AC Resonance
AM Radio

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Joseph F. Alward, PhD    
Department of  Physics  
University of the Pacific		  

 Applets included in this eLecture:  Animation of Electromagnetic Waves

 

 

 

 

 

 

 Important Equations and Concepts            
EMF = L DI /Dt
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LC resonant frequency:
 
f = (1/2p)/(LC)1/2  

 Lenz's Law
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The tunable AM radio is based on
LC resonance.

     

 

 

 

 Charging a Capacitor                   

 

 

 

 

 

 

  Shorting a Capacitor    

 

 

 

 

 

 

 

  Magnetic Field in an Inductor        

 

 

 

 

 

 

 

 

  Inductors Resist Changes in Current    

 

 

 

 

 

 

 

  Inductance and Inductors            

     EMF = L DI / Dt 		L = "inductance"

L is measured
in Henries

 
       Joseph Henry
           (1797-1878)		 Example:  L = 500 mH

I0 = 3 A
I  =  0 A
Dt = 0.001 s

DI = I - I0
     = 0 - 3
     = -3 A

EMF = 500 x 10-3 (-3) /0.001
         = -1500 V

 

 

 

 

 

 

  LC Circuit                                                              

Once current begins to flow in the inductor,
it will not let it fall to zero even after the
plates are neutral.

 

  

 

 

 

  LC Circuit                                                                                            

Plates are empty of charge, but the current
keeps going--by Lenz's Law.

 

 

 

 

  LC Resonance                                                                                     
Frequency = (1/2p) / (LC)1/2 
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Example:  

C = 100 mF
L =  5 mH

LC = (5 x 10-3)(100 x 10-6)
      = 50 x 10-8

f = (1/2p) / (LC)1/2
f  = 225 Hz

 

 

 

 

 

 

  Locating Buried Metal                                                                 

Iron in nearby metals changes the inductance
of both inductors, but the one closer to the
iron changes more.  The resonant frequencies
of the LC circuits are now different.
 Slightly different resonant frequencies are
 superimposed, giving beats which indicate the
 presence of magnetizable material.

 

 

 

 

 

 

 Generating Electromagnetic Waves         

                             Click here to see animation.

 

 

 

 

 

 

 Electromagnetic Waves                         

c = 186, 000 miles/second

   = 3 x 108 meters/second

 

 

 

 

 

 

 

 

  Unmodulated Carrier Wave     

 

 

 

 

 

 

 

 

   Modulating Carrier Wave        

 

 

 

 

 

 

 

 

  Transmitter and Receiver                              

 

 

 

   Amplitude Modulation                                  

 

 

 

 

   The Straight Wire Antenna      

  Oscillating electric field component of EM wave causes electrons in wire
  to oscillate up and down.

 

  

 

 

 

 

   Loop Antenna                                             


 Changing magnetic flux induces emf in loop.  If the frequency of the electro-
 magnetic wave is the same as the frequency of oscillation of the LC circuit,
 the circuit is said to be "in resonance" with the wave.

 

 

 

 

 

  Loop Antenna         

 

 

 

 

 

 

  The Diode            

 

 

 

  Diodes Rectify Alternating Voltage           

 

 

 

   Rectification                                                 

 

 

  

 

 

 

  The AM Radio Circuit                                     

 

 

 

 

 

 

   Radio Parts                                                                     

 

 

 

 

 

  An AM Radio