Ch0221 - Chapter 21 The Nature of Electromagnetic Waves 21...

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Chapter 21 The Nature of Electromagnetic Waves 186 21 The Nature of Electromagnetic Waves When we left off talking about the following circuit: we had recently closed the switch and the wire was creating a magnetic field which was expanding outward. The boundary between that part of the universe in which the magnetic field is already established and that part of the universe in which the magnetic field has not yet been established is moving outward at the speed of light, c = 3.00 × 10 8 m/s. Between that boundary and the wire we have a region in which there exists a steady unmoving magnetic field. Note that it was the act of creating the current that caused the magnetic field “edge” that is moving at the speed of light. In changing from a no-current situation to one in which there was current in the wire, charged particles in the wire went from no net velocity in the along-the-wire direction to a net velocity along the wire, meaning, that the charged particles were accelerated. In other words, accelerated charged particles cause light. We can also cause light by means of the angular acceleration of particles having a magnetic dipole moment, but, the short answer to the question about what causes light, is, accelerated charged particles . Here’s a simple circuit that one might use to intentionally cause light: The vertical arrangement of wires on the right is referred to as a dipole antenna. As the AC power source alternately causes charge to surge upward in both parts of the antenna, and then downward, the dipole antenna creates electric and magnetic fields that oscillate sinusoidally in both time and space. The fields propagate through space away from the antenna at the speed of light. v = c B E I UP
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The Nature of Electromagnetic Waves 187 The charged particles oscillating up and down in the antenna causes waves of electric and magnetic fields known as light. The frequency of the waves is the same as the frequency of oscillations of the particles which is determined by the frequency of the power source. The speed of the waves is the speed of light c = 3.00 × 10 8 m/s, because the waves are light. For any kind of wave, the frequency, wavelength, and wave speed are related by: f v λ = which, in the case of light reads: = c Here’s a quick pictorial review of some properties of waves. In the case of light, we have electric and magnetic fields oscillating in synchronization with each other. It is customary to characterize the waves in terms of the electric field. I’ll do that here, but, one should keep in mind that the magnetic field oscillates and moves in the same manner that the electric field does, but, at right angles to the electric field. max Peak-to-Peak Amplitude
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Ch0221 - Chapter 21 The Nature of Electromagnetic Waves 21...

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