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WIRELESS CONTROL:SENDING AND RECEIVING ELECTROMAGNETIC WAVES
Tony Hyun KimSpring 2008, 6.UAT
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Objectives
Explain the basic physics of wireless control. Focus on ELECTROMAGNETIC WAVES
Demonstrate the physics with early 20th century technology.
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Objectives
Explain the basic physics of wireless control. Focus on ELECTROMAGNETIC WAVES
Demonstrate the physics with early 20th century technology.
Basic principle:Sudden charge motion emits EM waves
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The Basics:Electric field of a stationary charge
+q
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The Basics:Electric field of a stationary charge
+q
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The Basics:Electric field of a stationary charge
+q
Boring: The field is static, and radial
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The Basics:Electric field of a stationary charge
+q
Question:How does this picture change, when we move the charge?
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Two basic physical facts
1. “Information transfer” is NOT instantaneous.
1. For electric phenomena, the “transfer rate” is c = 300,000,000 m/s = 3 x 108 m/s
2. In “free space,” field lines don’t disappear.
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E-Field of an ACCELERATED chargeY
Xt = 0 second
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E-Field of an ACCELERATED chargeY
X
t = 1 second
1 cm
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E-Field of an ACCELERATED chargeY
X
t = 0 second
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E-Field of an ACCELERATED chargeY
X
t = 0 second
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E-Field of an ACCELERATED chargeY
X
?
t = 1 second
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E-Field of an ACCELERATED chargeY
XRadius = c * (1 sec) = 3 x 108 m
t = 1 second
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E-Field of an ACCELERATED chargeY
X
t = 1 second
Radius = c * (1 sec) = 3 x 108 m/s
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E-Field of an ACCELERATED chargeY
X
t = 2 second
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E-Field of an ACCELERATED chargeY
X
? t = 2 second
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E-Field of an ACCELERATED chargeY
X
t = 2 second
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E-Field of an ACCELERATED chargeY
X
t = 2 second
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E-Field of an ACCELERATED chargeY
X
t > 2 second
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Let’s see that again
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E-Field of an ACCELERATED chargeY
X
t = 2 second
“Transverse”Electric field!
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E-Field of an ACCELERATED chargeY
X
t > 2 second
And it moves out!
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That’s how electric fields are radiated!
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Can we arrange for sudden charge motion?
Use a relic from the previous century: A “spark gap transmitter”
Basic Idea:1. High voltage2. Build-up of charge3. Breakdown: Sudden discharge across
junction
Focus here!
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Spark gap transmitter: 1. High voltage
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Spark gap transmitter: 2. Charge buildup
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Spark gap transmitter: 3. Breakdown
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Spark gap transmitter
The apparatus is a realization of the sudden charge motion we described earlier.
Y
X
t > 0 second
Top down view of
the transmitt
er
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Wireless control of LEDs
Want to take advantage of the basic physics to do something useful.
Use a device that responds to electric fields: A “coherer”: a circuit with an antenna
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Coherer
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Conclusion
Accelerated charges radiate electromagnetic waves.
Described and demonstrated a simple experimental setup to accelerate charges.
Used this physics to control LEDs wirelessly.
Acknowledgements: Robert Moffatt (Physics ’09)