# Electric current
**Electric current** is the rate at which electric charge flows past a point or through a surface, written \( I = dq/dt \) and measured in the [SI base unit](https://www.edgepedia.org/si-base-unit) the ampere (A): one ampere is one coulomb of charge passing a given area per second.[1](https://openstax.org/books/university-physics-volume-2/pages/9-1-electrical-current) One coulomb equals about \( 6.241 \times 10^{18} \) elementary charges.[2](https://www.nist.gov/si-redefinition/ampere-introduction) Current is a macroscopic, measurable quantity; the microscopic motion of the carriers is described by the current density vector \( \vec{J} \), with units of amperes per square meter, whose direction is the direction of net flow of positive charge.[3](https://phys.libretexts.org/Bookshelves/University_Physics/Book%3A_Introductory_Physics_-_Building_Models_to_Describe_Our_World_(Martin_Neary_Rinaldo_and_Woodman)/19%3A_Electric_Current/19.01%3A_Current) • [4](https://openstax.org/books/university-physics-volume-2/pages/9-2-model-of-conduction-in-metals)
| Key fact | Detail |
|---|---|
| Definition | \( I = dq/dt \); 1 A = 1 C/s; one coulomb ≈ \( 6.241 \times 10^{18} \) elementary charges[1](https://openstax.org/books/university-physics-volume-2/pages/9-1-electrical-current) • [2](https://www.nist.gov/si-redefinition/ampere-introduction) |
| SI definition (2019) | The ampere is defined by fixing \( e = 1.602\,176\,634 \times 10^{-19} \) C = A s, effective 20 May 2019[5](https://www.bipm.org/en/si-base-units/ampere) • [2](https://www.nist.gov/si-redefinition/ampere-introduction) |
| Drift vs signal | Carriers drift at ~\( 10^{-4} \) m/s while signals propagate at ~\( 10^{8} \) m/s, a factor of about \( 10^{12} \)[4](https://openstax.org/books/university-physics-volume-2/pages/9-2-model-of-conduction-in-metals) |
| Direction | Conventional current points the way positive charge would flow, opposite to electron motion[1](https://openstax.org/books/university-physics-volume-2/pages/9-1-electrical-current) |
| Everyday scale | Hair dryer ~15 A, home breakers 15–20 A, arc welding up to ~200 A, lightning 100,000 A or more[2](https://www.nist.gov/si-redefinition/ampere-introduction) |
| Human safety | NASA crew limits: ≤40 mA DC and ≤8 mA peak AC to avoid catastrophic effects; ≤2 mA DC and ≤0.5 mA AC for startle scenarios[6](https://www.nasa.gov/wp-content/uploads/2023/12/ochmo-tb-021-electrical-shock.pdf) |
| Ampacity | The maximum current, in amperes, a conductor can carry continuously under conditions of use without exceeding its temperature rating (NEC Article 100)[7](https://ampacitychart.com/) |
## Conduction mechanisms
**Metals.** In a metal the valence band is partially filled and serves as the conduction band, so electrons move freely under an electric field.[8](https://www.britannica.com/science/electricity/Conductors-insulators-and-semiconductors) Conductivity is the product of carrier number density, charge, and mobility, \( \sigma = n \cdot e \cdot \mu \); metallic conductivities span roughly \( 10^{4} \) to \( 10^{6} \) S/cm (resistivity 1–100 µΩ·cm), and are effectively infinite in superconductors.[9](https://www.eolss.net/sample-chapters/c05/E6-39A-03-02.pdf)
**Drift velocity versus signal speed.** The individual charges in a wire move slowly. A worked example gives a drift velocity of \( -4.54 \times 10^{-4} \) m/s for 20.00 A in a copper wire with free charge density \( 8.34 \times 10^{28} \)/m³ and cross-section \( 3.30 \times 10^{-6} \) m²;[4](https://openstax.org/books/university-physics-volume-2/pages/9-2-model-of-conduction-in-metals) a 12-gauge copper wire carrying 10.0 A gives \( 2.23 \times 10^{-4} \) m/s, a typical order of magnitude.[10](http://www.physics.mcgill.ca/~hilke/142/Chapter%2027.pdf) Yet most electrical signals travel at speeds on the order of \( 10^{8} \) m/s, a significant fraction of the speed of light, because the electric field itself propagates through the insulation at roughly 60% to 90% of \( c \) depending on the dielectric.