This document provides an overview of electromagnetic waves physics: 1) Electromagnetic waves consist of oscillating electric and magnetic fields that propagate perpendicular to each other at the speed of light. 2) Radio antennas generate oscillating electric fields that produce electromagnetic waves, allowing transmission and reception of radio signals. 3) The electromagnetic spectrum encompasses all types of electromagnetic waves including light, radio waves, microwaves, x-rays, which only differ in their wavelength and frequency. 4) Maxwell's equations unified electric and magnetic phenomena and predicted the existence of electromagnetic waves traveling at the speed of light.

1. Electromagnetic Waves Physics 102: Lecture 14

9. Radio antenna Generator creates oscillating current up and down metal rods I x y This is called an electric dipole antenna This is an electric dipole! + -

10. Oscillating E field Electric dipole antenna creates an oscillating electric field In which direction does the E-field point at this time? NOT QUITE! E-fields do NOT appear everywhere in space instantaneously, they travel at a finite speed c ... and now?

14. Recall fundamental constants of electricity and magnetism: “ Permeability of free space” (magnetism) “ Permittivity of free space” (electricity) Speed of EM wave in vacuum Now multiply them: Note: 1T = 1 N/Cm/s (from F = qvBsin( θ )) 1A = 1 C/s (from I = Δ Q/ Δ t) c =

18. EM Waves Practice Shown below is the E field of an EM wave broadcast at 96.1 MHz and traveling to the right. (1) What is the direction of the magnetic field? (2) Label the two tic marks on the x axis (in meters). Perpendicular to E, v: Into/out of the page 3.1 6.2 Example E x

19. This picture only represents EM wave along one line (x-axis) Representing EM wave: Wavefronts Imagine a slice in y-z plane E-field & B-field same everywhere along plane x z y y z  Wavefronts – surfaces at crests of EM wave

20. Doppler Effect Now the car is moving to the left. Observed wavelength  o different! A police car emits light of wavelength  e Moving toward observer: f o = f e (1 + u / c ) Only relative velocity matters: u = v 1 + v 2 moving in opposite directions u = v 1 – v 2 moving in same direction  = f/c Moving away from observer: f o = f e (1 – u / c ) Wavefronts u  e  o  e  o  e

21. ACT: Doppler Practice V = 32 m/s V = 50 m/s In the jeep, the frequency of the light from the troopers car will appear: (1) higher (more blue) (2) Lower (more red) What value should you use for u in the equation? (1) 32 (2) 50 (3) 50+32 (4) 50-32