport of a quantity-like quantity between states - quasi through their interface in the state space (for example, charged and neutral currents in the weak interaction ).

If the quantity-like quantity is a conserved quantity , then a flow is the only possibility to change the quantity of this quantity in a volume element (because no sources and sinks can exist that would cause other changes).


The word "current" often means the strength of the current . It is defined as the magnitude of the quantity that moves through a (surface) area per time interval :

The current strength is a directed quantity which always applies to a single volume and its edge, i.e. H. its surface, is related. The surface of the volume is understood as an oriented surface . The current intensity is the measure of the flow of this volume also , so that shows signs of its size value , the current direction of.

Current density

The current density is a vector quantity . Its magnitude , also called intensity , is the amount that leaves the volume per time interval and (upper) area , and its direction is that of the mean drift speed of the movement:

Conversely, the current strength results mathematically as the area integral over the current density, clearly by adding up all current densities perpendicular to the surface:


where denotes the entire edge of the volume , i.e. the surface of the volume.

Mathematical formulation

The (extended) continuity equation applies :


  • the density of the quantity in a volume
  • the density of the rate of generation of the quantity-like quantity in the volume
  • the derivative of time
  • the divergence operator .

Based on the Gaussian integral theorem then applies


The quantity-like size and its density are connected via the volume integral :



For example, currents are considered in mechanics, thermodynamics, acoustics, optics, electricity, neutron physics and particle physics. The terms mass flow and volume flow are common for the movement of matter . For incompressible fluids like water, the volume can be viewed as a quantity.

If the quantitative quantities of thermal energy or electrical charge are carried along by moving matter, one speaks of convection current .

The Karlsruhe physics course strongly expands on such analogies and also advocates the idea of ​​an entropy and momentum stream in order to avoid Newton's idea of force and counterforce .


  • Hans Moor: Physical principles of construction and energy , vdf, Zurich 1993, vol. 1, p. 12f. ISBN 9783728118240
  • Norbert Pucker: Physical basics of energy technology , Springer, Vienna 1986, p. 20f. ISBN 978-3-211-81948-7

Individual evidence

  1. Heiner Schwarze et al. (Ed.): Everything flows , volume 1 in: Praxis der Wissenschaft im Schule , 61: 2012.