Magnetic Flux
Magnetic flux describes the lines of a magnetic field emanating from, and in the region of space surrounding, a magnetized object. Quantitatively, magnetic flux describes the magnetic field lines of force that traverse a given cross-sectional area.
Magnetic fields exert forces on moving charged electrical particles. In this regard magnetic field lines are lines of force along which moving charged particles will be deflected. Magnetic fields are represented by continuous magnetic flux lines thatare depicted to emerge from north-seeking magnetic poles and enter south-seeking magnetic poles.
Magnetic fields are characterized by magnetic flux density with the density of magnetic flux lines directly related to the magnitude of the magnetic field. The greater the magnitude of the field (field potential) the more dense the magnetic flux lines. At magnetic poles the magnitude of the magnetic field strength is at a maximum and the magnetic field lines are dense. Accordingly, as distance from the pole increases the field lines diverge and become less dense. The quantitative unit of magnetic flux is the weber (Wb). Because magnetic flux can arise due to the motion of electrons (current), at the atomic level magnetic flux can be shown to be quantized. (i.e, related to Planck's constant and the quantum electrical charge).
Magnetic flux density is a vector (designated B) used to characterize magnetic field strength. The magnetic flux density vector measures the total magnetic field, including the fields created by other magnetic materials. The force on a charged particle moving through a magnetic field (i.e., magnetic flux lines) is related to the charge on the particle, the charged particle's velocity and the magnetic flux density: F = qv x B.
Because both magnetic flux and electric flux measure flow in terms of quantity per unit area per unit time, magnetic flux is defined in a very similar manner to the definition of flux through an electric field. There are, however, fundamental differences between magnetic and electrical fields. In the magnetic field there are no magnetic charges or poles to serve as the points of origin or termination for the magnetic field lines. In fact, there is no point of origin or termination for any magnetic field line. Magnetic field lines are continuous loops and the number of magnetic field lines leaving a surface must, therefore, equal the number of magnetic field lines entering a surface.
Because magnetic field lines form closed loops it is impossible to isolate magnetic poles (i.e., it is impossible to divide a magnet, one simply forms two new magnets). A surface that encloses one pole of a magnet has zero magnetic flux because each magnetic field line leaving the surface must reenter the surface.
Surrounding permanent magnets or conductive wires in which there is an established, steady, and unidirectional current flow there is a magnetostatic field (i.e., a stationary magnetic field). The magnetostatic field is uniform in all directions. In such a uniform field the magnetic field lines are equally spaced and parallel. The magnetic field surrounding a conducting wire carrying a alternating current (AC) or variable direct current (DC), however, is not magnetostatic and the magnetic field changes in response to variations in current flow.
A changing magnetic flux induces an emf in a loop of conducting coil. In the process of mutual inductance, emf is induced in one coil by altering current in another coil. For example, as current flows through the first in a series of coils it produces a magnetic field. Accordingly, magnetic flux induces an emf in the next coil in the series. The emf magnitude depends on the magnitude of magnetic flux and upon the distance, orientation and composition of the coil. As current flows in the second coil, the induction process is repeated for a third coil, and all subsequent coils, in the series. The emf induced in the induced conductive coil is directly proportional to the change in magnetic flux. In turn, the magnetic flux is directly proportional to the change in current in the inducing coil.
Magnetic induction resulting from magnetic flux is used in transformers and ammeters designed for indirect measurement of current.
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