[4](https://openstax.org/books/university-physics-volume-2/pages/9-2-model-of-conduction-in-metals) • [11](https://electricalflux.com/learn-components/electrical-current-flow-electrons-ions-practical) The gap between the two speeds is about \( 10^{12} \).[4](https://openstax.org/books/university-physics-volume-2/pages/9-2-model-of-conduction-in-metals) [Individual](https://www.edgepedia.org/individual) conduction electrons also need not travel the full length of a circuit.[12](https://phys.libretexts.org/Courses/Berea_College/Electromagnetics_I_(Messina)/06%3A_Steady_Current_and_Conductivity)
**Other media.** Carriers are not always electrons. Ions are the prevalent carriers in electrolytes and biological systems.[13](https://phys.libretexts.org/Courses/University_of_California_Davis/UCD%3A_Physics_9C__Electricity_and_Magnetism/3%3A_Direct_Current_Circuits/3.1%3A_Moving_Charge) Electrolytic conduction is the convective movement of oppositely directed streams of ions, a term from Faraday, carrying their charges through the liquid; through liquid metals such as mercury, by contrast, current flows as in solid metals without changing the conductor's state.[14](https://www.gutenberg.org/cache/epub/32063/pg32063-images.html) In semiconductors the band gap is smaller than in insulators, and at room temperature a few thermally excited electrons cross into the conduction band, leaving mobile holes; in insulators the large gap prevents field-driven motion unless electrons gain enough energy to cross it.[8](https://www.britannica.com/science/electricity/Conductors-insulators-and-semiconductors) Vacuum, ionized gas, and liquid electrolyte conduction are each distinct mechanisms from metallic conduction.[15](https://iopscience.iop.org/book/mono/978-0-7503-5854-5/chapter/bk978-0-7503-5854-5ch2)
**Conduction versus convection current.** A convection current consists of charged particles moving in response to mechanical forces, such as wind-borne charged cloud, rather than being guided by the electric field; [Ohm's law](https://www.edgepedia.org/ohms-law) applies only to conduction current.[12](https://phys.libretexts.org/Courses/Berea_College/Electromagnetics_I_(Messina)/06%3A_Steady_Current_and_Conductivity)
## Ohm's law and non-ohmic behavior
Ohm's law in microscopic form, \( J = \sigma E \), is an empirical relationship valid only for certain (ohmic) materials; junction diodes are a standard non-ohmic example.[10](http://www.physics.mcgill.ca/~hilke/142/Chapter%2027.pdf) The classical Drude-type derivation shows why: current density comes out proportional to the electric field, yielding \( V = IR \) from microscopic quantities.[16](https://www.macmillanlearning.com/studentresources/college/physics/tiplermodernphysics6e/classial_concept_review/chapter_10_ccr_11_classical_theory_of_electical_conduction.pdf)
**Gas discharges** are strongly non-ohmic. Conduction through ionized gas obeys Ohm's law only when the current is very small; the current then reaches a constant "saturation" value before rising again near spark potentials, curve shapes obtained by von Schweidler (1899) and J. S. Townsend (1901).[14](https://www.gutenberg.org/cache/epub/32063/pg32063-images.html)
For alternating current, the skin effect pushes current toward the conductor's surface, reducing the effective cross-section, and raising resistance, and the proximity effect raises resistance further when two AC-carrying conductors are near each other; at commercial power frequencies both matter for large conductors.[17](https://codexery.com/electricity-and-magnetism/e/electricity-and-magnetism/electrical-conductor/)
