- Forward


Electric and Electromagnetic (EM) Waves
An Introduction


Prof. David Bernstein
James Madison University

Computer Science Department
bernstdh@jmu.edu

Print

Electricity Basics
Back SMYC Forward
  • Electric Charge:
    • Can be positive or negative
    • By conventions, a single proton has a charge of +1 while a single electron has a charge of -1
    • In the general case, electric charge is usually denoted by \(q\)
  • Laws/Properties:
    • Similar/opposite charges repel/atrtract (Coulomb's Law)
    • The total quantity of electric charge in a space is zero (Conservation)
    • An electric charge exerts a force in the electric field surrounding it
Electricity Basics (cont.)
Back SMYC Forward
  • Electric Current:
    • A flow of electric charge through a conductive medium
    • The charge is often carried by electrons moving in a wire
  • Conductive Metals:
    • Contain (a large number of) free electrons (i.e., electrons bound to the metal lattice but not to individual atoms)
    • When subjected to electric force at opposite ends these free electrons move (towards the positive end) forming a current
Electricity Basics (cont.)
Back SMYC Forward
  • Direct Current (DC):
    • The flow of electric charge is in one direction
  • Alternating Current (AC):
    • The flow of electric charge reverses direction periodically (forming a wave in which the voltage changes over time)
    • The typical waveform for residential/commercial power is sinusoidal (at 50-60Hz)
Electromagnetism Basics
Back SMYC Forward
  • An electric current produces a magnetic field
    • images/electric-current.gif
    • The energy that is radiated is lost
  • A changing magnetic field produces an electric current
Electromagnetism Basics (cont.)
Back SMYC Forward

A charge, \(q\), moving with velocity, \(v\), in an electric field, \(E\), and magnetic field, \(B\), experiences a force called the Lorentz force (or electromagnetic force) of:

\(F = qE + qv \times B\)

images/electric-current_force.gif
Pulses
Back SMYC Forward
  • Mechanical Pulse (A Review):
    • A disturbance that moves through a medium
  • A Generalization:
    • A disturbance that moves through space
Observations About Electromagnetic Pulses
Back SMYC Forward
  • A source of an electromagnetic pulse creates electric and magnetic fields, perpendicular to each other, that travel away from the source
  • The energy of an EM pulse is stored in the electric and magnetic fields
  • EM pulses can travel through a vacuum
Electromagnetic Waves
Back SMYC Forward

An Illustration

images/em-wave.gif
Electromagnetic Waves (cont.)
Back SMYC Forward
  • An electromagnetic wave carries no mass (but, as mentioned above, does carry energy)
  • The energy carried by an electromagnetic wave is proportional to the frequency of the wave
  • An electromagnetic wave has momentum, and can exert pressure (known as radiation pressure)
Electromagnetic Waves (cont.)
Back SMYC Forward
  • Recall the Velocity of a Wave:
    • \(v = \lambda f\)
  • Velocity of an EM Wave in a Vacuum:
    • The speed of light (usually denoted by \(c\))
    • \(3 \times 10^8\) m/sec; 186,000 mi/sec; 186 mi/millisec
  • Velocity of an EM Wave in a Copper/Fiber:
    • About \(2/3 c\)
The Electromagnetic Spectrum
Back SMYC Forward

International Telecommunication Union (ITU) Bands

Band Frequency
Gamma Rays 1022
1021
1020
1019
1018
1017
X-Rays 1016
Ultraviolet 1015
Visible Light 1014
Infrared 1013
THF 1012 (1THz)
EHF 1011
SHF 1010
UHF 10 9 (1GHz)
VHF 10 8
10 7
High 10 6 (1MHz)
Medium 10 5
Low 10 4
Very Low 10 3 (1KHz)
10 2
Extremely Low 10 1
10 0 (1Hz)
There's Always More to Learn
Back -