Electric and Electromagnetic (EM) Waves

Electric and Electromagnetic (EM) Waves
An Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Electricity Basics

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.)

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.)

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

An electric current produces a magnetic field
- The energy that is radiated is lost
A changing magnetic field produces an electric current

Electromagnetism Basics (cont.)

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:

$F = qE + qv \times B$

Pulses

Mechanical Pulse (A Review):
- A disturbance that moves through a medium
A Generalization:
- A disturbance that moves through space

Observations About Electromagnetic Pulses

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

An Illustration

Electromagnetic Waves (cont.)

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.)

Notation:
- $\lambda$ is the wavelength (meters per cycle)
- $f$ is the frequency (cycles per second)
- $v$ is the velocity (meters per second)
An Important Relationship:
- $v = \lambda f$
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

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	10⁹ (1GHz)
VHF	10⁸
	10⁷
High	10⁶ (1MHz)
Medium	10⁵
Low	10⁴
Very Low	10³ (1KHz)
	10²
Extremely Low	10¹
	10⁰ (1Hz)

There's Always More to Learn