## Current and magnetism
In 1820 [Hans Christian Ørsted](https://www.edgepedia.org/hans-christian-rsted) showed that a compass needle near an electric current deflects from north, demonstrating that current produces a magnetic field encircling the wire.[18](https://www.nist.gov/si-redefinition/ampere/ampere-history) Inspired by that demonstration, [André-Marie Ampère](https://www.edgepedia.org/andre-marie-ampere) found that two parallel current-carrying wires attract or repel depending on whether their currents flow in the same or opposite directions, with the force growing with wire length and current.[18](https://www.nist.gov/si-redefinition/ampere/ampere-history) Historical scholarship holds that the very concept of electric current was elaborated only after Ørsted's 1820 discovery and Ampère's subsequent work, including his experiment with a compass above a [Voltaic pile](https://www.edgepedia.org/voltaic-pile) and above the connecting wire; Ampère created a new quantity, the current, identical in pile and wire and characterized only by direction and intensity, defined experimentally by the oriented deviation of a magnetic needle.[19](https://pubs.aip.org/aapt/ajp/article/85/5/369/1038150/The-key-role-of-Oersted-s-and-Ampere-s-1820)
## Measurement
AC current is commonly expressed as an RMS value; a True RMS meter is needed for accurate measurement of non-sinusoidal current waveforms. AC clamp meters use a current transformer, while DC clamp meters use a Hall-effect sensor.[20](https://electricalflux.com/learn-components/electrical-current-ampacity-wire-sizing) Typical device currents give the scale: a 120 V space heater draws 12.5 A, a hardwired Level 2 EV charger 32 to 48 A at 240 V, and a 3-ton central air conditioner 18 A with locked-rotor current up to 90 A.[20](https://electricalflux.com/learn-components/electrical-current-ampacity-wire-sizing)
**Ampacity** is defined in NEC Article 100 as the maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating; circuit breakers and fuses are rated in amperes accordingly.[7](https://ampacitychart.com/) • [1](https://openstax.org/books/university-physics-volume-2/pages/9-1-electrical-current)
## By the numbers
A 1,800-watt hair dryer draws about 15 amps, home circuit breakers are rated 15 to 20 amps for a single line, arc welding uses up to around 200 amps, and a lightning bolt can carry 100,000 amps or more.[2](https://www.nist.gov/si-redefinition/ampere-introduction) At the small end, quantum current standards work in the nanoampere range: a unified quantum realization determined currents of 9.33 to 252 nA with a lowest uncertainty of 4.3 µA/A at 83.9 nA.[21](https://www.nature.com/articles/s41928-025-01421-2) A striking contrast sits at the top of the scale: an average lightning bolt carries only around 5 coulombs of charge despite currents of tens of thousands of amps, because the strike lasts only a few tens of milliseconds.[2](https://www.nist.gov/si-redefinition/ampere-introduction)
## Safety and physiological effects
The thresholds for shock effects are set in milliamps. NASA standards require limiting current through a crewmember to ≤40 mA DC and ≤8 mA peak AC to avoid catastrophic physiological effects, and to ≤2 mA DC and ≤0.5 mA AC to avoid potentially catastrophic conditions from startle reactions.[6](https://www.nasa.gov/wp-content/uploads/2023/12/ochmo-tb-021-electrical-shock.pdf) Those values were chosen from the IEC TR 60479-2, Figure 7 threshold for maintaining muscle control if shocked, protecting 99.5% of the population using the most sensitive population (women).[6](https://www.nasa.gov/wp-content/uploads/2023/12/ochmo-tb-021-electrical-shock.pdf) The IEC/TS 60479-1 curves a, b, and c1 remain the prime reference for physiological thresholds, which relate to touch current thresholds through body impedance according to Ohm's law.[22](https://assets.vde-verlag.de/iec-normen/preview-pdf/info_iec60479-5%7Bed1.0%7Db.pdf)
