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Re: Re: What Is Magnetic Field

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Posted by Alexander on February 17, 2001 21:47:41 UTC

No contradiction. STATIC electric field is different from MOVING one (due to relativistic space contraction in the direction of motion). THE DIFFERENCE BETWEEN ELECTRIC FIELD OF STATIC CHARGE AND ELECTRIC FIELD OF MOVING CHARGE IS WHAT WE CALL (=LABEL) "A MAGNETIC FIELD". Due to relaticistic nature of this field, it depends not only on the magnitude of velocity of the moving charge but also on the relative position of observer vesus the charge - for example, it is zero on the axis of motion and non-zero anywhere else: B = q[v x r]/r ([v x r] = vector product of the velocity of charge and position of the observer in respect to the charge.

It is convenien labeling, because after that you may consider complex moving (or changing which is the same) electric field as if it would havre two distinctive components: "electrostatic" E and a "magnetic" B (although it is ideologically incorrect, and less confusing term is "electromagnetic" field)

If charge is accelerating, then its field is no longer constant (=does not depend on time) as it was when charge was moving with constant velocity. Now this field it is changing in time too. Now in addition to relativity (magnetic component) we have to take into consideration mass (inertia) of the field as it has energy in it and energy has inertial and gravitational properties. This means that if you suddenly shift non moving before charge from point A to point B, then the electric field far from the charge due to inertia does not change and will still continue to point to the point A. But, of course the electric field next to the charge should point to the charge itself, i.e. to the new position B. So, somewhere in between the field should change direction from A to B. And this change of direction starts moving as an expanding spherical shell with the center at the point B. Turns out that this shell expands with finite speed, and careful measurements of this speed had shown that this "disturbed" area where electric field changes direction from old position to new always moves with the same speed (about 3x10^8 m/sec). People who experimented with moving charges (mainly with electrons) noticed that because this shell moves without changing the shape of "disturbance" envelope, i.e. like a moving of pressure disturbance in the air or like a disturbance of surface of water across a pond after you thrown a rock in it, decided to use similar equations to describe details of its motion as traditionally used for air or water motion (called wave equations). They also decided that instead long expression "disturbance of electric field of charge due to accelerated motion of the charge" it is more convenient to label it by some shorter label. Some called it "electromagnetic disturbance", some "electromagnetic wave", some "radiowave", some "elecromagnetic radiation", some "radio signal", etc. Later they discovered that although equation of e/m wave (which can be derived from Maxwell equations for E and B components of moving charge) indeed very accurately describes
behaviour of electromagnetic radiation as it propagates via vacuum and all kind of matter. But when it comes to influence other electric charges, this influence is always descrete: wave energy can change only by certain non-zero number. So e/magnetic field energy appeared to occupy only a discrete set of levels, much like levels of energy of electron in an atom, but levels unlike in atom are equidistantly spaced. Measuring spacing between states of energy of electromagnetic disturbance revealed that if this disturbance was created by charge oscillating with constant frequency f (thus, the disturbance also oscillates with the same frequency f), then the spacing between energy levels of such e/magnetic field is equal hf where h is the Plank constant. Because of equal spacing for monofrequent e/magnetic field (they call it "monochromatic" field or "monochromatic radiation") they decided to label this spacing by a word "photon".

Later the new branch of physics describing this phenomenon emerged (and was labeled "electrodynamics") gave more accurate and more coplete quantum mechanical description of many properties of this e/magnetic field than classic Maxwellian equations did.

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