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Electromagnetic Waves

This overview explores electromagnetic (EM) waves, their ability to travel through space, and their significant applications in radio, TV, and communication devices like cell phones. We delve into Maxwell's equations, which describe the relationship between electric and magnetic fields, and highlight Hertz's experimental confirmation of light as an EM wave. The text discusses wave production mechanisms, such as changing electric fields generating magnetic fields, and includes calculations for wavelengths across different frequencies. Finally, we address the speed of light and its implications in communication technologies.

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Electromagnetic Waves

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Presentation Transcript

  1. Electromagnetic Waves Chapter 22

  2. Overview • Electromagnetic waves could travel through space • Radio • TV • Cell Phones • Prediction – light is an EM wave

  3. Maxwell’s Equations • James Maxwell (1831–1879) • Brought fields to the forefront of physics • 4 equations that explained all of EM • Gauss’s law relates the electric field to the electric charge • The lines of the magnetic fields are always continuous; they don’t begin or end with charges • An electric field is produced by a changing magnetic field (Faraday’s Law) • A magnetic field is produced by a current or changing electric field

  4. Production of EM Waves • A changing electric field produces a magnetic field • Changing magnetic fields produce more changing electric fields • The cycle continues and it propagates through space (EM Wave)

  5. Production of EM Wave Cont. • Two conducting rods connected by a battery • Two rods assume a charge • Create an electric field • Momentary current • Current creates a magnetic field • When the current stops, the magnetic field stops • The electric field remains x x x x E x x x x

  6. Production of EM Wave Cont. x • Replace the battery with an ac source • Direction of current and charges constantly change • Electric and magnetic fields change constantly x + ˜ ˜ - x x x x - + x x

  7. Production of EM Wave Cont. • Magnitude of E and B vary with 1/r • Energy is proportional to E2 and B2 • Intensity of wave is proportional to 1/r2 • E and B are always perpendicular to one another • Transverse waves • Fields – do not require a medium

  8. Production of EM Wave Cont. • Accelerating charges give rise to EM waves • Charges are changing direction • Speed of EM wave • 3E8 m/s = speed of light • Light must be an EM wave

  9. Light as an EM Wave • Hertz (1887) • First to detect EM waves experimentally • Traveled at 3E8 m/s • Showed all characteristics of light: reflection, refraction, and interference • Confirmed Maxwell’s work • Wavelengths • 400 – 750 nm • c = fλ • c = speed of light (3E8 m/s) • f = frequency (Hz) • λ = wavelength (m)

  10. EM Spectrum • Light is only portion of EM spectrum that we can see • Lower end = Radio waves • Higher end = Gamma rays • Some EM waves are produced by accelerating electrons • X-rays: Crash electrons into a metal target • Light bulbs: Energy into filament and accelerating electrons off the surface • Infrared and Ultraviolet • Surround visible spectrum • Infrared: Heat from fire • Ultraviolet: Causes damage to cells

  11. Example • Calculate the wavelength of a 60 Hz EM wave. • Calculate the wavelength of the radio signal FM 93.3 MHz. • Calculate the wavelength of a beam of visible red light from a laser at 4.74E14 Hz.

  12. EM Waves in a Medium • EM waves can be transmitted through wires as well as space • Takes time for the EM wave to be set up • If wire is separated by air, the electrical signal travels along the wire at 3E8 m/s • If wire is separated by another medium, the speed of the electrical signal is determined by the permittivity of free space and the permeability of free space

  13. Example • When you speak on the telephone from LA to a friend in New York (4000 km apart), how long does it take your voice to travel to the receiver?

  14. Measuring the Speed of Light Observer • Galileo • Could not distinguish between human reaction time and speed of light • Roemer • Determined by the period of Jupiter’s moon Io • Period changed depending on whether Earth was approaching or moving away from Jupiter’s orbit • Michelson • Used rotating mirror 8 sided rotating mirror Stationary Mirror Light Source

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