em spectrum physics 102 professor lee carkner lecture 27
TRANSCRIPT
PAL #26 EM Waves Changing C of LC oscillator
f and C have an inverse square root relationship
Increase the frequency
Decrease the frequency
Double the frequency
Tune in WVIK (90.3 MHz) with L = 1X10-8 H (LC)½ = 1/2f C = [1/(2)()(90.3X106)]2(1/1X10-8) = 3.1X10-10 F
Energy in EM Wave
Each has an energy density: uE = uB =
The total energy density is the sum of each term
This energy density is not very useful
Called the intensity, I
Intensity
Use Erms or Brms to get average 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 = I = c0Erms
2
Units: joules per second per square meter (W/m2)
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 = Ps / 4r2
Light (like sound) falls off with an inverse square law
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
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:
p = 2U/c
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
pr = I/c (total absorption)pr = 2I /c (total reflection)
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
Today’s PAL Consider a light sail craft near the Earth with a mass
of 5 tons (5000 kg) and a sail area of 2.25 x 108 m2 (about 15 by 15 km). The sail is made from a lightweight, thin, and highly reflective fabric and is powered by sunlight (distance to sun = 1.5 X 1011 meters, power of sun 3.9 X 1026 W).
a) What is the intensity of sunlight for the spacecraft?
b) How much light pressure does the spacecraft feel?
c) What is the acceleration of the spacecraft (remember F = ma, p = F/A)?
The Electromagnetic Spectrum
Electromagnetic radiation can have any wavelength
This is where our Sun produces the most light, so our eyes have evolved to detect it
We will use the terms “light”, “photons” and “electromagnetic (EM) radiation or waves” interchangeably
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
The EM Spectrum Radio
Low energy but good penetration
Millimeter (microwave) Also good for communication
Infrared Emitted strongly by living things
Visible What your eyes see
The EM spectrum
Ultraviolet High energy
X-ray Strongly penetrating
Gamma Ray Hard to produce Get from H bombs, etc.
Light from the Sun
The sun (like all stars) is actually producing EM waves with a variety of wavelengths via many different processes
The sun’s visible light is thermal (blackbody)
radiation
The sun produces more yellow and red light than blue, so the sun looks orange
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