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Encyclopedia > Electric current
Electromagnetism
Electricity · Magnetism
Electrodynamics
Electric current
Lorentz force law
Electromotive force
(EM) Electromagnetic induction
Displacement current
Maxwell's equations
(EMF) Electromagnetic field
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The amount of electric current (measured in amperes) through some surface, e.g., a section through a copper conductor, is defined as the amount of electric charge (measured in coulombs) flowing through that surface over time. If Q is the amount of charge that passed through the surface in the time t, then the average current I is:

$I = frac{Q}{t}$

By making the measurement time T shrink to zero, we get the instantaneous current i(t) as:

$i(t) = frac{dQ}{dt}$

The ampere, the measure of electric current, is an SI base unit so that the coulomb, the measure of electric charge, is derived from the definition of the ampere. For other uses, see Ampere (disambiguation). ... The SI system of units defines seven SI base units: physical units defined by an operational definition. ...

## Current in a metal wire

In solid conductive metal, a large population of electrons are mobile or free electrons. These electrons are bound to the metal lattice but not to any individual atom. Even without an external electric field applied, these electrons move about randomly due to thermal energy but on average, there is zero net current within the metal. Given an imaginary plane through which the wire passes, the number of electrons moving from one side to the other in any period of time is exactly equal to the number passing in the opposite direction. This box:      For other uses, see Solid (disambiguation). ... In science and engineering, conductors, such as copper or aluminum, are materials with atoms having loosely held valence electrons. ... In physics, the free electron model is a possible model for the behaviour of electrons in a crystal structure. ... In thermal physics, thermal energy is the energy portion of a system that increases with its temperature. ... For other uses, see Electron (disambiguation). ...

A typical metal wire for electrical conduction is the stranded copper wire.

The current I in amperes can be calculated with the following equation: For other uses, see Ampere (disambiguation). ...

$I = {Q over t}$

where

$Q !$ is the electric charge in coulombs (ampere seconds)
$t !$ is the time in seconds

It follows that: This box:      Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. ... The coulomb (symbol: C) is the SI unit of electric charge. ... Look up time in Wiktionary, the free dictionary. ... This article is about the unit of time. ...

$Q=It !$ and $t = {Q over I}$

## Current density

Main article: Current density

Current density is a measure of the density of electrical current. It is defined as a vector whose magnitude is the electric current per cross-sectional area. In SI units, the current density is measured in amperes per square meter. In electricity, current is the rate of flow of charges, usually through a metal wire or some other electrical conductor. ... For other uses, see Density (disambiguation). ... This article is about vectors that have a particular relation to the spatial coordinates. ... Look up si, Si, SI in Wiktionary, the free dictionary. ... For other uses, see Ampere (disambiguation). ... A square metre (US spelling: square meter) is by definition the area enclosed by a square with sides each 1 metre long. ...

## The drift speed of electric charges

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. Electrons are the charge carriers in metals and 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: This article is about metallic materials. ... In physics, the space surrounding an electric charge or in the presence of a time-varying magnetic field has a property called an electric field. ...

$I=nAvQ !$

where

$I !$ is the electric 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. For other uses, see Copper (disambiguation). ... 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: 1. ... The speed of light in a vacuum is an important physical constant denoted by the letter c for constant or the Latin word celeritas meaning swiftness.[1] It is the speed of all electromagnetic radiation, including visible light, in a vacuum. ...

However, we know that electrical signals are electromagnetic waves which propagate at very high speed outside the surface of the conductor (moving at the speed of light, as can be deduced from Maxwell's Equations). For example, in AC power lines, the waves of electromagnetic energy propagate through the space between the wires which is usually filled with insulating material, moving from a source to a distant load, even though the electrons in the wires only move back and forth over a tiny distance. The velocity of the flowing charges is quite low. The associated electromagnetic energy travels at a speed which is much faster. The velocity factor is a measure of the speed of electromagnetic propagation compared to the speed of light in a vacuum. The velocity factor is affected by the nature of the insulating medium surrounding the conductor, and also the magnetic properties of the materials of the conductor and its surroundings. In information theory, a signal is the sequence of states of a communications channel that encodes a message. ... Electromagnetism is the physics of the electromagnetic field: a field, encompassing all of space, composed of the electric field and the magnetic field. ... For thermodynamic relations, see Maxwell relations. ... Power line redirects here. ... 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) is a parameter that characterizes the speed at which an electrical or radio signal passes through a medium. ...

