EM Waves

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
Maxwells 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