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

X

t = 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)