Resonance Electromagnetic Waves

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ResonanceElectromagnetic Waves
Physics 102 Lecture 14
2
Hour Exam IIMonday October 27

• Material covered
• lectures through 14 (resonance)
• discussions through 7
• homework through 7
• labs through 6
• Review, Sunday October 26, 3-430
• will go over Fall 2007 Exam

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ReviewImpedance Triangle and Resonance
Vmax,gen Imax Z
Z
(XL-XC)
f
R
XL and XC point opposite. When adding, they tend
to cancel! When XL XC they completely cancel
and Z R. This is resonance!
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4
Resonance
R is independent of f
R
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5
Resonance
R is independent of f
XL increases with f
R
XL
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Resonance
R is independent of f
XL increases with f
XC increases with 1/f
R
XC
XL
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Resonance
R is independent of f
XL increases with f
XC increases with 1/f
Z
R
Z XL and XC subtract
XC
XL
Resonance XL XC
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Resonance
R is independent of f
XL increases with f
XC increases with 1/f
Imax Vgen,max/Z
Z XL and XC subtract
Current
Z
f0
Resonance XL XC
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Preflight 14.1
L

• As the frequency of the circuit is either raised
  above or lowered below the resonant frequency,
  the impedance of the circuit

R
C
Always increases
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Only increases for lowering the frequency
Only increases for raising the frequency
Z
Z is minimum at fo!
Any other frequency will have higher Z!
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ACT Resonance
L
R

• The AC circuit to the right is being driven at
  its resonance frequency. Compare the maximum
  voltage across the capacitor with the maximum
  voltage across the inductor.
• VC gt VL
• VC VL
• VC lt VL
• Depends on R

C
At resonance XL XC. Since everything has the
same current we can write XL XC XLImax
XCImax VL VC
Also VGenerator is in phase with current!
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Preflight 14.3

• At the resonant frequency, which of the following
  is true?

VL
I leads Vgenerator
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I lags Vgenerator
Vgen
f 0
I is in phase with Vgenerator
VR
VC
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Resonance in Radios

• An AC circuit with R 2 W, L 0.30 mH and
  variable capacitance is connected to an antenna
  to receive radio signals at the resonance
  frequency. If you want to listen to music
  broadcasted at 96.1 MHz, what value of C should
  be used?

L
Example
R
C
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ACT Radios

• Your radio is tuned to FM 96.1 MHz and want to
  change it to FM 105.9 MHz, which of the following
  will work.
• Increase Capacitance
• Decrease Capacitance
• Neither, you need to change both

Higher frequency needs smaller capacitor so it
can develop voltage quicker.
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Summary of Resonance

• At resonance
• Z is minimum (R)
• Imax is maximum (Vgen,max/R)
• Vgen is in phase with I
• XL XC VL -VC
• At lower frequencies
• XC gt XL Vgen lags I
• At higher frequencies
• XC lt XL Vgen lead I

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This concludes the material covered on HE2. The
material to follow will be covered on HE3.
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Electromagnetic Waves

• Light, Radio, TV, Microwaves, Satellites, X-Rays

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Electromagnetic Waves
Generator creates E field up and down.
-
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Electromagnetic Waves
Generator creates E field up and down.
- -
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Electromagnetic Waves
Generator creates E field up and down.
-
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Electromagnetic Waves
Generator creates E field up and down.

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Electromagnetic Waves
Generator creates E field up and down.
-
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Electromagnetic Waves
Generator creates E field up and down.
- -
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Electromagnetic Waves
Generator creates E field up and down.
-
http//www.colorado.edu/physics/2000/waves_particl
es/wavpart4.html
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ACT EM Waves

• Which direction should I orient my antenna to
  receive a signal from a vertical transmission
  tower?
• 1) Vertical 2) Horizontal 3) 45 Degrees

-


Alternating E field moves charges up and down
thru antenna!
demo
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Electromagnetic Waves(Preflight 14.5)
Generator also creates B field into and out of
the page!
Nice work! 61 got preflight correct.
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Electromagnetic Waves

• Transverse (vs. sound waves longitudinal)
• E perpendicular to B and always in phase E
  B increase and decrease at same times
• Can travel in empty space (sound waves cant!)
  
• Speed of light v c 1/ v(e0 m0) 3 x
  108 m/s (186,000 miles/second!)
• Frequency f v/l c/l Period T 1/f
• wave travels one wavelength l in one period T

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Preflight 14.6 14.12
Which of the following are transverse waves?
?

• sound
• light
• radio
• X-ray
• microwave
• water waves
• The Wave (i.e. at football games)

?
?
All but sound!
?
?
?
?
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EM Waves Practice
Example
Shown below is the E field of an EM wave
broadcast at 96.1 MHz and traveling to the
right. (1) What is the direction of the magnetic
field? (2) Label the two tic marks on the x
axis (in meters).
Perpendicular to E, v Into/out of the page
3.1
6.2
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Doppler Effect

• Doppler Example Audio
• Doppler Example Visual
• fo fe (1 u/c) moving toward each other
• fo fe (1 - u/c) moving away from each other
• Notes
• These are easier than equations for sound!
• If f increases then ? decreases (and vice versa),
  since speed is always c (c ?f)

• Finding relative velocity
• u v1 v2 moving in opposite directions
• u v1 v2 moving in same direction

demos
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ACT Doppler Practice
Example
V 32 m/s
V 50 m/s
In the jeep, the frequency of the light from the
troopers car will appear (1) higher (more
blue) (2) Lower (more red) What value should
you use for u in the equation? (1) 32 (2)
50 (3) 5032 (4) 50-32
Cars are getting closer together fo fe (1
u/c)
Cars are moving in same directions u v1 v2
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See you Wednesday!