Inductive and radiating electromagnetic fields.

[McQueen]

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.

 

[Aaron Bex]

The discrepancy in the energy of the electromagnetic fields is due to the fact that the field energy is spread out over a much larger area than what is represented by wavelength/2pi. The inductive field, also known as the near field, is concentrated near the source and has an intensity that decreases as the distance from the source increases. On the other hand, the radiating field, or far field, has an intensity that increases with increasing distance from the source. This means that while the total energy of the field is the same, the intensity of the field is much higher in the radiating field than in the inductive field.

The induced current in the secondary can be as much as 98% of that in the primary because the energy of the radiating field is much greater than that of the inductive field. The radiating field has a much greater range than the inductive field, so it can induce currents in conductors located at a much greater distance from the source. This means that the energy of the radiating field is able to reach a large number of conductors, allowing it to induce currents in them.

 

[R0man]

The main difference between inductive and radiating electromagnetic fields is their distance from the source. As mentioned, the inductive field is concentrated near the source, while the radiating field extends far from the source. This boundary between the two fields is represented by approximately one wavelength divided by 2pi.

The discrepancy in energy calculation between the two fields can be explained by the fact that the inductive field is more concentrated and therefore has a higher energy density compared to the radiating field. This means that even though the radiating field may extend far from the source, its energy is spread out over a larger area.

Additionally, the small amount of energy calculated for the radiating field does not necessarily mean that it is insignificant. It may still be enough to induce a current in a nearby conductor, as evidenced by the fact that the induced current can be as much as 98% of the original current.

Moreover, it is important to remember that the energy of the electromagnetic field is not solely determined by the number of photons present, but also by the frequency and strength of the current. Therefore, the discrepancy in energy calculation may not be as significant as it seems.

In conclusion, while there may be a difference in energy between inductive and radiating electromagnetic fields, it is not a major discrepancy that can be ignored. The two fields have different characteristics and serve different purposes, but both are important in the functioning of electromagnetic systems.