EM Waves Physics 102 Professor Lee Carkner Lecture 26

PAL #25 RLC Circuits  Power dissipated by RLC circuit  Find , X C and X L   = 2  f = 2  (60) = 377 rad/s  X L =  L = (377)(130X10-3) =  X C = 1/  C = 1/(377)(7X10-6) =  Find Z to find I rms and P  Z = (R 2 + (X L – X C ) 2 ) ½  Z = ( (49.1 – 378.9) 2 ) ½ =  I rms = V rms /Z = 120/1053 =  P = IV cos  = IV (R/Z) = (0.114)(120)(1000/1053) =

How would you change R, C and  to increase the rms current through a RC circuit? A)Increase all three B)Increase R and C, decrease  C)Decrease R, increase C and  D)Decrease R and , increase C E)Decrease all three

How would you change R, L and  to increase the rms current through a RL circuit? A)Increase all three B)Increase R and L, decrease  C)Decrease R, increase L and  D)Decrease R and , increase L E)Decrease all three

Would increasing  always increase the current through an RLC circuit? A)No, since the capacitive reactance decreases B)Yes, since the capacitive reactance increases C)Yes, since the inductive reactance decreases D)No, since the inductive reactance increases E)No, continually raising  does not continually raise I

Maxwell’s Laws   James Clerk Maxwell unified the two fields in 1865   Maxwell thought that a changing electric field should produce a magnetic field  Could the two fields continuously create each other?

Hertz and Oscillators   A charged capacitor is placed in a circuit with an inductor   This oscillation occurs at the resonance frequency: f 0 = 1/[2  (LC) ½ ]

LC Circuit

Oscillators and EM Waves  Hertz found that if he set up an oscillation in one circuit and then put another one near- by (with the same frequency) it would also have oscillations   First circuit transmits electromagnetic waves   Radio transmitter and receiver

Generating EM Waves   The alternating current will make one end of the rod positive, then neutral, then negative   This changing electric field generates a changing magnetic field  These fields propagate out from the rod as an EM wave

AC and EM Wave

Structure of an EM Wave   The magnetic field is at right angles to the plane of the the E field   The directions of E, B and c are at right angles to each other

AC Generation of E and B Field

Propagating EM Wave

Radio  EM waves can be received the same way they are generated   This current can be large if the frequency of the wave matches the natural frequency of the circuit   First person to make use of radio waves for communication was Marconi  Sent first wireless message from US to England in 1903

Radio Reception

EM Waves in Nature   Hertz’s radio waves are just one example   Frequency is related to wavelength and velocity v = f = 3 X 10 8 m/s = c c = speed of light

The Electromagnetic Spectrum

c   Why?  It is built into the universe  c = 1/(     ) ½  The value of c is due to the way in which electric and magnetic fields penetrate space  c = E/B

The Doppler Effect  When you observe a moving object, the wavelengths of light you observe change  Moving away --  Moving towards --  Example: the change in a car’s sound as it moves past you   By measuring the shift of lines in a spectrum, you can determine how fast the object is moving

Doppler Effect

Doppler Effect for Light   However, at low speeds (u<<c, where u is the relative velocity between source and detector) the equations reduce to the classical form: f’ = f (1 ± u/c)  In astronomy it is easier to measure the wavelength rather than the frequency (through the shift of spectral lines):  c, the speed of light in vacuum, is constant (3 X 10 8 m/s)

Spectral Line Shifts  When we observe a spectrum of a object, we compare the observed wavelengths to standard ones   Find how the wavelength has shifted (  and thus find u  For objects moving away from us the spectral lines move to larger wavelengths   For objects moving towards us the spectral lines move to shorter wavelengths  More shift, moving faster towards

Red Shifted Spectrum

Expansion of the Universe  In the early 20th century astronomers discovered that all distant galaxies are red shifted  All galaxies are moving away from all others   In the past, everything in the universe must have been much closer together 

Next Time  Read  Homework Ch 22 P 13, 18, 29, 31