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Classical electromagnetism or classical electrodynamics is a branch of theoretical physics that studies the interactions between electric charges and currents using an extension of the classical Newtonian model. The theory provides an excellent description of electromagnetic phenomena whenever the relevant length scales and field strengths are large enough that quantum mechanical effects are negligible. For small distances and low field strengths, such interactions are better described by quantum electrodynamics.

Fundamental physical aspects of classical electrodynamics are presented in many texts, such as those by Feynman, Leighton and Sands,^[1]Griffiths,^[2]Panofsky and Phillips,^[3] and Jackson.^[4]

History[edit]

The physical phenomena that electromagnetism describes have been studied as separate fields since antiquity. For example, there were many advances in the field of optics centuries before light was understood to be an electromagnetic wave. However, the theory of electromagnetism, as it is currently understood, grew out of Michael Faraday's experiments suggesting an electromagnetic field and James Clerk Maxwell's use of differential equations to describe it in his A Treatise on Electricity and Magnetism (1873). For a detailed historical account, consult Pauli,^[5] Whittaker,^[6] Pais,^[7] and Hunt.^[8]

Lorentz force[edit]

The electromagnetic field exerts the following force (often called the Lorentz force) on charged particles:

F=qE+qv×Bdisplaystyle mathbf F =qmathbf E +qmathbf v times mathbf B "/>

where all boldfaced quantities are vectors: F is the force that a particle with charge q experiences, E is the electric field at the location of the particle, v is the velocity of the particle, B is the magnetic field at the location of the particle.

The above equation illustrates that the Lorentz force is the sum of two vectors. One is the cross product of the velocity and magnetic field vectors. Based on the properties of the cross product, this produces a vector that is perpendicular to both the velocity and magnetic field vectors. The other vector is in the same direction as the electric field. The sum of these two vectors is the Lorentz force.

Therefore, in the absence of a magnetic field, the force is in the direction of the electric field, and the magnitude of the force is dependent on the value of the charge and the intensity of the electric field. In the absence of an electric field, the force is perpendicular to the velocity of the particle and the direction of the magnetic field. If both electric and magnetic fields are present, the Lorentz force is the sum of both of these vectors.

Although the equation appears to suggest that the Electric and Magnetic fields are independent, the equation can be rewritten in term of four-current (instead of charge) and a single tensor that represents the combined Electromagnetic field (Fμνdisplaystyle F^mu nu )

fα=FαβJβ.displaystyle f_alpha =F_alpha beta J^beta .!

The electric field E[edit]

The electric field E is defined such that, on a stationary charge:

F=q0Edisplaystyle mathbf F =q_0mathbf E "/>

where q₀ is what is known as a test charge. The size of the charge doesn't really matter, as long as it is small enough not to influence the electric field by its mere presence. What is plain from this definition, though, is that the unit of E is N/C (newtons per coulomb). This unit is equal to V/m (volts per meter); see below.

In electrostatics, where charges are not moving, around a distribution of point charges, the forces determined from Coulomb's law may be summed. The result after dividing by q₀ is:

E(r)=14πε0∑i=1nqi(r−ri)|r−ri|3displaystyle mathbf E(r) =frac 14pi varepsilon _0sum _i=1^nfrac q_ileft(mathbf r -mathbf r _iright)mathbf r -mathbf r _iright

where n is the number of charges, q_i is the amount of charge associated with the ith charge, r_i is the position of the ith charge, r is the position where the electric field is being determined, and ε₀ is the electric constant.

If the field is instead produced by a continuous distribution of charge, the summation becomes an integral:

E(r)=14πε0∫ρ(r′)(r−r′)|r−r′|3d3r′displaystyle mathbf E(r) =frac 14pi varepsilon _0int frac rho (mathbf r' )left(mathbf r -mathbf r' right)leftmathrm d^3 mathbf r'

where ρ(r′)displaystyle rho (mathbf r' ) is the charge density and r−r′displaystyle mathbf r -mathbf r' is the vector that points from the volume element d3r′displaystyle mathrm d^3 mathbf r' to the point in space where E is being determined.

Both of the above equations are cumbersome, especially if one wants to determine E as a function of position. A scalar function called the electric potential can help. Electric potential, also called voltage (the units for which are the volt), is defined by the line integral

φ(r)=−∫CE⋅dldisplaystyle varphi mathbf (r) =-int _Cmathbf E cdot mathrm d mathbf l "/>

where φ(r) is the electric potential, and C is the path over which the integral is being taken.

