Inductive and radiating electromagnetic fields.

by McQueen
Tags: electromagnetic, fields, inductive, radiating
McQueen is offline
Feb7-04, 09:44 AM
P: 265
I have been having some difficulty in trying to understand the difference between inductive and radiating electromagnetic fields. When an alternating current is present in an electrical conductor , two types of fields are observed an inductive field and a radiating field . The inductive field of an ac cuurent is often called the near field because it is concentrated near the source. Similarly, the radiating field is referred to as the far field because its effects extend far from the source. The boundary between the inductive field and the radiative field is generally represented as being approximately wavelength/2pi . Yet the energy of this EM radiation whether inductive or radiative when calculated using the formula e = hc/(wavelength) is found to be phenomenally small . Taking the normal household supply of 60 Hz we get 6.62 x 10 -39 x 3 x 10 8 / 5 x 10 6 which works out to 2.481402 x 10 –13 eV. If we take 1 eV = 1.6 x 10 –19 J. Then in terms of Joules this would mean : 1.6 x 10 –19 x 2.481402 x 10 –13 J = 3.969 x 10 –32 J. this is a phenomenally small amount of energy. To gain some idea of just how small this number is , if the positive of this number is taken it comes close to the number of atoms in the entire Universe ( 1044). Can this discrepancy really be ignored , because it means in effect that even if the field had 10 23 (i.e the number of free electrons in a conductor 10 23 cm 3 ) photons in it the whole energy of the field would amount to hardly 10 –9 J. While carrying out this calculation remember that 1 Coulomb ( or 1 Ampere ) of current means a flow of 6.25 x 10 18 electrons /sec , so the figure of 10 23 photons , going by the figures , could be representative of a current far in excess of 10 5 amps , as compared to what we actually have i.e , 3.969 x 10 –32 J. Yet this same field is supposed to give rise to currents that are 98% of the original current. That is the induced current in the secondary can be as much as 98% of that in the primary. To me it doesn’t make sense , especially because qualitatively there is supposed to exist no difference between the inductive field and the radiative field except for the distance represented by wavelength/2pi.
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