# Electromagnetic field

The**electromagnetic field**(

**EMF**) is composed of two related vectorial fields, the electric field and the magnetic field. This means that the vectorss (

**E**and

**B**) that characterize the field each have a value defined at each point of space and time. If only

**E**, the electric field, is nonzero and is constant in time, the field is said to be an electrostatic field.

**E** and **B** are linked by Maxwell's equations.

Electromagnetic fields can be explained with a quantum basis by quantum electrodynamics.

## Behaviour of the electromagnetic field (a hydrodynamic interpretation)

(where*r*is distance of the fluid away from the proton) so that the total flux of liquid going through any (imaginary) sphere which contains that proton is the area of the sphere times the speed of the fluid flowing through it: .

Magnetic liquid, on the other hand, has no sources or sinks: nothing can pour out or suck up magnetic fluid. Magnetic fluid is incompressible, which means that its density does not change: it is not possible to compress a lot of magnetic fluid into a smaller space, or to squash it out of a given volume. (Electric fluid is also incompressible, but it has sources and sinks.) If magnetic fluid is standing still, it can be stirred up, making it move in closed circles and closed loops (see vortical motion).

For the magnetic fluid to keep moving in the same loop, though, some force has to keep stirring it up: otherwise the energy of its circular motion will dissipate and the magnetic fluid will stop moving and will return to rest.

If electric fluid starts to accelerate in a certain direction, it will cause a vortex of magnetic fluid to move in circles around the direction in which the electric fluid is accelerating (according to the right hand rule). As soon as the electric fluid stops accelerating, the vortex of magnetic fluid vanishes.

Notice that: electric fluid will not accelerate spontaneously. Something has to force it to accelerate. This same thing then causes (indirectly) the magnetic vortex to be stirred up. A magnetic vortex will not arise spontaneously.

Finally, if magnetic fluid accelerates in a certain direction, it causes electric fluid to move in a vortex which circles around the direction of acceleration in the direction opposite to the right hand rule.

Summarily: an acceleration of the electric fluid causes a positive vortex of magnetic "liquid" to move around it, but an acceleration of the magnetic liquid causes a negative vortex of electric liquid to flow around it.

Why the opposite signs? The opposite signs create a negative feedback loop (see Lenz's law.) An acceleration of electric fluid causes a positive magnetic vortex. This means that the magnetic fluid has been accelerated to produce this circular flow. But this causes a negative vortex of electric fluid around the magnetic vortex. This reactive vortical acceleration of electric fluid is in the direction opposite of the original acceleration of electric fluid: hence a negative feedback loop:

- .

## Flaw in the velocity field interpretation

The fluid analogy does not work in this sense: that objects immersed in a moving fluid (e.g. a river) tend to be pushed by that fluid in such a way that the velocity of the object aligns with the velocity of the fluid. Once the velocities are aligned, the fluid's motion should vanish from the object's point of view.However, the force of an electric field on a charged particle is , and this force is independent of the velocity of the particle, which means that the particle will accelerate continually in the direction of the field. If the field is the velocity field of a fluid then the fluid would be causing the object to accelerate continually in the direction of the fluid's motion, to the point that the object's speed becomes way larger than the fluid it is immersed in. This is paradoxical.

From the continually accelerating object's point of view (see principle of relativity), if its speed has already surpassed the speed of the fluid, then the fluid is moving backwards, so the field should be pointing in the direction opposite to the direction in which the object keeps accelerating. This means that that the object should stop accelerating and begin decelerating, until its speed aligns with the speed of the electric fluid.

## The field as a stream of moving photons

An alternative interpretation would be that the field is not actually a velocity field, but a flux density field of photonic fluid, which is constantly moving at the same speed: the speed of light, independent of the speed of the observer (the charged object). Photonic fluid never changes speed but can change net direction and the intensity of its net movement in that direction.The velocity field interpretation is related to the hypothesis of a luminiferous aether through which electromagnetic waves would propagate. The existence of the aether was disproved by the Michelson-Morley experiment and the necessity of having an aether vanished when it was replaced by Einstein's theory of relativity.

According to special relativity, the Lorentz force equation reduces to the equation

*On The Electrodynamics Of Moving Bodies*.)

The speed of light is invariant under a Lorentz transformation, but the velocity of light is changed. The component of the velocity of light parallel to the boost is left unchanged, but the transversal component is rotated: it is accelerated in a direction parallel to the boost. The addition of special relativity allows the combination of the electric and magnetic fields into a single tensor field. The tensor character of this combined electromagnetic field implies that the field is anisotropic with respect to the velocity of the charged particle on which it produces a force: the Lorentz force varies with the velocity of the charged particle.

## Light and Electromagnetic Waves

Electrically charged particles are constantly emitting (or absorbing) photonic fluid, which is more commonly known as light. So how is light related to electromagnetic waves? E-M waves are undulatory movement patterns of light which can always be observed to be emitted by electric charges undergoing acceleration.If a charged particle is at rest, then it does not emit electromagnetic waves. Instead, it is surrounded by an electrostatic field. If the charged particle is in inertial motion, then the electrostatic field is joined by a magnetostatic field. These pair of static fields produce a movement of electromagnetic energy (i.e. a field of non-zero Poynting vectors), which is similar to an electromagnetic wave, except that that the fields are not oscillating.

E-M waves are propagating, expanding, harmonic, oscillating, accelerations of the photonic fluid. Since the photonic fluid itself moves at the speed of light (by definition), then E-M waves can move no faster than the speed of light. E-M waves move at a speed close to the speed of light, depending on the medium through which they move (e.g. faster in air than through water, and faster through water than through a glass lens).

Water waves are waves of water, and air sound waves are waves of air, and electromagnetic waves are transverse waves of light.

Ocean currents are movements of water, and wind is the movement of air, and the electromagnetic field is the movement of light.

## The Electromagnetic Field as a Feedback Loop

The behavior of the electromagnetic field can be resolved into four different parts of a loop: (1) the electric and magnetic fields are generated by electric charges, (2) the electric and magnetic fields interact only with each other, (3) the electric and magnetic fields produce forces on electric charges, (4) the electric charges move in space.The feedback loop can be summarized in a list, including phenomena belonging to each part of the loop:

- charges generate fields
- Gauss's Law: charges generate electric fields
- Ampère's Law: currents generate magnetic fields ()

- the fields interact with each other
- displacement current: 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
- Maxwell-Hertz equations: simplified version of Maxwell's equations
- electromagnetic wave equation

- fields act upon charges
- 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
- charges move
- continuity equation: current is movement of charges

- continuity equation: current is movement of charges

**See also:** electromagnetic interaction | photoelectric effect | intensity | bremsstrahlung | polar aurora | polarized light | holography | electromagnetic radiation | electromagnetic spectrum | electromagnetic spectroscopy | electrodynamics | Fresnel equations | antenna.

## External Link

*On the Electrodynamics Of Moving Bodies*by A. Einstein, June 30, 1905. Hosted by Fourmilab.*On the Electrodynamics Of Moving Bodies*(PDF version)