EM Spectrum

Physics 102Professor Lee

CarknerLecture 25

Final Exam

Covers: Chapters 21-22 (2/3) Chapters 13-20 (1/3)

Structured like previous tests About 15 multiple choice, 6 problems

Practice problems online Sample equation sheet on web page

PAL #24 EM Waves Radio tuner with L = 10 nH to tune in f

= 90.3 MHz f = 1/2(LC)½

LC = (1/2f)2

C = (1/2f)2(1/L) C = (1/(2)()(90.3X106))2(1/10X10-9) C = 3.11X10-10 F

What value of L will tune in 106.5 MHz? L = (1/2f)2(1/C) L = (1/(2)()(106.5X106))2(1/3.11X10-10) L = 7.2X10-9 H = 7.2 nH

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, east and west

Energy in EM Wave

Each has an energy density (J per m3): uE = ½0E2

uB = ½(1/0)B2

u = uE + uB

We want to know how much energy we

get per unit time over a certain area Called the intensity, I

Intensity of Flashlight

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 = (½0Erms2 + ½0Erms

2)c

I = c0Erms2

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

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 Since pressure (pr) is force per unit area the

pressure becomes:pr = I/c (total absorption)

pr = 2I /c (total reflection)

Light Sail

A light sail is a very large, very thin, very reflective piece of fabric to which a spacecraft is attached

Can also use a laser Light sail powered craft need

no engines or fuel Do need very large sails

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 Earth’s atmosphere Our eyes

Atmospheric Transmission

Gamma+

X-ray blocked

Infrared blocked

O2, N2

AbsorptionH2O, CO2

Absorption

Sensitivity of Your Eye

Light from the Sun

The sun (like all stars) is actually producing EM waves with a variety of wavelengths via many different processes

Your eyes can only see the visible portion

Like the radiation produced by any hot object The sun produces more yellow and red light

than blue, so the sun looks orange

Solar Emission Spectrum

Radio Waves from the Sun

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

Next Time

Final exam: Section 1: Thursday 9am Section 2: Tuesday noon

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 CB) Half CC) Double CD) Quadruple CE) You cannot get exactly double f by

changing C

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

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 halvedB) wavelength is halved, speed is same

C) wavelength is same, speed is sameD) wavelength is same, speed is doubledE) wavelength is doubled, speed is

halved