Electromagnetic field

In the physics of electromagnetism, an electromagnetic field is a field composed of two related vector fields: the electric field and the magnetic field. When referred to as the electromagnetic field, the field is imagined to encompass all of space; typically an electromagnetic field is considered to be limited to a local area around an object in space.

Behaviour of the electromagnetic field

;(A hydrodynamic interpretation)

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The electric and magnetic vector fields can be thought of as being the velocities of a pair of incompressible fluids which permeate space. In the absence of charges these fluids would be at rest, so that their velocity fields would be zero. Since both fluids are incompressible, their densities do not change: it is not possible to compress magnetic or electric fluid into a smaller space.

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Electric charges act either as sources or sinks of the electric fluid. An electron is constantly absorbing electric fluid around it at some rate, call it ε. Protons are the reverse: they constantly pour electric "liquid" towards the surrounding space at rate ε, so liquid moves away from the proton with speed

Related Topics:
Electric charge - Sources or sinks - Electron - Proton

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: v = {epsilon over 4 pi r^2}

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(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: 4 pi r^2 cdot v = epsilon .

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Magnetic liquid, on the other hand, has no sources or sinks: there are no magnetic charges that could pour out or suck up magnetic fluid. If magnetic fluid is standing still, it can be stirred up, making it move in closed circles and closed loops (see vortical motion).

Related Topics:
No sources or sinks - Magnetic charges - Vortical

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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.

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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.

Related Topics:
Vortex - Right hand rule

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Notice that electric fluid will not accelerate spontaneously; something has to force it to accelerate. This same thing then indirectly causes the magnetic vortex to be stirred up: a magnetic vortex will not arise spontaneously.

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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.

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To summarise, 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.

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The opposite signs of acceleration 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:

Related Topics:
Negative feedback - Lenz's law

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: Delta E ightarrow + Delta B

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: - Delta E leftarrow + Delta B .

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If there were a positive feedback loop, the result might be similar to the high pitched resonant effect produced by a microphone too close to its speaker. The positive feedback would cause the original acceleration of electric fluid to amplify itself continually, while at the same time the vortices around it would amplify as well: an explosive maelstrom of movement of electromagnetic fluid. Fortunately, the laws of electromagnetism and conservation of energy being what they are, an initial disturbance (acceleration) of the electric fluid will cause a feedback loop that, being negative, will tend to extinguish itself at its source but which will propagate outwards in what is called an electromagnetic wave.

Related Topics:
Positive feedback - Conservation of energy - Electromagnetic wave

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