The nature of these three velocities can be clarified by analogy with the three similar velocities associated with gases. The low drift velocity of charge carriers is analogous to air motions; to wind. The large signal velocity is roughly analogous to the rapid propagation of sound waves, while the large random motion of charges is analogous to heat; to the high thermal velocity of randomly vibrating gas particles.

## Ohm's law

Ohm's law predicts the current in an (ideal) resistor (or other ohmic device) to be applied voltage divided by resistance: A voltage source, V, drives an electric current, I , through resistor, R, the three quantities obeying Ohms law: V = IR Ohms law states that, in an electrical circuit, the current passing through a conductor between two points is proportional to the potential difference (i. ... Resistor symbols (American) Resistor symbols (Europe, IEC) Axial-lead resistors on tape. ... An ohmic device is one that demonstrates the Ohms Law for the relationship of current through it and voltage across. ... International safety symbol Caution, risk of electric shock (ISO 3864), colloquially known as high voltage symbol. ... 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

For other uses, see Ampere (disambiguation). ... 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. ... Josephson junction array chip developed by NIST as a standard volt. ... Electrical resistance is a measure of the degree to which an electrical component opposes the passage of current. ... The ohm (symbol: Î©) is the SI unit of electric resistance. ...

## 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 charge carriers are immobile, and only the negatively charged electrons flow. Because the electron carries negative charge, the electron current is in the direction opposite that of the conventional (or electric) current. For other uses, see Electron (disambiguation). ...

Diagram showing conventional current notation. Electric charge moves from the positive side of the power source to the negative.

In other conductive materials, the electric current is due to the flow of charged particles 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 chloride 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. Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... An electrolyte is a substance which dissociates free ions when dissolved (or molten), to produce an electrically conductive medium. ... This article is about the electrically charged particle. ... English chemists John Daniell (left) and Michael Faraday (right), both credited to be founders of electrochemistry as known today. ... R-phrases 36 S-phrases none Flash point Non-flammable Related Compounds Other anions NaF, NaBr, NaI Other cations LiCl, KCl, RbCl, CsCl, MgCl2, CaCl2 Related salts Sodium acetate Supplementary data page Structure and properties n, Îµr, 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. ... For other uses, see Proton (disambiguation). ...

There are also materials where the electric current is due to the flow of electrons and yet it is conceptually easier to think of the current as due to the flow of positive "holes" (the spots that should have an electron to make the conductor neutral). This is the case in a p-type semiconductor. For the following two reasons the electron hole was introduced into calculations: If an electron is excited into higher state it leaves a hole in its old state. ... A semiconductor is a solid whose electrical conductivity is in between that of a conductor and that of an insulator, and can be controlled over a wide range, either permanently or dynamically. ...

## Examples

According to Ampère's law, an electric current produces a magnetic field.

Image File history File links This is a lossless scalable vector image. ... Image File history File links This is a lossless scalable vector image. ... An electric current produces a magnetic field. ... Magnetic field lines shown by iron filings Magnetostatics Electrodynamics Electrical Network Tensors in Relativity This box:      In physics, the magnetic field is a field that permeates space and which exerts a magnetic force on moving electric charges and magnetic dipoles. ...

## Electromagnetism

Electric current produces a magnetic field. The magnetic field can be visualized as a pattern of circular field lines surrounding the wire. Magnetic field lines shown by iron filings Magnetostatics Electrodynamics Electrical Network Tensors in Relativity This box:      In physics, the magnetic field is a field that permeates space and which exerts a magnetic force on moving electric charges and magnetic dipoles. ...

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 associated with the current. Devices used for this include Hall effect sensors, current clamps, current transformers, and Rogowski coils. Wire carrying current to be measured Restoring spring N and S are poles of magnet A galvanometer is a type of ammeter â€” an instrument for detecting and measuring electric current. ... Magnetic field lines shown by iron filings Magnetostatics Electrodynamics Electrical Network Tensors in Relativity This box:      In physics, the magnetic field is a field that permeates space and which exerts a magnetic force on moving electric charges and magnetic dipoles. ... Hall effect diagram, showing electron flow (rather than conventional current). ... Not to be confused with censure, censer, or censor. ... Current clamp is a way of recording biological voltages such as the action of an excitable cell with a microelectrode while keeping electrical current through the recording electrode very small (or at least constant). ... A CT for operation on a 110 kV grid A current transformer (CT) is a type of instrument transformer designed to provide a current in its secondary winding proportional to the current flowing in its primary. ... The Rogowski coil is an electrical device for measuring alternating current (AC). ...

