EM Waves

1
EM Waves

• Physics 102
• Professor Lee Carkner
• Lecture 26

2
PAL 25 RLC Circuits

• What will increase current?
• I V/Z
• Z (R2 (XL - XC)2)½
• Z (R2 (wL - (1/wC))2)½
• For RC circuit
• For RL circuit
• Increase V, decrease w, L and R

3
PAL 25 RLC Circuits

• RLC circuit
• Does not help to increase or decrease w
• Can minimize Z if XL XC
• wL 1/wC

4
Maxwells Laws

• James Clerk Maxwell unified the two fields in
  1865
• Maxwell thought that a changing electric field
  should produce a magnetic field
• Could the two fields continuously create each
  other?

5
Hertz and Oscillators

• A charged capacitor is placed in a circuit with
  an inductor
• This oscillation occurs at the resonance
  frequency
• f0 1/2p(LC)½

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LC Circuit
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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

8
Generating EM Waves

• 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

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AC and EM Wave
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Structure of an EM Wave

• The magnetic field is at right angles to the
  plane of the the E field
• The directions of E, B and c are at right angles
  to each other

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AC Generation of E and B Field
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Propagating EM Wave
13
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

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Marconi Station Cape Cod 1917
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Radio Reception
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Todays PAL

• If you have a LC oscillator receiving
  electromagnetic waves with a fixed L and a
  variable C, how would you change C to
• Increase the frequency
• Decrease the frequency
• Double the frequency
• If you want to tune in WVIK (90.3 MHz) with a
  tuner that has L 1X10-8 H, what value of C do
  you need?

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EM Waves in Nature

• Hertzs radio waves are just one example
• Frequency is related to wavelength and velocity
• c speed of light

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The Electromagnetic Spectrum
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c

• Why?
• We can write c as
• c 1/(m0e0)½
• We can also write c as
• c E/B

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Measuring c in Lab
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The Doppler Effect

• When you observe a moving object, the wavelengths
  of light you observe change
• Moving away --
• Moving towards --
• Example the change in a cars sound as it moves
  past you
• By measuring the shift of lines in a spectrum,
  you can determine how fast the object is moving

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Doppler Effect
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Doppler Effect for Light

• However, at low speeds (ultltc, where u is the
  relative velocity between source and detector)
  the equations reduce to the classical form
• In astronomy it is easier to measure the
  wavelength rather than the frequency (through the
  shift of spectral lines)
• u (Dl/l) c
• c, the speed of light in vacuum, is constant (3
  X 108 m/s)

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Spectral Line Shifts

• When we observe a spectrum of a object, we
  compare the observed wavelengths to standard ones
• Find how the wavelength has shifted (Dl) and thus
  find u
• More shift, moving faster away
• More shift, moving faster towards

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Red Shifted Spectrum
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Expansion of the Universe

• All galaxies are moving away from all others
• In the past, everything in the universe must have
  been much closer together

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Next Time

• Read 22.5-22.6
• Homework Ch 22 P 13, 18, 29, 31