Electromagnetism

4Induction

4.1. Introduction: variable regimes

Here, we are interested in circuits in motion, through which variable currents flow and in which there are conductor systems carrying variable charges over time.

Figure 4.1. Current–field relationships, in quasi-steady states

For a color version of all figures in this book, see www.iste.co.uk/gontrand/electromagnetism.zip.

We can see, below, the laws established in the framework of steady states:

[4.1]

[4.2]

However, following the appearance of phenomena that vary over time, differences appear.

Propagation phenomena

It takes some time for variations in charges and currents to manifest themselves at a distance. The maximum propagation velocity is that of light in a vacuum: 3×10⁸ m/s (10⁹ km/h).

To any periodic phenomenon, we can associate a wavelength λ = vT, where T = 1/f; T = period, f = frequency.

Figure 4.2. Magnetic field propagation

EXAMPLES.–

• – λ = 0.1 m ==> f = 3×10⁹ Hz: radar, close to klystrons – our microwaves (Klystron), GSM (Global System Mobil);
• – λ = 10 m ==> f = 3×10⁷ Hz: Television (video);
• – 20 m < λ< 3000 m ==> ...