## History and the 2019 redefinition
Ampère's manuscripts already contain the substance of the two modern theoretical definitions of current intensity: the ratio of charge to time, \( q/t \), and the ratio of electromotive force to resistance, \( E/R \).[19](https://pubs.aip.org/aapt/ajp/article/85/5/369/1038150/The-key-role-of-Oersted-s-and-Ampere-s-1820) Early practical units set the ohm at \( 10^{9} \) electromagnetic units of resistance and the volt at \( 10^{8} \) electromagnetic units of electromotive force, ratified and expanded by international congresses held between 1881 and 1904; a 1901 proposal by Giovanni Giorgi led toward the modern SI framework.[23](https://pubs.aip.org/aapt/ajp/article-pdf/82/1/60/13097949/60_1_online.pdf)
**The force-based ampere.** Until 2019 the ampere was defined by fixing the exact value of the attractive force between two current-carrying wires in an ideal configuration, which fixed the magnetic constant of vacuum at \( \mu_{0} = 4\pi \times 10^{-7} \) N/A².[24](https://ar5iv.labs.arxiv.org/html/1910.11310)
**The charge-based ampere.** In November 2018 the redefinition of the ampere, along with the kilogram, kelvin, and mole, was approved, and from 20 May 2019 the ampere is based on the elementary charge \( e \).[2](https://www.nist.gov/si-redefinition/ampere-introduction) The BIPM definition fixes the numerical value of \( e \) at exactly \( 1.602\,176\,634 \times 10^{-19} \) when expressed in coulombs, equal to A s, with the second defined by the caesium hyperfine frequency \( \Delta\nu_{\mathrm{Cs}} \); equivalently, one ampere is the flow of \( 1/(1.602\,176\,634 \times 10^{-19}) \) elementary charges per second.[5](https://www.bipm.org/en/si-base-units/ampere) • [25](https://beta.iopscience.iop.org/article/10.1088/1681-7575/ac0503/meta) Since that revision, any current source whose output can be expressed as \( e \cdot f \), with \( f \) a frequency, provides a realization of the ampere.[26](https://www.nature.com/articles/s41467-025-56413-9)
**Realizations.** The BIPM mise en pratique lists three practical routes: Ohm's law (A = V/Ω) using Josephson-effect volt and quantum Hall-effect ohm realizations; single electron transport (SET) devices using A = C/s; or a capacitor via \( I = C \cdot dU/dt \).[27](https://www.bipm.org/documents/20126/41489676/SI-App2-ampere.pdf) SET implementations still had technical limitations and often larger relative uncertainties than competing techniques at the time of that document.[27](https://www.bipm.org/documents/20126/41489676/SI-App2-ampere.pdf) Counting electrons directly is demanding because \( e \) is about a tenth of a billionth of a billionth of the charge moved per second in a 1-ampere current.[2](https://www.nist.gov/si-redefinition/ampere-introduction) Practical SET standards require error rates below 0.01 ppm and many sources in parallel to reach useful nanoampere currents; a source on an industry-grade 300 mm silicon MOS process has now measured 0.008 ppm.[28](https://arxiv.org/html/2601.03916) At the national-laboratory level, a programmable quantum current generator combining a [Josephson voltage standard](https://www.edgepedia.org/josephson-voltage-standard), a quantum Hall resistor, and a cryogenic current comparator realized the ampere with \( 10^{-8} \) relative uncertainty, and PTB and LNE realizations in the ±50 µA range agreed to −3.7 parts in \( 10^{7} \) with combined standard uncertainty 3.1 parts in \( 10^{7} \).[25](https://beta.iopscience.iop.org/article/10.1088/1681-7575/ac0503/meta)
## References
1. [9.1 Electrical Current, University Physics Volume 2, OpenStax](https://openstax.org/books/university-physics-volume-2/pages/9-1-electrical-current)
2. [Ampere: Introduction, NIST](https://www.nist.gov/si-redefinition/ampere-introduction)
3. [19.1 Current, Physics LibreTexts](https://phys.libretexts.org/Bookshelves/University_Physics/Book%3A_Introductory_Physics_-_Building_Models_to_Describe_Our_World_(Martin_Neary_Rinaldo_and_Woodman)/19%3A_Electric_Current/19.01%3A_Current)
