Electromagnetic waves are changing electric and magnetic fields that travel through space. Unlike mechanical waves, they do not require a material medium. In this lesson, we focus on waves traveling in one direction and use Maxwell’s equations to understand their orientation.
Mechanical waves, such as waves on a string or sound in air, need a medium to carry the disturbance. Maxwell’s equations predict a different kind of wave: a changing electromagnetic field that can propagate through free space, where there is no material medium.
All flashcards
Transverse electromagnetic wave
A wave whose electric and magnetic fields are perpendicular to its direction of propagation.
Free-space electromagnetic-wave speed
In vacuum, c=1/μ0ε0, approximately 3.00×108m/s.
Field-amplitude relationship in a free-space plane wave
The amplitudes satisfy E0/B0=c and the fields are in phase.
Electromagnetic radiation from a transmitting antenna
Accelerating charges, including charges driven by alternating current, produce electromagnetic waves.
Resonance in a radio tuner
A tuner responds strongly when its natural frequency matches or nears the incoming signal frequency.
Quiz questions
What propagation speed follows from the free-space electromagnetic wave equation?Answer: c=1/μ0ε0
Which direction of travel is represented by the phase kx−ωt?Answer: The positive x-direction.
How does the radiation intensity of an ideal dipole vary with direction?Answer: It is strongest perpendicular to the antenna axis and zero along the axis.
When does a receiver’s resonant tuner respond strongly to an incoming signal?Answer: When its natural frequency matches or nearly matches the signal frequency.
Which charge motion produces electromagnetic radiation as described for antennas?Answer: Accelerating charges, such as charges driven by alternating current.
Based on University Physics Volume 2 by Samuel J. Ling, William Moebs, Jeff Sanny (OpenStax), licensed CC BY 4.0. Adapted into a summary, flashcards and quizzes.