## Reference direction

When solving electrical circuits, the actual direction of current through a specific circuit element is usually unknown. Consequently, each circuit element is assigned a current variable with an arbitrarily chosen reference direction. When the circuit is solved, the circuit element currents may have positive or negative values. A negative value means that the actual direction of current through that circuit element is opposite that of the chosen reference direction.

## Electrical safety

The most obvious hazard is electrical shock, where a current passes through part of the body. It is the amount of current passing through the body 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. While a very small amount can cause a slight tingle, too much can cause severe burns if it passes through the skin or even cardiac arrest if enough passes through the heart. The effect also varies considerably from individual to individual. (For approximate figures see Shock Effects under electric shock.) For other uses, see Burn. ... Sign warning of possible electric shock hazard An electric shock can occur upon contact of a humans body with any source of voltage high enough to cause sufficient current flow through the muscles or hair. ...

Due to this and the fact that passing current cannot be easily predicted in most practical circumstances, any supply of over 50 volts should be considered a possible source of dangerous electric shock. In particular, note that 110 volts (a minimum voltage at which AC mains power is distributed in much of the Americas, and 4 other countries, mostly in Asia) can certainly cause a lethal amount of current to pass through the body. Type F mains power plug & socket The term mains usually refers to the general purpose alternating current (AC) electrical power supply (as in â€œIve connected the appliance to the mainsâ€). The term is not usually used in the United States and Canada. ... The terms line voltage/frequency or mains voltage/frequency commonly refer to the electrical voltage and frequency used in the low voltage parts (in particular wall sockets) of an alternating current electricity supply network. ...

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 tens of volts), are very hot and emit ultra-violet (UV) and infra-red radiation (IR). Proximity to an electric arc can therefore cause severe thermal burns, and UV is damaging to unprotected eyes and skin. A 3000 volt electricity arc between two nails Electricity arcs between the power rail and electrical pickup shoe on a London Underground train An electric arc can melt calcium oxide An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current... Manual Metal Arc welding, also known as stick or MMA welding is one of the most common forms of welding. ... For other uses, see Electrode (disambiguation). ... Note: Ultraviolet is also the name of a 1998 UK television miniseries about vampires. ... Image of a small dog taken in mid-infrared (thermal) light (false color) Infrared (IR) radiation is electromagnetic radiation of a wavelength longer than visible light, but shorter than microwave radiation. ...

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 metal coins can lead to a short circuit heating the battery and the coins which may inflict burns. NiCad, NiMh cells, and Lithium batteries are particularly risky because they can deliver a very high current due to their low internal resistance. A power cable is an assembly of two or more electrical conductors, usually held together with an overall sheath. ... Four double-A batteries In science and technology, a battery is a device that stores energy and makes it available in an electrical form. ... The nickel-cadmium battery (commonly abbreviated NiCd and pronounced nye-cad) is a popular type of rechargeable battery for portable electronics and toys using the metals nickel (Ni) and cadmium (Cd) as the active chemicals. ... A nickel metal hydride battery, abbreviated NiMH, is a type of rechargeable battery similar to a nickel-cadmium (NiCd) battery but has a hydrogen-absorbing alloy for the anode instead of cadmium. ... CR2032 lithium battery Lithium batteries are primary batteries that have lithium metal or lithium compounds as an anode. ... The output impedance, source impedance, or internal impedance of an electronic device is the opposition exhibited by its output terminals to the flow of an alternating current (AC) of a particular frequency as a result of resistance, induction and capacitance. ...

 Electronics Portal

Image File history File links Nuvola_apps_ksim. ... City lights viewed in a motion blurred exposure. ... In electricity, current is the rate of flow of charges, usually through a metal wire or some other electrical conductor. ... Direct current (DC or continuous current) is the continuous flow of electricity through a conductor such as a wire from high to low potential. ... Conduction is the movement of electrically charged particles through a transmission medium (electrical conductor). ... In special and general relativity, the four-current is the Lorentz covariant four-vector that replaces the electromagnetic current density where c is the speed of light, &#961; the charge density, and j the conventional current density. ... Since electric current is invisible and the processes at play in electronics are often difficult to understand in an intuitive way, it is common to teach electronics using analogies to more common sense objects and processes. ... ...

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.
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