Electric current

Electric current
SI unit		ampere
Derivations from other quantities	${\displaystyle I={V \over R},I={Q \over t}}$
Dimension	${\displaystyle {\mathsf {I}}}$

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An electric current is a flow of

The conventional symbol for current is I, which originates from the French phrase intensité du courant (current intensity).[5]^[6] Current intensity is often referred to simply as current.^[7] The I symbol was used by André-Marie Ampère, after whom the unit of electric current is named, in formulating Ampère's force law (1820).^[8] The notation travelled from France to Great Britain, where it became standard, although at least one journal did not change from using C to I until 1896.^[9]

The conventional direction of current, also known as conventional current,

A flow of positive charges gives the same electric current, and has the same effect in a circuit, as an equal flow of negative charges in the opposite direction. Since current can be the flow of either positive or negative charges, or both, a convention is needed for the direction of current that is independent of the type of charge carriers. Negatively charged carriers, such as the electrons (the charge carriers in metal wires and many other electronic circuit components), therefore flow in the opposite direction of conventional current flow in an electrical circuit.^[10][11]

A current in a wire or

${\displaystyle I}$ to represent the current, the direction representing positive current must be specified, usually by an arrow on the

^: 13 This is called the reference direction of the current ${\displaystyle I}$. When

Ohm's law states that the current through a conductor between two points is directly

$${\displaystyle I={\frac {V}{R}},}$$

where I is the current through the conductor in units of

In

In contrast,

Current can be measured using an ammeter.

Electric current can be directly measured with a

Current can also be measured without breaking the circuit by detecting the magnetic field associated with the current. Devices, at the circuit level, use various

• Shunt resistors^[18]
• Hall effect current sensor transducers
• Transformers (however DC cannot be measured)
• Magnetoresistive field sensors^[19]
• Rogowski coils
• Current clamps

Joule heating, also known as ohmic heating and resistive heating, is the process of

$${\displaystyle P\propto I^{2}R.}$$

This relationship is known as

In an electromagnet a coil of wires behaves like a magnet when an electric current flows through it. When the current is switched off, the coil loses its magnetism immediately. Electric current produces a magnetic field. The magnetic field can be visualized as a pattern of circular field lines surrounding the wire that persists as long as there is current.

Magnetic fields can also be used to make electric currents. When a changing magnetic field is applied to a conductor, an electromotive force (EMF) is induced,^[21]: 1004 which starts an electric current, when there is a suitable path.

When an electric current flows in a

For a steady flow of charge through a surface, the current I (in amperes) can be calculated with the following equation: $${\displaystyle I={Q \over t}\,,}$$ where Q is the electric charge transferred through the surface over a time t. If Q and t are measured in coulombs and seconds respectively, I is in amperes.

More generally, electric current can be represented as the rate at which charge flows through a given surface as: $${\displaystyle I={\frac {\mathrm {d} Q}{\mathrm {d} t}}\,.}$$

Electric currents in electrolytes are flows of electrically charged particles (ions). For example, if an electric field is placed across a solution of Na⁺ and Cl⁻ (and conditions are right) the sodium ions move towards the negative electrode (cathode), while the chloride ions move towards the positive electrode (anode). Reactions take place at both electrode surfaces, neutralizing each ion.

Water-ice and certain solid electrolytes called proton conductors contain positive hydrogen ions ("protons") that are mobile. In these materials, electric currents are composed of moving protons, as opposed to the moving electrons in metals.

In certain electrolyte mixtures, brightly coloured ions are the moving electric charges. The slow progress of the colour makes the current visible.^[23]

In air and other ordinary

Plasma is the state of matter where some of the electrons in a gas are stripped or "ionized" from their molecules or atoms. A plasma can be formed by high temperature, or by application of a high electric or alternating magnetic field as noted above. Due to their lower mass, the electrons in a plasma accelerate more quickly in response to an electric field than the heavier positive ions, and hence carry the bulk of the current. The free ions recombine to create new chemical compounds (for example, breaking atmospheric oxygen into single oxygen [O₂ → 2O], which then recombine creating ozone [O₃]).^[24]

Since a "

Superconductivity is a phenomenon of exactly zero

In a

Current density is the rate at which charge passes through a chosen unit area.

^: 22

In linear materials such as metals, and under low frequencies, the current density across the conductor surface is uniform. In such conditions,

ohmic device

): $${\displaystyle I={V \over R}\,,}$$

where ${\displaystyle I}$ is the current, measured in amperes; ${\displaystyle V}$ is the

; and ${\displaystyle R}$ is the

The mobile charged particles within a conductor move constantly in random directions, like the particles of a gas. (More accurately, a Fermi gas.) To create a net flow of charge, the particles must also move together with an average drift rate. Electrons are the charge carriers in most metals and they follow an erratic path, bouncing from atom to atom, but generally drifting in the opposite direction of the electric field. The speed they drift at can be calculated from the equation: $${\displaystyle I=nAvQ\,,}$$ where

• ${\displaystyle I}$ is the electric current
• ${\displaystyle n}$ is number of charged particles per unit volume (or charge carrier density)
• ${\displaystyle A}$ is the cross-sectional area of the conductor
• ${\displaystyle v}$ is the drift velocity, and
• ${\displaystyle Q}$ is the charge on each particle.

Typically, electric charges in solids flow slowly. 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 on 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 at about a tenth of the speed of light.

Any accelerating electric charge, and therefore any changing electric current, gives rise to an

The ratio of the speed of the electromagnetic wave to the speed of light in free space is called the velocity factor, and depends on the electromagnetic properties of the conductor and the insulating materials surrounding it, and on their shape and size.

The magnitudes (not the natures) of these three velocities can be illustrated by an analogy with the three similar velocities associated with gases. (See also hydraulic analogy.)

• The low drift velocity of charge carriers is analogous to air motion; in other words, winds.
• The high speed of electromagnetic waves is roughly analogous to the speed of sound in a gas (sound waves move through air much faster than large-scale motions such as convection)
• The random motion of charges is analogous to heat – the thermal velocity of randomly vibrating gas particles.

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