Quantities and units
Electromagnetic units are part of a system of electrical units based primarily upon the magnetic properties of electric currents, the fundamental SI unit being the ampere. The units are:
In the electromagnetic cgs system, electric current is a fundamental quantity defined via Ampère's law and takes the permeability as a dimensionless quantity (relative permeability) whose value in a vacuum is unity. As a consequence, the square of the speed of light appears explicitly in some of the equations interrelating quantities in this system.
SI electromagnetism units
| ||||
|---|---|---|---|---|
| Symbol | Name of Quantity | Derived Units | Unit | Base Units |
| I | electric current | ampere (SI base unit) | A | A (= W/V = C/s) |
| Q | electric charge | coulomb | C | A⋅s |
| U, ΔV, Δφ; E | potential difference; electromotive force | volt | V | kg⋅m2⋅s−3⋅A−1 (= J/C) |
| R; Z; X | electric resistance; impedance; reactance | ohm | Ω | kg⋅m2⋅s−3⋅A−2 (= V/A) |
| ρ | resistivity | ohm metre | Ω⋅m | kg⋅m3⋅s−3⋅A−2 |
| P | electric power | watt | W | kg⋅m2⋅s−3 (= V⋅A) |
| C | capacitance | farad | F | kg−1⋅m−2⋅s4⋅A2 (= C/V) |
| E | electric field strength | volt per metre | V/m | kg⋅m⋅s−3⋅A−1 (= N/C) |
| D | electric displacement field | coulomb per square metre | C/m2 | A⋅s⋅m−2 |
| ε | permittivity | farad per metre | F/m | kg−1⋅m−3⋅s4⋅A2 |
| χe | electric susceptibility | (dimensionless) | – | – |
| G; Y; B | conductance; admittance; susceptance | siemens | S | kg−1⋅m−2⋅s3⋅A2 (= Ω−1) |
| κ, γ, σ | conductivity | siemens per metre | S/m | kg−1⋅m−3⋅s3⋅A2 |
| B | magnetic flux density, magnetic induction | tesla | T | kg⋅s−2⋅A−1 (= Wb/m2 = N⋅A−1⋅m−1) |
| Φ | magnetic flux | weber | Wb | kg⋅m2⋅s−2⋅A−1 (= V⋅s) |
| H | magnetic field strength | ampere per metre | A/m | A⋅m−1 |
| L, M | inductance | henry | H | kg⋅m2⋅s−2⋅A−2 (= Wb/A = V⋅s/A) |
| μ | permeability | henry per metre | H/m | kg⋅m⋅s−2⋅A−2 |
| χ | magnetic susceptibility | (dimensionless) | – | – |
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