4. [9.2 Model of Conduction in Metals, University Physics Volume 2, OpenStax](https://openstax.org/books/university-physics-volume-2/pages/9-2-model-of-conduction-in-metals)
5. [Ampere, BIPM](https://www.bipm.org/en/si-base-units/ampere)
6. [NASA-STD-3001 Volume 2 Rev D, Electrical Shock Technical Brief](https://www.nasa.gov/wp-content/uploads/2023/12/ochmo-tb-021-electrical-shock.pdf)
7. [NEC Wire Ampacity Chart: Table 310.16](https://ampacitychart.com/)
8. [Electricity: Conductors, insulators, and semiconductors, Britannica](https://www.britannica.com/science/electricity/Conductors-insulators-and-semiconductors)
9. [Conductive Materials, Wires, and Cables, EOLSS](https://www.eolss.net/sample-chapters/c05/E6-39A-03-02.pdf)
10. [Chapter 27 course notes, McGill University](http://www.physics.mcgill.ca/~hilke/142/Chapter%2027.pdf)
11. [Why Electrical Current Is the Flow of Electrons (And When It Isn't), ElectricalFlux](https://electricalflux.com/learn-components/electrical-current-flow-electrons-ions-practical)
12. [Steady Current and Conductivity, Physics LibreTexts](https://phys.libretexts.org/Courses/Berea_College/Electromagnetics_I_(Messina)/06%3A_Steady_Current_and_Conductivity)
13. [3.1 Moving Charge, Physics LibreTexts (UC Davis)](https://phys.libretexts.org/Courses/University_of_California_Davis/UCD%3A_Physics_9C__Electricity_and_Magnetism/3%3A_Direct_Current_Circuits/3.1%3A_Moving_Charge)
14. [Conduction, Electric, Encyclopædia Britannica (1911)](https://www.gutenberg.org/cache/epub/32063/pg32063-images.html)
15. [Electric charge transport in media other than metals, IOPscience](https://iopscience.iop.org/book/mono/978-0-7503-5854-5/chapter/bk978-0-7503-5854-5ch2)
16. [Classical Theory of Electrical Conduction, Tipler (Macmillan)](https://www.macmillanlearning.com/studentresources/college/physics/tiplermodernphysics6e/classial_concept_review/chapter_10_ccr_11_classical_theory_of_electical_conduction.pdf)
17. [Electrical conductor, Codexery](https://codexery.com/electricity-and-magnetism/e/electricity-and-magnetism/electrical-conductor/)
18. [Ampere: History, NIST](https://www.nist.gov/si-redefinition/ampere/ampere-history)
19. [The key role of Oersted's and Ampère's 1820 electromagnetic experiments, American Journal of Physics](https://pubs.aip.org/aapt/ajp/article/85/5/369/1038150/The-key-role-of-Oersted-s-and-Ampere-s-1820)
20. [Ampacity, Wire Sizing, and Real-World Circuit Behavior, ElectricalFlux](https://electricalflux.com/learn-components/electrical-current-ampacity-wire-sizing)
21. [A unified realization of electrical quantities from the quantum SI, Nature Electronics](https://www.nature.com/articles/s41928-025-01421-2)
22. [IEC/TS 60479-5 preview, Effects of current on human beings and livestock](https://assets.vde-verlag.de/iec-normen/preview-pdf/info_iec60479-5%7Bed1.0%7Db.pdf)
23. [The Daniell cell, Ohm's law, and the emergence of the International System of Units, American Journal of Physics](https://pubs.aip.org/aapt/ajp/article-pdf/82/1/60/13097949/60_1_online.pdf)
24. [The ampere and the electrical units in the quantum era, arXiv](https://ar5iv.labs.arxiv.org/html/1910.11310)
25. [Improvements of the programmable quantum current generator, Metrologia](https://beta.iopscience.iop.org/article/10.1088/1681-7575/ac0503/meta)
26. [A primary quantum current standard based on the Josephson and the quantum Hall effects, Nature Communications](https://www.nature.com/articles/s41467-025-56413-9)
27. [Mise en pratique for the ampere, BIPM SI Brochure Appendix 2](https://www.bipm.org/documents/20126/41489676/SI-App2-ampere.pdf)
28. [A current source with metrological precision made on a 300mm silicon MOS process, arXiv](https://arxiv.org/html/2601.03916)
---
*Topic: Encyclopedia › Physical world and mathematics › Physics › Classical physics › Electromagnetism › Electric and magnetic fields › Magnetostatics › Steady currents and conductors*
*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license