Friday, 1 July 2011
From Wikipedia, the free encyclopedia
The field can be viewed as the combination of an electric field and a magnetic field. The electric field is produced by stationary charges, and the magnetic field by moving charges (currents); these two are often described as the sources of the field. The way in which charges and currents interact with the electromagnetic field is described by Maxwell's equations and the Lorentz force law.
From a classical perspective, the electromagnetic field can be regarded as a smooth, continuous field, propagated in a wavelike manner; whereas from the perspective of quantum field theory, the field is seen as quantized, being composed of individual particles.
Structure of the electromagnetic fieldThe electromagnetic field may be viewed in two distinct ways: a continuous structure or a discrete structure.
Continuous structureClassically, electric and magnetic fields are thought of as being produced by smooth motions of charged objects. For example, oscillating charges produce electric and magnetic fields that may be viewed in a 'smooth', continuous, wavelike fashion. In this case, energy is viewed as being transferred continuously through the electromagnetic field between any two locations. For instance, the metal atoms in a radio transmitter appear to transfer energy continuously. This view is useful to a certain extent (radiation of low frequency), but problems are found at high frequencies (see ultraviolet catastrophe).
Discrete structureThe electromagnetic field may be thought of in a more 'coarse' way. Experiments reveal that in some circumstances electromagnetic energy transfer is better described as being carried in the form of packets called quanta (in this case, photons) with a fixed frequency. Planck's relation links the energy E of a photon to its frequency ν through the equation:
This quantum picture of the electromagnetic field (which treats it as analogous to harmonic oscillators) has proved very successful, giving rise to quantum electrodynamics, a quantum field theory describing the interaction of electromagnetic radiation with charged matter. It also gives rise to Quantum optics, which is different from quantum electrodynamics in that the matter itself is modelled using quantum mechanics rather than Quantum field theory
The electromagnetic field as a feedback loopThe behavior of the electromagnetic field can be resolved into four different parts of a loop:
- the electric and magnetic fields are generated by electric charges,
- the electric and magnetic fields interact only with each other,
- the electric and magnetic fields produce forces on electric charges,
- the electric charges move in space.
The feedback loop can be summarized in a list, including phenomena belonging to each part of the loop:
- charged particles generate electric and magnetic fields
- the fields interact with each other
- changing electric field acts like a current, generating 'vortex' of magnetic field
- Faraday induction: changing magnetic field induces (negative) vortex of electric field
- Lenz's law: negative feedback loop between electric and magnetic fields
- fields act upon particles
- Lorentz force: force due to electromagnetic field
- electric force: same direction as electric field
- magnetic force: perpendicular both to magnetic field and to velocity of charge
- Lorentz force: force due to electromagnetic field
- particles move
- current is movement of particles
- particles generate more electric and magnetic fields; cycle repeats
Mathematical descriptionThere are different mathematical ways of representing the electromagnetic field. The first one views the electric and magnetic fields as three-dimensional vector fields. These vector fields each have a value defined at every point of space and time and are thus often regarded as functions of the space and time coordinates. As such, they are often written as (electric field) and (magnetic field).
If only the electric field () is non-zero, and is constant in time, the field is said to be an electrostatic field. Similarly, if only the magnetic field () is non-zero and is constant in time, the field is said to be a magnetostatic field. However, if either the electric or magnetic field has a time-dependence, then both fields must be considered together as a coupled electromagnetic field using Maxwell's equations.
With the advent of special relativity, physical laws became susceptible to the formalism of tensors. Maxwell's equations can be written in tensor form, generally viewed by physicists as a more elegant means of expressing physical laws.
The behaviour of electric and magnetic fields, whether in cases of electrostatics, magnetostatics, or electrodynamics (electromagnetic fields), is governed in a vacuum by Maxwell's equations. In the vector field formalism, these are:
- (Gauss's law)
- (Gauss's law for magnetism)
- (Faraday's law)
- (Ampère-Maxwell law)
The Lorentz force law governs the interaction of the electromagnetic field with charged matter.
When a field travels across to different media, the properties of the field change according to the various boundary conditions. These equations are derived from Maxwell's equations. The tangential components of the electric and magnetic fields as they relate on the boundary of two media are as follows:
Properties of the field
Reciprocal behavior of electric and magnetic fieldsThe two Maxwell equations, Faraday's Law and the Ampère-Maxwell Law, illustrate a very practical feature of the electromagnetic field Faraday's Law may be stated roughly as 'a changing magnetic field creates an electric field'. This is the principle behind the electric generator.
Ampere's Law roughly states that 'a changing electric field creates a magnetic field'. Thus, this law can be applied to generate a magnetic field and run an electric motor.
Light as an electromagnetic disturbanceMaxwell's equations take the form of an electromagnetic wave in an area that is very far away from any charges or currents (free space) – that is, where ρ and are zero. It can be shown, that, under these conditions, the electric and magnetic fields satisfy the electromagnetic wave equation:
Relation to and comparison with other physical fields
|This section requires expansion.|
Electromagnetic and gravitational fieldsSources of electromagnetic fields consist of two types of charge – positive and negative. This contrasts with the sources of the gravitational field, which are masses. Masses are sometimes described as gravitational charges, the important feature of them being that there is only one type (no negative masses), or, in more colloquial terms, 'gravity is always attractive'.
The relative strengths and ranges of the four interactions and other information are tabulated below:
|Chromodynamics||Strong interaction||gluon||1038||1||10−15 m|
|Flavordynamics||Weak interaction||W and Z bosons||1025||1/r5 to 1/r7||10−16 m|