Unfortunately, this definition has a caveat. From Maxwell's equations, it is clear that ∇ × E is not always zero, and hence the scalar potential alone is insufficient to define the electric field exactly. As a result, one must add a correction factor, which is generally done by subtracting the time derivative of the A vector potential described below. Whenever the charges are quasistatic, however, this condition will be essentially met.

From the definition of charge, one can easily show that the electric potential of a point charge as a function of position is:

φ(r)=14πε0∑i=1nqi|r−ri|displaystyle varphi mathbf (r) =frac 14pi varepsilon _0sum _i=1^nfrac q_imathbf r -mathbf r _irightsum_i="1^n fracq_i left"
"/>

where q is the point charge's charge, r is the position at which the potential is being determined, and r_i is the position of each point charge. The potential for a continuous distribution of charge is:

φ(r)=14πε0∫ρ(r′)|r−r′|d3r′displaystyle varphi mathbf (r) =frac 14pi varepsilon _0int frac rho (mathbf r' ),mathrm d^3 mathbf r' int fracrho(mathbfr')mathbfr-mathbfr', mathrmd^3mathbfr'
"/>

where ρ(r′)displaystyle rho (mathbf r' ) is the charge density, and r−r′displaystyle mathbf r -mathbf r' is the distance from the volume element d3r′displaystyle mathrm d^3 mathbf r' to point in space where φ is being determined.

The scalar φ will add to other potentials as a scalar. This makes it relatively easy to break complex problems down in to simple parts and add their potentials. Taking the definition of φ backwards, we see that the electric field is just the negative gradient (the del operator) of the potential. Or:

E(r)=−∇φ(r).displaystyle mathbf E(r) =-nabla varphi mathbf (r) .

From this formula it is clear that E can be expressed in V/m (volts per meter).

Electromagnetic waves[edit]

A changing electromagnetic field propagates away from its origin in the form of a wave. These waves travel in vacuum at the speed of light and exist in a wide spectrum of wavelengths. Examples of the dynamic fields of electromagnetic radiation (in order of increasing frequency): radio waves, microwaves, light (infrared, visible light and ultraviolet), x-rays and gamma rays. In the field of particle physics this electromagnetic radiation is the manifestation of the electromagnetic interaction between charged particles.

General field equations[edit]

As simple and satisfying as Coulomb's equation may be, it is not entirely correct in the context of classical electromagnetism. Problems arise because changes in charge distributions require a non-zero amount of time to be "felt" elsewhere (required by special relativity).

For the fields of general charge distributions, the retarded potentials can be computed and differentiated accordingly to yield Jefimenko's equations.

Retarded potentials can also be derived for point charges, and the equations are known as the Liénard–Wiechert potentials. The scalar potential is:

φ=14πε0q|r−rq(tret)|−vq(tret)c⋅(r−rq(tret))displaystyle varphi =frac 14pi varepsilon _0frac qmathbf r -mathbf r _q(t_ret)right"/>

where q is the point charge's charge and r is the position. r_q and v_q are the position and velocity of the charge, respectively, as a function of retarded time. The vector potential is similar:

A=μ04πqvq(tret)|r−rq(tret)|−vq(tret)c⋅(r−rq(tret)).displaystyle mathbf A =frac mu _04pi frac qmathbf v _q(t_ret)mathbf r -mathbf r _q(t_ret)right."/>

These can then be differentiated accordingly to obtain the complete field equations for a moving point particle.

Models[edit]

Branches of classical electromagnetism such as optics, electrical and electronic engineering consist of a collection of relevant mathematical models of different degrees of simplification and idealization to enhance the understanding of specific electrodynamics phenomena, cf.^[9] An electrodynamics phenomenon is determined by the particular fields, specific densities of electric charges and currents, and the particular transmission medium. Since there are infinitely many of them, in modeling there is a need for some typical, representative

(a) electrical charges and currents, e.g. moving pointlike charges and electric and magnetic dipoles, electric currents in a conductor etc.;

(b) electromagnetic fields, e.g. voltages, the Liénard–Wiechert potentials, the monochromatic plane waves, optical rays; radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, gamma rays etc.;

(c) transmission media, e.g. electronic components, antennas, electromagnetic waveguides, flat mirrors, mirrors with curved surfaces convex lenses, concave lenses; resistors, inductors, capacitors, switches; wires, electric and optical cables, transmission lines, integrated circuits etc.;

all of which have only few variable characteristics.

See also[edit]

• Electromagnetism


• Maxwell's equations


• Weber electrodynamics


• Wheeler–Feynman absorber theory

References[edit]

External links[edit]

• 
  Electromagnetic Field Theory by Bo Thidé