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Tony Hyun Kim Spring 2008, 6.UAT. Wireless control: Sending and receiving electromagnetic waves. Objectives. Explain the basic physics of wireless control. Focus on ELECTRO MAGNETIC WAVES Demonstrate the physics with early 20 th century technology. Objectives. - PowerPoint PPT Presentation
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WIRELESS CONTROL:SENDING AND RECEIVING ELECTROMAGNETIC WAVES
Tony Hyun KimSpring 2008, 6.UAT
Objectives
Explain the basic physics of wireless control. Focus on ELECTROMAGNETIC WAVES
Demonstrate the physics with early 20th century technology.
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
The Basics:Electric field of a stationary charge
+q
The Basics:Electric field of a stationary charge
+q
The Basics:Electric field of a stationary charge
+q
Boring: The field is static, and radial
The Basics:Electric field of a stationary charge
+q
Question:How does this picture change, when we move the charge?
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.
E-Field of an ACCELERATED chargeY
Xt = 0 second
E-Field of an ACCELERATED chargeY
X
t = 1 second
1 cm
E-Field of an ACCELERATED chargeY
X
t = 0 second
E-Field of an ACCELERATED chargeY
X
t = 0 second
E-Field of an ACCELERATED chargeY
X
?
t = 1 second
E-Field of an ACCELERATED chargeY
XRadius = c * (1 sec) = 3 x 108 m
t = 1 second
E-Field of an ACCELERATED chargeY
X
t = 1 second
Radius = c * (1 sec) = 3 x 108 m/s
E-Field of an ACCELERATED chargeY
X
t = 2 second
E-Field of an ACCELERATED chargeY
X
? t = 2 second
E-Field of an ACCELERATED chargeY
X
t = 2 second
E-Field of an ACCELERATED chargeY
X
t = 2 second
E-Field of an ACCELERATED chargeY
X
t > 2 second
Let’s see that again
E-Field of an ACCELERATED chargeY
X
t = 2 second
“Transverse”Electric field!
E-Field of an ACCELERATED chargeY
X
t > 2 second
And it moves out!
That’s how electric fields are radiated!
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!
Spark gap transmitter: 1. High voltage
Spark gap transmitter: 2. Charge buildup
Spark gap transmitter: 3. Breakdown
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
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
Coherer
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)