1. EM Spectrum Physics 102 Professor Lee Carkner Lecture 27

2. PAL #26 EM Waves  Radio tuner with L = 10 nH to tune in f = 90.3 MHz  f = 1/2  (LC) ½  LC = (1/2  f) 2  C = (1/2  f) 2 (1/L)  C = (1/(2)(  )(90.3X10 6 )) 2 (1/10X10 -9 )  C =  What value of L will tune in 106.5 MHz?  L = (1/2  f) 2 (1/C)  L = (1/(2)(  )(106.5X10 6 )) 2 (1/3.11X10 -10 )  L =

3. Direction of B Vectors  If the antenna is vertical the E vectors go up and down and the direction of propagation is north  B vectors are at right angles to E vectors and so are horizontal or east and west

4. If you have a LC oscillator receiving electromagnetic waves with a fixed L and a variable C, how would you change C to double the frequency? A)Quarter C B)Half C C)Double C D)Quadruple C E)You cannot get exactly double f by changing C

5. The E and B fields of an electromagnetic wave are oriented A)parallel to the direction of travel and to each other B)perpendicular to the direction of travel and to each other C)parallel to the direction of travel and perpendicular to each other D)perpendicular to the direction of travel and parallel to each other E)Always 180 degrees from each other and the direction of travel

6. If the frequency of the wave emitted by a radio transmitter is doubled, what happens to the wavelength and the speed? A)wavelength is halved, speed is halved B)wavelength is halved, speed is same C)wavelength is same, speed is same D)wavelength is same, speed is doubled E)wavelength is doubled, speed is halved

7. Energy in EM Wave   Each has an energy density:   u B = ½(1/  0 )B 2  The total energy density is the sum of each term   This energy density is not very useful   Called the intensity, I

8. Intensity of Flashlight

9. Intensity  The value of u depends on where the EM wave is in its cycle   The energy density of each field is the same   The intensity depends on how much energy the wave delivers, which depends on the energy density and the speed:  I = uc = (½  0 E rms 2 + ½  0 E rms 2 )c I = c  0 E rms 2  Units: joules per second per square meter (W/m 2 )

10. Intensity of Light  We normally don’t know much about the E field of a light wave, but rather we know something about the source of the wave   As the waves travel out the power is spread out over a sphere of radius r (r is the distance away): I = P s / 4  r 2   Light (like sound) falls off with an inverse square law

11. Inverse Square Law

12. Radiation Pressure   If someone shines a flashlight on you, the light is trying to push you away   EM pressure is due to the fact that light has momentum which can be transmitted to an object through absorption or reflection

13. Momentum Transfer  The change in momentum due to light is given by:   Where  p is the momentum change and  U is the energy change   For reflection the momentum change is twice as much: 

14. Light Pressure  From Newton’s second law  The amount of energy delivered in time  t is:   where I is the intensity and A is the area  Since pressure (p r ) is force per unit area the pressure becomes: p r = I/c (total absorption) p r = 2I /c (total reflection)

15. Example: Light Sail  Radiation pressure can be used to power a spacecraft   The sail can gather light from a star to propel the spacecraft   Light sail powered craft need no engines or fuel 

16. EM Waves and Energy  The energy of an EM wave is proportional to the frequency  where h = Planck’s constant = 6.63 X 10 -34 J s   Radio waves and X-rays produce different effects  They kinds of EM radiation we can observe depends on:   Our eyes

17. Atmospheric Transmission Gamma + X-ray blocked Infrared blocked O 2, N 2 Absorption H 2 O, CO 2 Absorption

18. Sensitivity of Your Eye

19. Light from the Sun  When you look at the sun (note, don’t look at the sun), it seems to be producing a uniform orange-yellow light   Your eyes can only see the visible portion  The sun’s visible light is thermal (blackbody) radiation   The sun produces more yellow and red light than blue, so the sun looks orange

20. Solar Emission Spectrum

21. Radio Waves from the Sun

22. Solar Spectrum  Sun emits most strongly at visible wavelengths   Produces stronger IR at cooler regions   Produces high energy radiation in outer layers   Also produces low energy radiation in magnetic loops 

23. Next Time  Read 24.10  Homework, Ch 22, P 21, Ch 24, P 53, 57  Final exam:  Monday, 6-8 pm, Room 102 Science  Please see me if you have a conflict