| Poynting Vector |
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| CATEGORIES ABOUT POYNTING VECTOR | |
| electromagnetic radiation | |
| optics | |
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It points in the direction of energy flow and its Magnitude is the power per unit area crossing a surface which is normal to it. (The fact that it ''points'' perhaps contributes to the frequency with which its name is often misspelled.) It is derived by considering the conservation of energy and taking into account that the magnetic field can do no work. It is given the symbol S (in bold because it is a Vector ) and, in SI units, is given by: :, where E is the Electric Field , '''H''' and '''B''' are the Magnetic Field and Magnetic Flux Density respectively, and is the Permeability of the surrounding medium. For example, Poynting vector near ideally conducting wire is parallel to the wire axis - so electric energy is flowing in space outside of wire. The Poynting vector becomes tilted toward wire for a resistive wire, indicating that energy flows from the e/m field into the wire, producing resistive Joule Heating in the wire. For an electromagnetic wave propagating in free space becomes , the Permeability Of Free Space . Since the electric and magnetic fields of an electromagnetic wave oscillate, the magnitude of the Poynting vector also oscillates. The average of the magnitude over a long time T (longer than the Period of the wave) is called the Irradiance or intensity, I: |
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