List of electromagnetism equations

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This article summarizes equations in the theory of electromagnetism.

Contents

Definitions

Lorentz force on a charged particle (of charge q) in motion (velocity v), used as the definition of the E field and B field. Lorentz force particle.svg
Lorentz force on a charged particle (of charge q) in motion (velocity v), used as the definition of the E field and B field.

Here subscripts e and m are used to differ between electric and magnetic charges. The definitions for monopoles are of theoretical interest, although real magnetic dipoles can be described using pole strengths. There are two possible units for monopole strength, Wb (Weber) and A m (Ampere metre). Dimensional analysis shows that magnetic charges relate by qm(Wb) = μ0qm(Am).

Initial quantities

Quantity (common name/s)(Common) symbol/sSI unitsDimension
Electric charge qe, q, QC = As[I][T]
Monopole strength, magnetic charge qm, g, pWb or Am[L]2[M][T]−2 [I]−1 (Wb)

[I][L] (Am)

Electric quantities

Continuous charge distribution. The volume charge density r is the amount of charge per unit volume (cube), surface charge density s is amount per unit surface area (circle) with outward unit normal n, d is the dipole moment between two point charges, the volume density of these is the polarization density P. Position vector r is a point to calculate the electric field; r' is a point in the charged object. Universal charge distribution.svg
Continuous charge distribution. The volume charge density ρ is the amount of charge per unit volume (cube), surface charge density σ is amount per unit surface area (circle) with outward unit normal , d is the dipole moment between two point charges, the volume density of these is the polarization density P. Position vector r is a point to calculate the electric field; r is a point in the charged object.

Contrary to the strong analogy between (classical) gravitation and electrostatics, there are no "centre of charge" or "centre of electrostatic attraction" analogues.[ citation needed ]

Electric transport

Quantity (common name/s)(Common) symbol/sDefining equationSI unitsDimension
Linear, surface, volumetric charge densityλe for Linear, σe for surface, ρe for volume.

C mn, n = 1, 2, 3[I][T][L]n
Capacitance C

V = voltage, not volume.

F = C V1[I]2[T]4[L]2[M]1
Electric current IA[I]
Electric current density JA m2[I][L]2
Displacement current densityJdA m2[I][L]2
Convection current densityJcA m−2[I][L]−2

Electric fields

Quantity (common name/s)(Common) symbol/sDefining equationSI unitsDimension
Electric field, field strength, flux density, potential gradientEN C−1 = V m−1[M][L][T]−3[I]−1
Electric flux ΦEN m2 C−1[M][L]3[T]−3[I]−1
Absolute permittivity;εF m−1[I]2 [T]4 [M]−1 [L]−3
Electric dipole moment p

a = charge separation directed from -ve to +ve charge

C m[I][T][L]
Electric Polarization, polarization density PC m−2[I][T][L]−2
Electric displacement field, flux densityDC m−2[I][T][L]−2
Electric displacement fluxΦDC[I][T]
Absolute electric potential, EM scalar potential relative to point

Theoretical:
Practical: (Earth's radius)

φ ,VV = J C−1[M] [L]2 [T]−3 [I]−1
Voltage, Electric potential differenceΔφVV = J C−1[M] [L]2 [T]−3 [I]−1

Magnetic quantities

Magnetic transport

Quantity (common name/s)(Common) symbol/sDefining equationSI unitsDimension
Linear, surface, volumetric pole densityλm for Linear, σm for surface, ρm for volume.

Wb mn

A m(n + 1),
n = 1, 2, 3

[L]2[M][T]−2 [I]−1 (Wb)

[I][L] (Am)

Monopole currentImWb s1

A m s1

[L]2[M][T]3 [I]1 (Wb)

[I][L][T]1 (Am)

Monopole current densityJmWb s1 m2

A m1 s1

[M][T]3 [I]1 (Wb)

[I][L]1[T]1 (Am)

Magnetic fields

Quantity (common name/s)(Common) symbol/sDefining equationSI unitsDimension
Magnetic field, field strength, flux density, induction fieldBT = N A−1 m−1 = Wb m−2[M][T]−2[I]−1
Magnetic potential, EM vector potentialAT m = N A−1 = Wb m3[M][L][T]−2[I]−1
Magnetic flux ΦBWb = T m2[L]2[M][T]−2[I]−1
Magnetic permeability V·s·A1·m1 = N·A2 = T·m·A1 = Wb·A1·m1[M][L][T]−2[I]−2
Magnetic moment, magnetic dipole moment m, μB, Π

Two definitions are possible:

using pole strengths,

using currents:

a = pole separation

N is the number of turns of conductor

A m2[I][L]2
Magnetization MA m−1[I] [L]−1
Magnetic field intensity, (AKA field strength)HTwo definitions are possible:

most common:

using pole strengths, [1]

A m−1[I] [L]−1
Intensity of magnetization, magnetic polarizationI, JT = N A−1 m−1 = Wb m−2[M][T]−2[I]−1
Self Inductance LTwo equivalent definitions are possible:

