lecture24

lecture24 - Physics 2102 Gabriela Gonzlez Electromagnetic...

This preview shows pages 1–6. Sign up to view the full content.

Electromagnetic waves Physics 2102 Gabriela González

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
A solution to Maxwell’s equations in free space: ) sin( t x k E E m ϖ - = ) sin( t x k B B m - = k = c , speed of propagation. mph s m B E c m m 163 , 187 954 , 462 , 299 1 0 0 = = = = ε μ Visible light, infrared, ultraviolet, radio waves, X rays, Gamma rays are all electromagnetic waves. Electromagnetic waves http://phys23p.sl.psu.edu/CWIS/
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) 2 1 1 | | E c EB S 0 0 = = μ The Poynting vector E B S Units: Watt/m2 For a wave, since E is perpendicular to B: B E S × = 0 1 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.

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
__ __________ 2 2 2 ___ ) ( sin 1 1 t kx E c E c S I m ϖ μ - = = = 0 0 The average of sin2 over one cycle is ½: 2 2 1 m E c I 0 = 2 1 rms E c I 0 = Both fields have the same energy density. B E u B cB E u = = = = 0 0 0 0 ε 2 0 2 2 2 1 ) ( 2 1 2 1 or, 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 . The total EM energy density is then 0 2 2 0 / B E u = =
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 8x20m? I=1kW/m2

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 11/18/2011 for the course PHYSICS 2102 taught by Professor Dowling during the Fall '10 term at LSU.

Page1 / 21

lecture24 - Physics 2102 Gabriela Gonzlez Electromagnetic...

This preview shows document pages 1 - 6. Sign up to view the full document.

View Full Document
Ask a homework question - tutors are online