Lecture 10/12 Chapter 25

Question 1

Maxwell’s
Predictions

Answer 1

o Maxwell thought that electric and
magnetic fields play symmetric
roles in nature
o Since a varying magnetic field
induces an emf and hence an
electric field (Faraday’s Law), he
hypothesized that a changing
electric field would produce a
magnetic field
o Maxwell calculated the speed of
light to be 3x108 m/s
o He concluded that visible light and
all other electromagnetic waves
consist of fluctuating electric and
magnetic fields, with each varying
field inducing the other

Question 2

Hertz’s Confirmation of
Maxwell’s Predictions

Answer 2

o Hertz measured the speed of the
waves from the transmitter
o He used the waves to form an
interference pattern and
calculated the wavelength
o From v = f λ, v was found
o v was very close to 3 x 108 m/s,
the known speed of light
o This provided evidence in support of
Maxwell’s theory

Question 3

Quick Quiz

A 7.55 x 1014 Hz electromagnetic wave travels in
carbon tetrachloride with a speed of 2.05 × 108 m/s.
What is the wavelength of the wave in this
material?

A) 272 nm
B) 301 nm
C) 338 nm
D) 361 nm
E) 397 nm

Question 4

Quick Quiz

Maxwell guessed that visible light was
an electromagnetic wave because of its:

a. frequency.
b. wavelength.
c. speed.
d. energy.

Question 5

Production of
Electromagnetic Waves

Answer 5

Electromagnetic fields are produced by oscillating charges

o When a charged particle undergoes an acceleration, it must radiate
energy
o EM waves are radiated by any circuit carrying alternating
current
o An alternating voltage applied to the wires of an antenna forces the
electric charge in the antenna to oscillate

Question 6

Charges and Fields

Answer 6

o Stationary charges produce only electric fields
o Charges in uniform motion (constant velocity) produce electric and
magnetic fields
o Charges that are accelerated produce electric and magnetic fields
and electromagnetic waves

Question 7

EM Waves Produced by Antenna

Answer 7

o Two rods are connected to an ac
source, charges oscillate between
the rods (a)
o As oscillations continue, the rods
become less charged, the field near
the charges decreases and the field
produced at t = 0 moves away from
the rod (b)
o The charges and field reverse (c)
o The oscillations continue (d)

o Because the oscillating charges in
the rod produce a current, there is
also a magnetic field generated
o As the current changes, the
magnetic field spreads out from the
antenna
o The magnetic field is perpendicular
to the electric field

Question 8

Quick Quiz

For a certain electromagnetic wave in free space, at one
instant the electric field vector points in the +z direction
while the magnetic field vector points in the +x direction, as
shown in the figure. In what direction is this wave traveling?

A) +x
B) -x
C) +y
D) -y
E) +z

Point the fingers of your right
hand in the direction of curl
your fingers toward and your
thumb will point in the direction
of propagation.

Question 9

Electromagnetic Waves
are Transverse Waves

Answer 9

o The E and B fields are perpendicular to each other
o Both fields are perpendicular to the direction of motion
o Therefore, em waves are transverse waves
o The ratio of the electric field to the magnetic field is equal to the speed of light

C=E/B

Question 10

Properties of EM Waves

Answer 10

o Electromagnetic waves travel at the speed of light
o Because em waves travel at a speed that is precisely the speed of
light, light is an electromagnetic wave

o Electromagnetic waves carry energy as they travel through space, and
this energy can be transferred to objects placed in their path

o Energy carried by em waves is shared equally by the electric and magnetic fields

o Electromagnetic waves transport linear momentum as well as energy

o If a total energy U is absorbed by a given area, the momentum, p, that it receives is: p=U/c

Question 11

Quick Quiz

The Earth is 1.49 × 1011 m from the Sun. If the solar radiation at the top of the Earth’s atmosphere is 1 340 W/m2, what is the total power output of the Sun?

a. 7.48 × 1027 W
b. 2.34 × 1030 W
c. 6.62 × 1026 W
d. 3.74 × 1026 W

Answer 11

d. 3.74 × 1026 W

Question 12

The EM Spectrum

Answer 12

o Forms of electromagnetic waves exist that are distinguished by their frequencies and wavelengths

o c = ƒλ

o Wavelengths for visible light range from 400 nm to 700 nm

o There is no sharp division between one kind of em wave and the next

Question 13

Radio Waves (f ~ 106 Hz to 109 Hz; » ~ 300 m to 0.3m)

Answer 13

o Used in radio and television communication systems
o Generated by charges accelerating through conducting wires

Question 14

Microwaves (f ~ 109 Hz to 1012 Hz; » ~ 300 mm to 0.3mm)

Answer 14

o Wavelengths from about 1 mm to 30 cm
o Generated by electronic devices
o Well suited for radar systems
o Microwave ovens are an application

Question 15

Infrared waves (f ~ 1012 Hz to 4.3x1014 Hz; » ~ 0.3 mm to 700 nm)

Answer 15

o Incorrectly called “heat waves”
o Produced by hot objects and molecules
o Readily absorbed by most materials

Question 16

Visible light (f ~ 4.3x1014 Hz to 7.5x1014 Hz; » ~ 700 nm to 400 nm)

Answer 16

o Part of the spectrum detected by the human eye
o Most sensitive at about 560 nm (yellow-green)
o Produced by rearrangement of electrons in atoms and molecules

Question 17

Ultraviolet light (f ~ 7.5x1014 Hz to 1017 Hz; » ~ 400 nm to 3 nm)

Answer 17

o Covers about 400 nm to 0.6 nm
o Sun is an important source of uv light
o Most uv light from the sun is absorbed in the stratosphere by ozone

Question 18

X-rays (f ~ 1017 Hz to 1020 Hz; » ~ 3 nm to 0.003 nm)

Answer 18

o Most common source is acceleration of high-energy electrons striking a metal target
o Used as a diagnostic tool in medicine

Question 19

Gamma rays (f ~ 1020 Hz and higher; » ~ 0.003 nm and smaller)

Answer 19

o Emitted by radioactive nuclei
o Highly penetrating and cause serious damage when absorbed by living tissue

Question 20

Notes on the EM Spectrum

Answer 20

o Looking at objects in different portions of the spectrum can produce
different information

Question 21

Quick Quiz

A radar pulse sent out to an airplane at a
distance of 20.0 km will return as an echo to the
source in what time interval? (c = 3.00 × 108 m/s)

a. 33.3 × 10−6 s
b. 66.7 × 10−6 s
c. 133 × 10−6 s
d. 0.033 3 × 10−6 s

Answer 21

c. 133 × 10−6 s

Question 22

Polarization

Answer 22

Direction of electric field

The polarization of an electromagnetic wave
refers to the direction of its electric field.

 Polarized light has its electric fields all in the same direction.

 Unpolarized light has its electric fields in random directions(ie. all directions ).

Question 23

Liquid Crystals
displays

Answer 23

LCDs use liquid crystals, whose direction of polarization can be rotated depending on the voltage across them.