Generation of Magnetic Fields

SOME THEORETICAL DEVELOPMENTS ON MAGNETIC FIELD GENERATION AND DECAY 

The discovery of the activity cycle of the sun, and of stars, supports the contention that magnetic fields in cosmic objects must be continually regenerated. The decay of the field, due to the finite electrical conductivity of matter, is sufficiently rapid to require that if a field is observed at all, it must not be primordial. 

The electric field in a moving medium is due to two contributing physical processes, one from the induced field due to the motion if there is already a magnetic field present, and the other due to the finite rate of dissipation of the field from the potential difference and finite electrical conductivity. That is, if a current is set up, the strength of the current is assumed due to an electric field. 

The Maxwell equations govern the temporal evolution of the fields. For the electric field, the rate of change of the field with time is 

Assuming that the electric field in the moving medium is vanishingly small—that the conductivity is very high (although not infinite)—we obtain 

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Principle of Operation. Electrical current flows are induced in all conductors when exposed to an a-c field. These currents generate a magnetic field surrounding the conductors which oppose the field being produced by the a-c field with a force sufficient to repel the conductor. Eigure 16 illustrates this principle by showing a rotor consisting of many poles. 

The early development of electric motors and generators can be traced to the 1820 discovery by Hans Christian Oersted that electricity in motion generates a magnetic field. Oersted proved the long-suspected... 

Figure 2.2 Effect of 180 pulse on phase imperfections resulting from magnetic field inhomogeneities. Spin-echo generated by 180 refocusing pulse removes the effects of magnetic field inhomogeneities.

Figure 7.22 The principle of creating a two-dimensional NMR image. A number of profiles of the sample are obtained in different orientations in the presence of magnetic field gradients pointing in different directions (designated by arrows). The x -gradient yields an x -profile, and a /gradient generates a y -profile. A combination of these profiles produces a two-dimensional image.

Low resolution spectrometers utilize a low-field permanent magnet to generate the polarizing magnetic field. The range of magnetic field strengths is from 0.01 to... 

In the hadronic phase due to the entrainment effect around each neutron vortex appears a cluster of proton vortices which generates mean magnetic field B = 4 1014G. The magnetic flux of such a cluster is... 

One henry is the inductance when one ampere flowing in the coil generates a magnetic field strength of one weber. 

As you learned from the previous section, three quantum numbers—n, 1, and mi—describe the energy, size, shape, and spatial orientation of an orbital. A fourth quantum number describes a property of the electron that results from its particle-like nature. Experimental evidence suggests that electrons spin about their axes as they move throughout the volume of their atoms. Like a tiny top, an electron can spin in one of two directions, each direction generating a magnetic field. The spin quantum number (mj specifies the direction in which the electron is spinning. This quantum number has only two possible values or —... 

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