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EM Waves

Explore the principles of AC circuits, Maxwell's laws, EM waves, oscillators, radio transmission, electromagnetic spectrum, and spectral analysis. Understand how changing components affects current in RC and RL circuits.

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EM Waves

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  1. EM Waves Physics 102 Professor Lee Carkner Lecture 24

  2. Three AC Circuits • DVmax = 10 V, f = 1Hz, R = 10 • DVrms = 0.707 DVmax = (0.707)(10) = 7.07 V • R = 10 W • Irms = DVrms/R = 0.707 A • Imax = Irms/0.707 = 1 A • Phase Shift = 0 • When V = 0, I = 0 • DVmax = 10 V, f = 1Hz, C = 10 F • DVrms = 0.707 DVmax = (0.707)(10) = 7.07 V • XC = 1/(2pfC) = 1/[(2)(p)(1)(10)] = 0.016 W • Irms = DVrms/XC = 441.9 A • Imax = Irms/0.707 = 625 A • Phase Shift = ¼ cycle (-p/2) • When V = 0, I = I max = 625 A

  3. Three AC Circuits • DVmax = 10 V, f = 1Hz, L = 10 H • DVrms = 0.707 DVmax = (0.707)(10) = 7.07 V • XL = 2pfL = (2)(p)(1)(10) =62.83 W • Irms = DVrms/XL = 0.11 A • Imax = Irms/0.707 = 0.16 A • Phase Shift = ¼ cycle (+p/2) • When V = 0, I = I max = 0.16 A

  4. For capacitor, V lags I For inductor, V leads I

  5. Current and Power Z = (R2 + (XL - XC)2)½ DV = IZ • The voltage through any one circuit element depends only on its value of R, XC or XL however cos f = IR/IZ = R/Z • The phase angle is also related to the power Pav = IrmsVrms cos f

  6. Maxwell’s Laws • In the late 18th - early 19th centuries the relationship between electricity and magnetism was uncertain • It was known that a changing magnetic field produced an electric field • Could the two fields continuously create each other?

  7. Hertz and Oscillators • In 1887 Heinrich Hertz experimented with an LC oscillator • The energy is transferred back and forth between the inductor and capacitor f0 = 1/[2p(LC)½]

  8. Oscillators and EM Waves • Hertz found that if he set up an oscillation in one circuit and then put another one near-by (with the same frequency) it would also have oscillations • First circuit transmits electromagnetic waves • Radio transmitter and receiver

  9. The alternating current will make one end of the rod positive, then neutral, then negative • This changing electric field generates a changing magnetic field • These fields propagate out from the rod as an EM wave

  10. AC and EM Wave

  11. Structure of an EM Wave • The magnetic field is at right angles to the plane of the E field • The directions of E, B and c are at right angles to each other

  12. Radio • EM waves can be received the same way they are generated • This current can be large if the frequency of the wave matches the natural frequency of the circuit • First person to make use of radio waves for communication was Marconi • Sent first wireless message from US to England in 1903

  13. Radio Reception

  14. EM Waves in Nature • We are familiar with many kinds of EM waves • The major difference between them is frequency v = fl = 3 X 108 m/s = c c = speed of light

  15. The Electromagnetic Spectrum

  16. c • EM waves in a vacuum travel only at the speed of light • Why? • We can write c as: c = 1/(m0e0)½ • We can also write c as c = E/B

  17. Spectrum • A plot of intensity versus wavelength is called a spectrum • Each transition produces or absorbs a single wavelength of radiation

  18. Types of Spectra • For a dense gas (or a solid or liquid) the atoms collide so much that they blur the lines into a continuous blackbody spectrum • A low density gas excited by collisions or radiation will produce an emission spectrum • A low density gas in front of a source of continuous radiation will produce an absorption spectrum • A continuum with dark lines at specific wavelengths

  19. Using Spectra • Each kind of atom produces light at specific wavelengths • If the atoms are moving, the lines will be Doppler shifted • We can measure this wavelength shift to find the velocity of the object

  20. Elemental Emission Spectra

  21. Stellar Doppler Shift

  22. Expansion of the Universe • In the early 20th century astronomers discovered that all distant galaxies are red shifted • All galaxies are moving away from all others • In the past, everything in the universe must have been much closer together

  23. Next Time • Read 22.5-22.6 • Homework Ch 22 P 13, 18, 29, 31

  24. How would you change R, C and w to increase the rms current through a RC circuit? • Increase all three • Increase R and C, decrease w • Decrease R, increase C and w • Decrease R and w, increase C • Decrease all three

  25. How would you change R, L and w to increase the rms current through a RL circuit? • Increase all three • Increase R and L, decrease w • Decrease R, increase L and w • Decrease R and w, increase L • Decrease all three

  26. Would increasing w always increase the current through an RLC circuit? • No, since the capacitive reactance decreases • Yes, since the capacitive reactance increases • Yes, since the inductive reactance decreases • No, since the inductive reactance increases • No, continually raising w does not continually raise I

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