Current (electricity)
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In electricity, current refers to electric current, which is the flow of electric charge. Lightning is an example of an electric current, as is the solar wind, the source of the polar aurora. Probably the most familiar form of electric current is the flow of conduction electrons in a metallic wire. This is how the electric company delivers electricity. In electronics, electric current is most often the flow of electrons through conductors and devices such as resistors, but it is also the flow of ions inside a battery or the flow of holes within a semiconductor.
The symbol typically used for the amount of current (the amount of charge flowing per unit of time) is I, from the German word Intensität, which means 'intensity'. The SI unit of electrical current is the ampere. Electric current is therefore sometimes informally referred to as amperage, by analogy with the term voltage. Though this is a valid term, some engineers frown on it.
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Conventional current
Conventional current was defined early in the history of electrical science as a flow of positive charge. In solid metals, like wires, the positive charges are immobile, and only the negatively charged electrons flow in the direction opposite conventional current, but this is not the case in most non-metallic conductors. In other materials, charged particles flow in both directions at the same time. Electric currents in electrolytes are flows of electrically charged atoms (ions), which exist in both positive and negative varieties. For example, an electrochemical cell may be constructed with salt water (a solution of sodium chloride) on one side of a membrane and pure water on the other. The membrane lets the positive sodium ions pass, but not the negative chlorine ions, so a net current results. Electric currents in plasma are flows of electrons as well as positive and negative ions. In ice and in certain solid electrolytes, flowing protons constitute the electric current. To simplify this situation, the original definition of conventional current still stands.
There are also instances where the electrons are the charge that is physically moving, but where it makes more sense to think of the current as the movement of positive "holes" (the spots that should have an electron to make the conductor neutral). This is the case in a p-type semiconductor.
The speed of an electric current
The charged particles whose movement causes an electric current do not always move in straight lines. In metals, for example, they follow an erratic path, bouncing from atom to atom, but generally drifting in the direction of the electric field. The speed at which they drift can be calculated from the equation:
- <math>I=nAvQ<math>
where
- I is the current
- n is number of charged particles per unit volume
- A is the cross-sectional area of the conductor
- v is the drift velocity, and
- Q is the charge on each particle.
For example, in a copper wire of cross-section 0.5 mm², carrying a current of 5 A, the drift velocity of the electrons is of the order of a millimetre per second. To take a different example, in the near-vacuum inside a cathode ray tube, the electrons travel in near-straight lines ("ballistically") at about a tenth of the speed of light.
However, we know that an electric signal travels much faster than this; usually close to the speed of light. These results show that the speed of the charged particles is not necessarily related to the speed of the electric signal. To understand how signals travel faster than the particles that carry them, it is necessary to understand the properties of electromagnetic waves (see article).
Current density
Current density is the current per unit (cross-sectional) area.
Mathematically, current is defined as the net flux through an area. Thus:
- <math>
\phi = j \cdot A <math>
where, in the MKS or SI system of measurement,
- φ is the current, measured in amperes
- j is the "current density" measured in amperes per square metre
- A is the area through which the current is flowing, measured in square metres
The current density is defined as:
- <math>
j=\int_i n_i \cdot x_i \cdot \mathbf{u_i} <math>
where
- n is the particle density (number of particles per unit volume)
- x is the mass, charge, or any other characteristic whose flow one would like to measure.
- u is the average velocity of the particles in each volume
Current density is an important consideration in the design of electrical and electronic systems. Most electrical conductors have a finite, positive resistance, making them dissipate power in the form of heat. The current density must be kept sufficiently low to prevent the conductor from melting or burning up, or the insulating material failing. In superconductors, excessive current density may generate a strong enough magnetic field to cause spontaneous loss of the superconductive property.
Electromagnetism
Every electric current produces a magnetic field. The magnetic field can be visualized as a pattern of circular field lines surrounding the wire.
Electric current can be directly measured with a galvanometer, but this method involves breaking the circuit, which is sometimes inconvenient. Current can also be measured without breaking the circuit by detecting the magnetic field it creates. Devices used for this include Hall effect sensors, current clamps and Rogowski coils.
Ohm's law
Ohm's law predicts the current in an (ideal) resistor (or other ohmic device) to be the quotient of applied voltage over electrical resistance:
- <math>
I = \frac{V}{R} <math>
where
- I is the current, measured in amperes
- V is the potential difference measured in volts
- R is the resistance measured in ohms
Electrical safety
The danger of an electric shock depends on the current (in milliamperes), duration and the current's path in the body:
- 1 mA causes a tingle
- 5 mA causes a slight shock
- 50 to 150 mA may result in death, e.g. through rhabdomyolysis (muscle breakdown) and resultant acute renal failure
- 1-4 A causes ventricular fibrillation
- 10 A causes cardiac arrest (only at this current will a typical home fuse break the circuit)
Currents through the heart and the nervous system are the most dangerous. As most dangerous sources are voltage sources, the current present depends on the resistance of the body between the points of contact and any current limiting built into the source.
SI electricity units
Template:SI electromagnetism units
See also
- Alternating current
- Direct current
- electrical conduction for more information on the physical mechanism of current flow in materials
External links
- Which direction does electricity really flow? (http://amasci.com/amateur/elecdir.html)ar:تيار كهربائي
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