FACTOID # 18: Alaska spends more money per capita on elementary and secondary education than any other state.
 
 Home   Encyclopedia   Statistics   States A-Z   Flags   Maps   FAQ   About 
   
 
WHAT'S NEW
 

SEARCH ALL

FACTS & STATISTICS    Advanced view

Search encyclopedia, statistics and forums:

 

 

(* = Graphable)

 

 


Encyclopedia > Current (electricity)

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 utility companies deliver 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. Electricity is a property of matter that results from the presence or movement of electric charge. ... Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interactions. ... Lightning over Pentagon City in Arlington County, Virginia Lightning is a powerful natural electrostatic discharge produced during a thunderstorm. ... The plasma in the Solar Wind meeting the heliopause Ion storm redirects here. ... Aurora borealis Polar aurorae are optical phenomena characterized by colorful displays of light in the night sky. ... Electrical conduction is the movement of electrically charged particles through matter. ... Properties The electron is a fundamental subatomic particle which carries a negative electric charge. ... Two digital voltmeters The field of electronics is the study and use of systems that operate by controlling the flow of electrons or other electrically charged particles in devices such as thermionic valves and semiconductors. ... In science and engineering, conductors are materials that contain movable charges of electricity. ... Resistor symbols A pack of resistors A resistor is a two-terminal electrical or electronic component that resists the flow of current, producing a voltage drop between its terminals in accordance with Ohms law. ... An ion is an atom or group of atoms with a net electric charge. ... Four double-A (AA) rechargeable batteries In science and technology, a battery is a device that stores energy and makes it available in an electrical form. ... In solid state physics, an electron hole (usually referred to simply as a hole) is the absence of an electron from the otherwise full valence band. ... A semiconductor is a material with an electrical conductivity that is intermediate between that of an insulator and a conductor. ...

Contents


Relation between current and charge

The symbol typically used for the amount of current (the amount of charge Q flowing per unit of time t) is I, from the German word Intensität, which means 'intensity'.

I = {dQ over dt}

Formally this is written as

i(t) = {dq(t) over dt} or inversely as q(t) = int_{-infty}^{t} i(x), dx

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. Properties The electron is a fundamental subatomic particle which carries a negative electric charge. ... An electrolyte is a substance which dissociates free ions when dissolved (or molten), to produce an electrically conductive medium. ... An ion is an atom or group of atoms with a net electric charge. ... English chemists Jonh Daniell (left) and Michael Faraday(right), both are credited to as founders of electrochemistry as is known today. ... Flash point Non-flammable R/S statement R: none S: none RTECS number VZ4725000 Supplementary data page Structure and properties n, εr, etc. ... A Plasma lamp In physics and chemistry, a plasma is an ionized gas, and is usually considered to be a distinct phase of matter. ... Properties In physics, the proton (Greek proton = first) is a subatomic particle with an electric charge of one positive fundamental unit (1. ...


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. In solid state physics, an electron hole (usually referred to simply as a hole) is the absence of an electron from the otherwise full valence band. ... A semiconductor is a material with an electrical conductivity that is intermediate between that of an insulator and a conductor. ...


The SI unit of electrical current is the ampere. Electric current is therefore sometimes informally referred to as amperage or ampage, by analogy with the term voltage. Though this is a valid term, some engineers frown on it. The International System of Units (abbreviated SI from the French language name Système International dUnités) is the modern form of the metric system. ... The ampere (symbol: A) is the SI base unit of electrical current equal to one coulomb per second. ... This article may be too technical for most readers to understand. ...


The speed of an electric current (Drift Speed)

The mobile charged particles within a conductor move constantly in random directions. In order for a net flow of charge to exist, the particles must also move together with an average drift rate. For example, during currents in metals the particles 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: Hot metal work from a blacksmith In chemistry, a metal (Greek: Metallon) is an element that readily forms ions (cations) and has metallic bonds, and metals are sometimes described as a lattice of positive ions (cations) in a cloud of electrons. ... In physics, an electric field or E-field is an effect produced by an electric charge (or a time-varying magnetic field) that exerts a force on charged objects in the field. ...

I=nAvQ !

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.

Electric currents in solid matter are typically very slow flows. 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. General Name, Symbol, Number copper, Cu, 29 Chemical series transition metals Group, Period, Block 11, 4, d Appearance metallic brown Atomic mass 63. ... A wire is a single, usually cylindrical, elongated strand of drawn metal. ... The drift velocity is the average velocity that a particle, such as an electron, attains, due to an electric field. ... Cathode ray tube employing electromagnetic focus and deflection Cutaway rendering of a color CRT The cathode ray tube or CRT, invented by Karl Ferdinand Braun, is the display device that was traditionally used in most computer displays, video monitors, televisions and oscilloscopes. ... Cherenkov effect in a swimming pool nuclear reactor. ...


However, we know that electric current signals are waves which propagate at very high speed. As with any wave, the speed of the waves in a medium have little relation to the speed of that medium as it moves. For example, in AC power lines, the waves of current propagate rapidly from a source to a distant load, while the charges themselves only move back and forth over a tiny distance. The velocity of flowing charges can be quite low. Yet, any changes in electric current can travel at the speed of light, though it might be slower in certain media. The percentage of speed in a medium compared to the speed of light in vacuum is called velocity factor, and is proportional to refractive index. In information theory, a signal is a flow of information. ... Transmission lines in Lund, Sweden Electric power transmission is one process in the delivery of electricity to consumers. ... If an electric circuit has a well-defined output terminal, the circuit connected to this terminal (or its input impedance) is the load. ... Velocity of Propagation (VoP) or velocity factor is a parameter that characterizes the speed at which an electrical or radio signal passes through a medium. ... The refractive index of a material is the factor by which the phase velocity of electromagnetic radiation is slowed relative to vacuum. ...


