# Concepts in Physics-Lecture 18 Electromagnetism spring 2008-Physics, Lecture notes for Concepts in Physics. University of California (CA) - UCLA

## Concepts in Physics

Description: A changing magnetic field produces an electric field, and a changing electric field produces a magnetic field.Electromagnetism, Electromagnetic Induction, Electromagnetic Waves, Magnetic fields, Electromagnets, Induced Current, Induced Current, Electromagnetic Spectrum, FCC, Radio Waves, Polarization, Waves, Radio Waves, time-varying,  frequency, frequency-space, wavelength, media, international allocation, propagation, radiation, Murphy, Concepts in Physics
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Electromagnetism & EM Waves 05/19/08
Lecture 18 1
UCSD Physics 10
Electromagnetism
Electromagnetism
Electromagnetic Induction
Electromagnetic Induction
Electromagnetic
Electromagnetic
Waves
Waves
Spring 2008 2
UCSD Physics 10
Electromagnetism
Electromagnetism
Electricity and magnetism are different facets of
Electricity and magnetism are different facets of
electromagnetism
electromagnetism
a moving electric charge produces magnetic fields
changing magnetic fields move electric charges
This connection first elucidated by Faraday, Maxwell
This connection first elucidated by Faraday, Maxwell
Einstein saw electricity and magnetism as frame-
Einstein saw electricity and magnetism as frame-
dependent facets of
dependent facets of unified
unified
electromagnetic
electromagnetic force
force
Spring 2008 3
UCSD Physics 10
Magnetic fields from electricity
Magnetic fields from electricity
A static distribution of charges produces an electric
A static distribution of charges produces an electric
field
field
Charges in
Charges in motion
motion (an electrical current) produce a
(an electrical current) produce a
magnetic field
magnetic field
electric current is an example of charges (electrons) in motion
Spring 2008 4
UCSD Physics 10
Electromagnets
Electromagnets
Arranging wire in a coil and running a current
Arranging wire in a coil and running a current
through produces a magnetic field that looks a lot
through produces a magnetic field that looks a lot
like a bar magnet
like a bar magnet
called an electromagnet
putting a real magnet inside, can shove the magnet back
and forth depending on current direction: called a
solenoid
Electromagnetism & EM Waves 05/19/08
Lecture 18 2
Spring 2008 5
UCSD Physics 10
Induced Current
Induced Current
The next part of the story is that a
The next part of the story is that a changing
changing
magnetic field
magnetic field produces an electric current in a
produces an electric current in a
loop surrounding the field
loop surrounding the field
called electromagnetic induction, or Faraday’s Law
Spring 2008 6
UCSD Physics 10
The Electromagnetic Connection
The Electromagnetic Connection
A
A changing
changing magnetic field produces an electric field, and a
magnetic field produces an electric field, and a
changing electric field produces a magnetic field.
changing electric field produces a magnetic field.
Electric and Magnetic fields can produce forces on charges
Electric and Magnetic fields can produce forces on charges
An
An accelerating
accelerating charge produces electromagnetic waves
charge produces electromagnetic waves
Both electric and magnetic fields can transport energy
Both electric and magnetic fields can transport energy
Electric field energy used in electrical circuits, e.g., released in
lightning
Magnetic field carries energy through transformer, for example
Spring 2008 7
UCSD Physics 10
Interrelated electric and magnetic fields traveling through space
Interrelated electric and magnetic fields traveling through space
All electromagnetic radiation travels at c
c = 3
= 3
10
108
8 m/s
m/s in
in
vacuum
vacuum
the
the cosmic speed limit!
cosmic speed limit!
real number is 299792458.0 m/s exactly
Spring 2008 8
UCSD Physics 10
What
What
s
s
Waving
Waving
in EM waves?
in EM waves?
What medium transports sound waves?
What medium transports sound waves?
Can there be sound waves in the vacuum of outer space?
What medium transports water waves?
What medium transports water waves?
A topic of considerable debate in the late 1800
A topic of considerable debate in the late 1800
s and early
s and early
1900
1900
s
s
Led to the concept of the
Led to the concept of the
luminiferous
luminiferous ether
ether
an invisible
an invisible
jello
jello
that was thought to vibrate electromagnetically
that was thought to vibrate electromagnetically
Experiments that sought this ether didn
Experiments that sought this ether didn
t find it!
t find it!
This was quite a surprise
This was quite a surprise
Electromagnetic waves travel through empty space!
Electromagnetism & EM Waves 05/19/08
Lecture 18 3
Spring 2008 9
UCSD Physics 10
AM and FM radio waves (including TV signals)
AM and FM radio waves (including TV signals)
Microwaves
Microwaves
Light
Light
X-rays
X-rays
Gamma rays
Gamma rays
What distinguishes these from one another?
What distinguishes these from one another?
Spring 2008 10
UCSD Physics 10
Uses of Electromagnetic Waves
Uses of Electromagnetic Waves
Communication systems
Communication systems
One-way and two-way
Cooking (with microwaves)
Cooking (with microwaves)
Medical Imaging (X rays)
Medical Imaging (X rays)
Night Vision
Night Vision
(infrared)
(infrared)
µwave, IR, visible, UV, gamma)
wave, IR, visible, UV, gamma)
All that we experience through our eyes is conveyed by
Spring 2008 11
UCSD Physics 10
The Electromagnetic Spectrum
The Electromagnetic Spectrum
Relationship between frequency, speed and
Relationship between frequency, speed and
wavelength
wavelength
f
f
= c
= c
f
f is frequency,
is frequency,
is wavelength,
is wavelength, c
c is speed of light
is speed of light
are better suited to different purposes
are better suited to different purposes
The frequency of a radio wave determines its
The frequency of a radio wave determines its
propagation characteristics through various media
propagation characteristics through various media
Spring 2008 12
UCSD Physics 10
US Frequency Allocation
US Frequency Allocation
the FCC
the FCC
(300 MHz has a wavelength of 1 meter)
“Radio” frequency-space is allocated to the hilt!
Here’s a sample region from 300–600 MHz
International allocation gets tricky
Electromagnetism & EM Waves 05/19/08
Lecture 18 4
Spring 2008 13
UCSD Physics 10
If charges oscillate back and forth, get time-varying fields
If charges oscillate back and forth, get time-varying fields
E
+
+
+
+
+
+
+
+
Spring 2008 14
UCSD Physics 10
If charges oscillate back and forth, get time-varying magnetic fields
If charges oscillate back and forth, get time-varying magnetic fields
too
too
Note that the magnetic fields are perpendicular to the electric field
Note that the magnetic fields are perpendicular to the electric field
vectors
vectors
B
+
+
+
+
+
+
+
+
Spring 2008 15
UCSD Physics 10
B
E
Transmitting
antenna
Spring 2008 16
UCSD Physics 10
Receiving antenna works best
when ‘tuned’ to the
wavelength of the signal, and
has proper polarization
Electrons in antenna are “jiggled”
by passage of electromagnetic wave
B
E
Optimal antenna length is one quarter-wavelength (/4)