List of electromagnetism equations

Last updated

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.

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

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

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 circuitsCircuit equation

Capacitor charge

Capacitor discharge

RL circuitsCircuit equation

Inductor current rise

Inductor current fall

LC circuitsCircuit equation

Circuit equation

Circuit resonant frequency

Circuit charge

Circuit current

Circuit electrical potential energy

Circuit magnetic potential energy

RLC CircuitsCircuit 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

Related Research Articles

<span class="mw-page-title-main">Electromagnetism</span> Fundamental interaction between charged particles

In physics, electromagnetism is an interaction that occurs between particles with electric charge via electromagnetic fields. The electromagnetic force is one of the four fundamental forces of nature. It is the dominant force in the interactions of atoms and molecules. Electromagnetism can be thought of as a combination of electrostatics and magnetism, which are distinct but closely intertwined phenomena. Electromagnetic forces occur between any two charged particles. Electric forces cause an attraction between particles with opposite charges and repulsion between particles with the same charge, while magnetism is an interaction that occurs between charged particles in relative motion. These two forces are described in terms of electromagnetic fields. Macroscopic charged objects are described in terms of Coulomb's law for electricity and Ampère's force law for magnetism; the Lorentz force describes microscopic charged particles.

<span class="mw-page-title-main">Electromagnetic field</span> Electric and magnetic fields produced by moving charged objects

An electromagnetic field is a physical field, mathematical functions of position and time, representing the influences on and due to electric charges. The field at any point in space and time can be regarded as a combination of an electric field and a magnetic field. Because of the interrelationship between the fields, a disturbance in the electric field can create a disturbance in the magnetic field which in turn affects the electric field, leading to an oscillation that propagates through space, known as an electromagnetic wave.

<span class="mw-page-title-main">Mathematical physics</span> Application of mathematical methods to problems in physics

Mathematical physics refers to the development of mathematical methods for application to problems in physics. The Journal of Mathematical Physics defines the field as "the application of mathematics to problems in physics and the development of mathematical methods suitable for such applications and for the formulation of physical theories". An alternative definition would also include those mathematics that are inspired by physics, known as physical mathematics.

<span class="mw-page-title-main">Magnetic monopole</span> Hypothetical particle with one magnetic pole

In particle physics, a magnetic monopole is a hypothetical elementary particle that is an isolated magnet with only one magnetic pole. A magnetic monopole would have a net north or south "magnetic charge". Modern interest in the concept stems from particle theories, notably the grand unified and superstring theories, which predict their existence. The known elementary particles that have electric charge are electric monopoles.

In physics, chemistry and biology, a potential gradient is the local rate of change of the potential with respect to displacement, i.e. spatial derivative, or gradient. This quantity frequently occurs in equations of physical processes because it leads to some form of flux.

In physics, semiclassical refers to a theory in which one part of a system is described quantum mechanically, whereas the other is treated classically. For example, external fields will be constant, or when changing will be classically described. In general, it incorporates a development in powers of Planck's constant, resulting in the classical physics of power 0, and the first nontrivial approximation to the power of (−1). In this case, there is a clear link between the quantum-mechanical system and the associated semi-classical and classical approximations, as it is similar in appearance to the transition from physical optics to geometric optics.

In theoretical physics and applied mathematics, a field equation is a partial differential equation which determines the dynamics of a physical field, specifically the time evolution and spatial distribution of the field. The solutions to the equation are mathematical functions which correspond directly to the field, as functions of time and space. Since the field equation is a partial differential equation, there are families of solutions which represent a variety of physical possibilities. Usually, there is not just a single equation, but a set of coupled equations which must be solved simultaneously. Field equations are not ordinary differential equations since a field depends on space and time, which requires at least two variables.

<span class="mw-page-title-main">Magnetic scalar potential</span> Magnetic analog of electric potential valid outside materials

Magnetic scalar potential, ψ, is a quantity in classical electromagnetism analogous to electric potential. It is used to specify the magnetic H-field in cases when there are no free currents, in a manner analogous to using the electric potential to determine the electric field in electrostatics. One important use of ψ is to determine the magnetic field due to permanent magnets when their magnetization is known. The potential is valid in any region with zero current density, thus if currents are confined to wires or surfaces, piecemeal solutions can be stitched together to provide a description of the magnetic field at all points in space.

<span class="mw-page-title-main">Gauss's law for magnetism</span> Foundational law of classical magnetism

In physics, Gauss's law for magnetism is one of the four Maxwell's equations that underlie classical electrodynamics. It states that the magnetic field B has divergence equal to zero, in other words, that it is a solenoidal vector field. It is equivalent to the statement that magnetic monopoles do not exist. Rather than "magnetic charges", the basic entity for magnetism is the magnetic dipole.

In physics, relativistic quantum mechanics (RQM) is any Poincaré covariant formulation of quantum mechanics (QM). This theory is applicable to massive particles propagating at all velocities up to those comparable to the speed of light c, and can accommodate massless particles. The theory has application in high energy physics, particle physics and accelerator physics, as well as atomic physics, chemistry and condensed matter physics. Non-relativistic quantum mechanics refers to the mathematical formulation of quantum mechanics applied in the context of Galilean relativity, more specifically quantizing the equations of classical mechanics by replacing dynamical variables by operators. Relativistic quantum mechanics (RQM) is quantum mechanics applied with special relativity. Although the earlier formulations, like the Schrödinger picture and Heisenberg picture were originally formulated in a non-relativistic background, a few of them also work with special relativity.

<span class="mw-page-title-main">Field (physics)</span> Physical quantities taking values at each point in space and time

In physics, a field is a physical quantity, represented by a scalar, vector, or tensor, that has a value for each point in space and time. For example, on a weather map, the surface temperature is described by assigning a number to each point on the map; the temperature can be considered at a certain point in time or over some interval of time, to study the dynamics of temperature change. A surface wind map, assigning an arrow to each point on a map that describes the wind speed and direction at that point, is an example of a vector field, i.e. a 1-dimensional (rank-1) tensor field. Field theories, mathematical descriptions of how field values change in space and time, are ubiquitous in physics. For instance, the electric field is another rank-1 tensor field, while electrodynamics can be formulated in terms of two interacting vector fields at each point in spacetime, or as a single-rank 2-tensor field.

<i>Classical Electrodynamics</i> (book) Graduate textbook by J.D. Jackson

Classical Electrodynamics is a textbook written by theoretical particle and nuclear physicist John David Jackson. The book originated as lecture notes that Jackson prepared for teaching graduate-level electromagnetism first at McGill University and then at the University of Illinois at Urbana-Champaign. Intended for graduate students, and often known as Jackson for short, it has been a standard reference on its subject since its first publication in 1962.

Electromagnetism is one of the fundamental forces of nature. Early on, electricity and magnetism were studied separately and regarded as separate phenomena. Hans Christian Ørsted discovered that the two were related – electric currents give rise to magnetism. Michael Faraday discovered the converse, that magnetism could induce electric currents, and James Clerk Maxwell put the whole thing together in a unified theory of electromagnetism. Maxwell's equations further indicated that electromagnetic waves existed, and the experiments of Heinrich Hertz confirmed this, making radio possible. Maxwell also postulated, correctly, that light was a form of electromagnetic wave, thus making all of optics a branch of electromagnetism. Radio waves differ from light only in that the wavelength of the former is much longer than the latter. Albert Einstein showed that the magnetic field arises through the relativistic motion of the electric field and thus magnetism is merely a side effect of electricity. The modern theoretical treatment of electromagnetism is as a quantum field in quantum electrodynamics.