Propagation of electromagnetic oscillations. Speed of electromagnetic waves
Analysing Faraday's discovery, Maxwell concluded that any change of a magnetic field creates a vortex electric field around it. From symmetry he put forward the reverse hypothesis: any change of an electric field creates a vortex magnetic field around it. So alternating electric and magnetic fields in space can create each other, i.e. propagate as an electromagnetic wave. About 10 years after Maxwell's death, in 1886–1889, H. Hertz produced and detected such waves with a spark vibrator: a spark between the balls of a second vibrator showed that a wave had arrived. A closed circuit hardly emits waves; if the capacitor plates are pulled apart to form an open oscillating circuit (an antenna), the field spreads into space. In an electromagnetic wave the vectors E and B are perpendicular to each other and to the direction of propagation (a transverse wave). In vacuum the speed is c ≈ 3·10⁸ m/s (the speed of light); the wavelength is λ = c/ν = c·T.
Draw E and B being mutually perpendicular: for a wave travelling along x, show E along y and B along z, over one wavelength. Then find from a device's documentation at which frequencies a mobile phone, Wi‑Fi and a radio work, and compute their wavelengths with λ = c/ν.