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

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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, Indu...
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Electromagnetism & EM Waves 05/19/08

Lecture 18 1

UCSD Physics 10

Electromagnetism

Electromagnetic Induction

Electromagnetic Waves

Spring 2008 2

UCSD Physics 10

Electromagnetism

• Electricity and magnetism are different facets of

electromagnetism

– a moving electric charge produces magnetic fields

– changing magnetic fields move electric charges

• This connection first elucidated by Faraday, Maxwell

• Einstein saw electricity and magnetism as frame-

dependent facets of unified electromagnetic force

Spring 2008 3

UCSD Physics 10

Magnetic fields from electricity

• A static distribution of charges produces an electric

field

• Charges in motion (an electrical current) produce a

magnetic field

– electric current is an example of charges (electrons) in motion

Spring 2008 4

UCSD Physics 10

Electromagnets

• Arranging wire in a coil and running a current

through produces a magnetic field that looks a lot

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

• The next part of the story is that a changing

magnetic field produces an electric current in a

loop surrounding the field

– called electromagnetic induction, or Faraday’s Law

Spring 2008 6

UCSD Physics 10

The Electromagnetic Connection

• A changing magnetic field produces an electric field, and a

changing electric field produces a magnetic field.

• Electric and Magnetic fields can produce forces on charges

• An accelerating charge produces electromagnetic waves

• 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

• All electromagnetic radiation travels at c = 3 108 m/s in

vacuum – the cosmic speed limit!

– real number is 299792458.0 m/s exactly

Spring 2008 8

UCSD Physics 10

What’s “Waving” in EM 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 radio waves?

• A topic of considerable debate in the late 1800’s and early

1900’s

• Led to the concept of the “luminiferous ether” – an invisible

“jello” that was thought to vibrate electromagnetically

• Experiments that sought this ether didn’t find it!

• 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)

• Microwaves

• Light

• X-rays

• Gamma rays

• What distinguishes these from one another?

Spring 2008 10

UCSD Physics 10

Uses of Electromagnetic Waves

• Communication systems

– One-way and two-way

• Cooking (with microwaves)

• Medical Imaging (X rays)

• “Night Vision” (infrared)

• Astronomy (radio, µwave, IR, visible, UV, gamma)

All that we experience through our eyes is conveyed by

Spring 2008 11

UCSD Physics 10

The Electromagnetic Spectrum

• Relationship between frequency, speed and

wavelength

f = c

f is frequency, is wavelength, c is speed of light

• Different frequencies of electromagnetic radiation

are better suited to different purposes

• The frequency of a radio wave determines its

propagation characteristics through various media

Spring 2008 12

UCSD Physics 10

US Frequency Allocation – 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

• Accelerating charges radiate EM energy

• 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 too

Note that the magnetic fields are perpendicular to the electric field vectors

B

+

+

+

+

+ +

+

+

Spring 2008 15

UCSD Physics 10

B

ETransmitting

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)

Electromagnetism & EM Waves 05/19/08

Lecture 18 5

Spring 2008 17

UCSD Physics 10

Questions

Why are car radio antennas vertical?

Why are cell phone antennas so short?

How do polarizing sunglasses work?

Spring 2008 18

UCSD Physics 10

Assignments

• Read Chapter 31 for Friday

• Q/O #4 due 5/23 by midnight

• HW 6 due 5/23: 22.E.1, 22.E.5, 22.E.11, 22.E.16,

22.E.20, 22.E.30, 22.E.33, 22.P.1, 23.E.3, 26.E.7,

26.E.9, 26.E.11