Current density

Current density is the current per unit (cross-sectional) area.


Mathematically, current is defined as the net flux through an area. Thus:

I = j cdot A

where, in the MKS or SI system of measurement, The International System of Units (abbreviated SI from the French language name Système International dUnités) is the modern form of the metric system. ...

I 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: The ampere (symbol: A) is the SI base unit of electrical current equal to one coulomb per second. ... A square metre (US spelling: square meter) is by definition the area enclosed by a square with sides each 1 metre long. ... A square metre (US spelling: square meter) is by definition the area enclosed by a square with sides each 1 metre long. ...

j=int_i n_i cdot x_i cdot mathbf{u_i}

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. A magnet levitating above a high-temperature superconductor (with boiling liquid nitrogen underneath) demonstrates the Meissner effect. ...


Electromagnetism

Every electric current produces a magnetic field. The magnetic field can be visualized as a pattern of circular field lines surrounding the wire. Current flowing through a wire produces a magnetic field (M) around 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. It has been suggested that Tangent galvanometer be merged into this article or section. ... Current flowing through a wire produces a magnetic field (M) around the wire. ... Hall effect diagram, showing electron flow (rather than conventional current). ... To meet Wikipedias quality standards, this article or section may require cleanup. ... The Rogowski coil is an electrical device for measuring alternating current (AC). ...


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: Ohms law, named after its discoverer Georg Ohm [1], states that the potential difference or voltage drop (U or V) between the ends of a conductor and the current (I) flowing through the conductor are proportional at a given temperature: The equation contains the proportionality constant R, which is... Resistor symbols A pack of resistors A resistor is a two-terminal electrical or electronic component that resists the flow of current, producing a voltage drop between its terminals in accordance with Ohms law. ... An ohmic device is one that demonstrates the Ohms Law for the relationship of current through it and voltage across. ... This article may be too technical for most readers to understand. ... Electrical resistance is a measure of the degree to which an electrical component opposes the passage of current. ...

I = frac{V}{R}

where

I is the current, measured in amperes
V is the potential difference measured in volts
R is the resistance measured in ohms

The ampere (symbol: A) is the SI base unit of electrical current equal to one coulomb per second. ... Potential difference is a quantity in physics related to the amount of energy that would be required to move an object from one place to another against various types of force. ... The volt (symbol: V) is the SI derived unit of electric potential difference. ... Electrical resistance is a measure of the degree to which an electrical component opposes the passage of current. ... Ohm may refer to: The scientist Georg Ohm. ...

Electrical safety

The most obvious hazard is electric shock, where a current through part of the body can cause effects from a slight tingle to cardiac arrest or severe burns. It is the current that passes that determines the effect, and this depends on the nature of the contact, the condition of the body part, the current path through the body and the voltage of the source. The effect also varies considerably from individual to individual. (For approximate figures see Shock Effects under Electric shock.) Because of this and because in practical situations the current that may pass cannot be predicted any supply of over 24 volts should be considered a possible source of dangerous electric shock. In particular note that 110 volts can certainly be lethal. Sign warning of possible electric shock hazard Electrocution redirects here; for deliberate execution by electric shock, see electric chair. ...


Electric arcs, which can occur with supplies of any voltage (for example, a typical arc welding machine has a voltage between the electrodes of just a few volts), are very hot and emit ultra-violet and infra-red radiation. Proximity to an electric arc can therefore cause severe burns while UV is damaging to the unprotected eye. Manual Metal Arc welding, also known as stick or MMA welding is one of the most common forms of welding. ...


Accidental electric heating can also be dangerous. An overloaded power cable is a frequent cause of fire. A battery as small as an AA cell placed in a pocket with change can lead to a short circuit heating the battery and the coins which may inflict burns. NiCad and NiMh cells are particularly risky because they can deliver a very high current due to their low internal resistance.


See also

city lights viewed in a motion blurred exposure. ... Direct current (DC or continuous current) is the continuous flow of electricity through a conductor such as a wire from high to low potential. ... Electrical conduction is the movement of electrically charged particles through matter. ... Categories: Electromagnetism ...

External links

  • Online Current Converter - convert between various units of current, such as ampere, biot, abampere, statampere, and so on

  Results from FactBites:
 
Electric current - Wikipedia, the free encyclopedia (1321 words)
Electric currents in electrolytes are flows of electrically charged atoms (ions), which exist in both positive and negative varieties.
The SI unit of electric current is the ampere (A), which is equal to a flow of one coulomb of charge per second.
It is the current that passes that determines the effect, and this depends on the nature of the contact, the condition of the body part, the current path through the body and the voltage of the source.
  More results at FactBites »

 
 

COMMENTARY     


Share your thoughts, questions and commentary here
Your name
Your comments

Want to know more?
Search encyclopedia, statistics and forums:

 


Press Releases |  Feeds | Contact
The Wikipedia article included on this page is licensed under the GFDL.
Images may be subject to relevant owners' copyright.
All other elements are (c) copyright NationMaster.com 2003-5. All Rights Reserved.
Usage implies agreement with terms, 1022, m