H = Wb A−1[L]2 [M] [T]−2 [I]−2
Mutual inductance MAgain two equivalent definitions are possible:

1,2 subscripts refer to two conductors/inductors mutually inducing voltage/ linking magnetic flux through each other. They can be interchanged for the required conductor/inductor;


H = Wb A−1[L]2 [M] [T]−2 [I]−2
Gyromagnetic ratio (for charged particles in a magnetic field)γHz T−1[M]−1[T][I]

Electric circuits

DC circuits, general definitions

Quantity (common name/s)(Common) symbol/sDefining equationSI unitsDimension
Terminal Voltage for power supply VterV = J C−1[M] [L]2 [T]−3 [I]−1
Load Voltage for CircuitVloadV = J C−1[M] [L]2 [T]−3 [I]−1
Internal resistance of power supplyRintΩ = V A−1 = J s C−2[M][L]2 [T]−3 [I]−2
Load resistance of circuitRextΩ = V A−1 = J s C−2[M][L]2 [T]−3 [I]−2
Electromotive force (emf), voltage across entire circuit including power supply, external components and conductorsEV = J C−1[M] [L]2 [T]−3 [I]−1

AC circuits

Quantity (common name/s)(Common) symbol/sDefining equationSI unitsDimension
Resistive load voltageVRV = J C−1[M] [L]2 [T]−3 [I]−1
Capacitive load voltageVCV = J C−1[M] [L]2 [T]−3 [I]−1
Inductive load voltageVLV = J C−1[M] [L]2 [T]−3 [I]−1
Capacitive reactance XCΩ−1 m−1[I]2 [T]3 [M]−2 [L]−2
Inductive reactance XLΩ−1 m−1[I]2 [T]3 [M]−2 [L]−2
AC electrical impedance Z

Ω−1 m−1[I]2 [T]3 [M]−2 [L]−2
Phase constantδ, φdimensionlessdimensionless
AC peak currentI0A[I]
AC root mean square currentIrmsA[I]
AC peak voltageV0V = J C−1[M] [L]2 [T]−3 [I]−1
AC root mean square voltageVrmsV = J C−1[M] [L]2 [T]−3 [I]−1
AC emf, root mean squareV = J C−1[M] [L]2 [T]−3 [I]−1
AC average powerW = J s−1[M] [L]2 [T]−3
Capacitive time constantτCs[T]
Inductive time constantτLs[T]

Magnetic circuits

Quantity (common name/s)(Common) symbol/sDefining equationSI unitsDimension
Magnetomotive force, mmfF,

N = number of turns of conductor

A[I]

Electromagnetism

Electric fields

Summary of electrostatic relations between electric potential, electric field and charge density. Here,
r
=
x
-
x
'
{\displaystyle \mathbf {r} =\mathbf {x} -\mathbf {x'} }
. Electrostatics relation triangle.svg
Summary of electrostatic relations between electric potential, electric field and charge density. Here, .

General Classical Equations

Physical situationEquations
Electric potential gradient and field

Point charge
At a point in a local array of point charges
At a point due to a continuum of charge
Electrostatic torque and potential energy due to non-uniform fields and dipole moments

Magnetic fields and moments

Summary of magnetostatic relations between magnetic vector potential, magnetic field and current density. Here,
r
=
x
-
x
'
{\displaystyle \mathbf {r} =\mathbf {x} -\mathbf {x'} }
. Magnetostatics relation triangle.svg
Summary of magnetostatic relations between magnetic vector potential, magnetic field and current density. Here, .

General classical equations

Physical situationEquations
Magnetic potential, EM vector potential
Due to a magnetic moment

Magnetic moment due to a current distribution
Magnetostatic torque and potential energy due to non-uniform fields and dipole moments

Electric circuits and electronics

Below N = number of conductors or circuit components. Subscript net refers to the equivalent and resultant property value.

Physical situationNomenclatureSeriesParallel
Resistors and conductors
  • Ri = resistance of resistor or conductor i
  • Gi = conductance of resistor or conductor i

Charge, capacitors, currents
  • Ci = capacitance of capacitor i
  • qi = charge of charge carrier i

Inductors
  • Li = self-inductance of inductor i
  • Lij = self-inductance element ij of L matrix
  • Mij = mutual inductance between inductors i and j

CircuitDC Circuit equationsAC Circuit equations
Series circuit equations
RC circuits Circuit equation

Capacitor charge

Capacitor discharge

RL circuits Circuit equation

Inductor current rise

Inductor current fall

LC circuits Circuit equation

Circuit equation

Circuit resonant frequency

Circuit charge

Circuit current

Circuit electrical potential energy

Circuit magnetic potential energy

RLC circuits Circuit equation

Circuit equation

Circuit charge

See also

Footnotes

  1. M. Mansfield; C. O'Sullivan (2011). Understanding Physics (2nd ed.). John Wiley & Sons. ISBN   978-0-470-74637-0.

Sources

Further reading