Electromagnetic Waves Slides - Little Shop of Physics

Electromagnetic
Waves
• Electromagnetic waves are
traveling waves of electric
and magnetic fields.
• Electromagnetic waves
come in a wide range of
wavelengths. There is more
to the spectrum than the
rainbow.
• Long wavelengths act like
waves, short wavelengths
like particles. Or, more
precisely, like photons.
Music: Battlefield Band, Start it All Over Again
Warming Up.
A metal wire is resting on
a U-shaped conducting
rail. The rail is fixed in
position, but the
wire is free to move.
If the magnetic field is increasing in strength, what is the
direction of the induced current?
a.Clockwise
b.Counterclockwise
What is the direction of the force on the wire?
a.To the left
b.To the right
A changing
magnetic field
induces an
electric field.
A changing
electric field
induces a
magnetic field
too.
One of the
10 coolest
things ever
discovered
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The Inductor
L is the
inductance;
the unit is the
Henry. (H)
LC
Circuits
Tuned Circuits
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Unifying Electricity and Magnetism
∇ ⋅ E = ρ
ε 0
∇ ⋅ B = 0
∇ × E = − ∂ B
∂t
∇ × B = µ 0
j + µ0ε 0
The electromagnetic pulse.
An accelerated charge emits a pulse of electric and
magnetic fields. This pulse travels through space.
Maxwell’s equations say the speed should be:
1
v = = 3.00 × 10
ε0µ 0
8 m/s
The electromagnetic wave.
An oscillating charge will emit an electromagnetic wave.
It’s a wave of electric and magnetic fields.
∂ E
∂t
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FM vs. AM
An FM station transmits at 100 MHz, corresponding to a
wavelength of 3.0 m.
An AM station transmits at 1000 kHz, which is 1.0 MHz.
What is the corresponding wavelength?
c = f λ
FM: Pick up electric field
FM & AM Radio
1
4 λ
Helpful Relationships
AM: Pick up magnetic field
E = hf = hc
λ
h = 4.14 × 10 −15 eVis
E (in eV) =
1240
λ (in nm)
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Atomic Energies
Red vs. Blue?
A red pen laser emits light
of wavelength 670 nm.
A blue pen laser emits light
of wavelength 470 nm.
Which has a higher photon energy, the red or the blue?
The
Electromagnetic
Spectrum
Particle-y
Depends
Wave-ish
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The Electromagnetic Spectrum
Wave Wavelength Frequency
FM Radio
Microwave
Far IR
Visible
Ultraviolet
10 µm
500 nm
290 nm
The
Electromagnetic
Spectrum
The
Electromagnetic
Spectrum
100 MHz
1.9 GHz
Particle-y
Depends
Wave-ish
Photon
energy
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Basic Relationships
The speed of electromagnetic waves
A. depends on the wavelength
B. depends on the photon energy
C. is the same as the speed of sound
D. is the same for all waves regardless of wavelength
Basic Relationships
A typical analog cell phone has a frequency of 850 MHz; a
digital phone a frequency of 1950 MHz. Compared to the
signal from an analog cell phone, the digital signal has
! A. ! longer wavelength and lower photon energy
! B. ! longer wavelength and higher photon energy
! C. ! shorter wavelength and lower photon energy
! D. ! shorter wavelength and higher photon energy
Basic Relationships
A radio tower emits two 50 W signals, one an AM signal
at a frequency of 850 kHz, one an FM signal at a
frequency of 85 MHz. Which signal has more photons per
second? Explain.
! A. ! The AM signal has more photons per second.
! B. ! The FM signal has more photons per second.
! C. ! Both signals have the same photons per second.
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Intensity
From
Chapter
15
Intensity is
a ratio of
power to
area:
I = P
a
My laser pointer has a total output power of about 1.0
mW. When I shine it on the screen, it spreads out to
make a spot about 1.0 mm in diameter. What is the
intensity of the light in this spot?
How does this compare to the intensity of sunlight on
the ground at high noon on a summer day (which is
approximately 1100 W/m 2 )?
Energy and Field Strength
E 0 =
2I
cε 0
B 0 = E 0
c
Intensity and Field Strength
E 0 = cB 0
The laser pointer from the previous problem has an
intensity of 1300 W/m 2 .
What electric and magnetic field strengths does this
intensity correspond to?
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Intensity Variation for Spherical Wave
Polarization
Navigating By The Sky
Polarization of skylight
I = P source
4πr 2
Bee eyes
detect
polarization
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The Microwave Oven
Inside the cavity of a microwave oven, the 2.4 GHz
electromagnetic waves have an intensity of 5.0 kW/m 2 .
a. What is the strength of the electric field?
b. The magnetic field?
A digital cell phone emits a 1.9 GHz
electromagnetic wave with total power
0.60 W.
• At a cell phone tower 2.0 km away,
what is the intensity of the wave?
(Assume that the wave spreads out
uniformly in all directions.)
• What are the electric and magnetic
field strengths at this distance?
Biological Effects of EM Radiation
Long wavelength
Short wavelength
Wave-ish Particle-y
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What are the photon energies corresponding
to the following wavelengths?
a. Near IR, 1000 nm
b. Far red end of the spectrum, 750 nm
c. Far blue end of the spectrum, 400 nm
d. Near UV, 290 nm
Atomic Energies
E (in eV) =
Atomic Radiation
Visible spectrum:
Approx. 400 - 750 nm
1240
λ (in nm)
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Visible Light, Near IR and UV
3 different cones
tuned to different
photon energies
The band gap of the silicon used to make the CCD
detector in a black and white security camera is 1.12 eV.
Photons with energy greater than this will be detected.
• What wavelength does this correspond to?
• In what part of the spectrum is this?
You Look Positively Radiant
A typical human has a surface area of about 1.8 m 2 . All
skin, regardless of color, has an emissivity of about e=0.97.
How much power does a person’s body radiate at normal
skin temperature? (About 33 °C, or 306 K)
What is the peak wavelength of the emission?
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Pit Vipers 1600 cells
100 Watts = 4 Watts?
Raw Processed
A typical incandescent lamp has a filament at a
temperature of approximately 2500 °C.
What is the peak wavelength of the emission?
Short wavelength = high photon energy
Germicidal lamps
254 nm
4.9 eV
Tanning beds
365 nm
3.4 eV
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Creating X rays
If an electron is accelerated through a 5.0 kV potential
difference, what is the maximum photon energy of the
resulting x ray? What is the wavelength?
Using X rays
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