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Electromagnetic Waves ISAT 241 Fall 2003 David J. Lawrence Waves (review) A wave is the propagation of a disturbance. A wave can be transverse or longitudinal (or mixed). Waves propagate (travel) with some speed v. Mechanical waves require a medium. Electromagnetic waves do not require a medium and can travel through empty space (a vacuum). Examples include radio, television, cell phones, radar, light, x-rays, and gamma rays. Electromagnetic Waves Electromagnetic waves are the propagation of disturbances of electric and magnetic fields. A time-varying electric field produces a magnetic field. A time-varying magnetic field produces an electric field. Electromagnetic waves travel through vacuum at “the speed of light”: c = 3 108 m/s Electromagnetic Waves The simplest electromagnetic waves are plane (not plain!) electromagnetic waves. Plane electromagnetic waves consist of oscillating electric and magnetic fields. The electric and magnetic fields are in the same plane. The electric and magnetic fields are perpendicular to each other. The electric and magnetic fields are perpendicular to the direction of propagation. Plane Electromagnetic Waves See Figure 24.3 on page 902: EB y E Ec Bc c B z x Note that E and B are not constant; they vary with x and t. Serway & Jewett, Principles of Physics Figure 24.3 Plane Electromagnetic Waves Also see Figure 24.6 on page 904: EB y E Ec Bc c B z x • Note that E and B are not constant; they vary with x and t. • Note that the wave is traveling in the direction of the cross product E x B Serway & Jewett, Principles of Physics Figure 24.6 Plane Electromagnetic Waves The speed of an electromagnetic wave in vacuum is given by 1 c moeo where mo = 4p 10-7 N/A2 = permeability of vacuum and eo = 8.854 10-12 C2/N-m2 = permittivity of vacuum c is “the speed of light” (in vacuum) Plane Electromagnetic Waves The wave equations for a plane electromagnetic wave are E E max cos(kx - wt ) B Bmax cos(kx - wt ) Recall the definitions of k, w, f, and l . Also w cfl k Plane Electromagnetic Waves The magnitudes of E and B in for a plane electromagnetic wave in vacuum are related by E E max c B Bmax S is called the Poynting vector 1 S EB μo The magnitude of the Poynting vector represents the power per unit area (W/m2) being transported by the wave at any instant of time. Serway & Jewett, Principles of Physics Figure 24.13