Lecture 33: FRI 03 APR Ch.33.1–3,5: E&M Waves Physics 2102 Jonathan Dowling James Clerk Maxwell (1831-1879) Lecture 33: FRI 03 APR Ch.33.1–3,5: E&M Waves

Maxwell, Waves and Light A solution to the Maxwell equations in empty space is a “traveling wave”… electric and magnetic “forces” can travel! The “electric” waves travel at the speed of light! Light itself is a wave of electricity and magnetism!

Electromagnetic Waves A solution to Maxwell’s equations in free space: Visible light, infrared, ultraviolet, radio waves, X rays, Gamma rays are all electromagnetic waves.

Radio waves are reflected by the layer of the Earth’s atmosphere called the ionosphere. This allows for transmission between two points which are far from each other on the globe, despite the curvature of the earth. Marconi’s experiment discovered the ionosphere! Experts thought he was crazy and this would never work.

Maxwell’s Rainbow The wavelength/frequency range in which electromagnetic (EM) waves (light) are visible is only a tiny fraction of the entire electromagnetic spectrum. Fig. 33-2 Fig. 33-1 (33-2)

The Traveling Electromagnetic (EM) Wave, Qualitatively An LC oscillator causes currents to flow sinusoidally, which in turn produces oscillating electric and magnetic fields, which then propagate through space as EM waves. Fig. 33-3 Next slide Oscillation Frequency: (33-3)

Mathematical Description of Traveling EM Waves Electric Field: Wave Speed: Magnetic Field: All EM waves travel a c in vacuum Fig. 33-5 Wavenumber: Angular frequency: Vacuum Permittivity: Vacuum Permeability: EM Wave Simulation Amplitude Ratio: Magnitude Ratio: (33-5)

The Poynting Vector: Points in Direction of Power Flow Electromagnetic waves are able to transport energy from transmitter to receiver (example: from the Sun to our skin). The power transported by the wave and its direction is quantified by the Poynting vector. John Henry Poynting (1852-1914) For a wave, since E is perpendicular to B: Units: Watt/m2 In a wave, the fields change with time. Therefore the Poynting vector changes too!! The direction is constant, but the magnitude changes from 0 to a maximum value. E B S

EM Wave Intensity, Energy Density A better measure of the amount of energy in an EM wave is obtained by averaging the Poynting vector over one wave cycle. The resulting quantity is called intensity. Units are also Watts/m2. The average of sin2 over one cycle is ½: or, Both fields have the same energy density. The total EM energy density is then

Solar Energy The light from the sun has an intensity of about 1kW/m2. What would be the total power incident on a roof of dimensions 8m x 20m ? I = 1kW/m2 is power per unit area. P=IA=(103 W/m2) x 8m x 20m=0.16 MegaWatt!! The solar panel shown (BP-275) has dimensions 47in x 29in. The incident power is then 880 W. The actual solar panel delivers 75W (4.45A at 17V): less than 10% efficiency…. The electric meter on a solar home runs backwards — Entergy Pays YOU!

EM Spherical Waves The intensity of a wave is power per unit area. If one has a source that emits isotropically (equally in all directions) the power emitted by the source pierces a larger and larger sphere as the wave travels outwards: 1/r2 Law! So the power per unit area decreases as the inverse of distance squared.

Example A radio station transmits a 10 kW signal at a frequency of 100 MHz. At a distance of 1km from the antenna, find the amplitude of the electric and magnetic field strengths, and the energy incident normally on a square plate of side 10cm in 5 minutes. Received energy: