Simple Nature - Light and Matter

div E = 4πkρdiv B = 0curl E = − ∂B∂tc 2 curl B = ∂E∂t + 4πk jAlthough all engineering and most scientific work these days is done in the SI (mks) system,one may still encounter the older cgs (centimeter-gram-second) system, especially in astronomyand particle physics. The mechanical units in this system include the dyne (g·cm/s 2 ) for force,and the erg (g·cm 2 /s 2 ) for energy. The system is extended to electrical units by taking k = 1as a matter of definition, so the Coulomb force law is F = q 1 q 2 /r 2 . This equation indirectlydefines a unit of charge called the elestrostatic unit, with 1 C = 2.998 × 10 9 esu, the factor of2.998 arising from the speed of light. The unit of voltage is the statvolt, 1 statvolt = 299.8 V.In this system, the electric and magnetic fields have the same units, dynes/esu, but to avoidconfusion, magnetic fields are normally written using the equivalent unit of gauss, 1 gauss=1dyne/esu=10 −4 T. The force on a charged particle is F = qE + q v c× B, which differs from themks version by the 1/c factor in the magnetic term. Maxwell’s equations are:Φ E = 4πq inΦ B = 0Γ E = − 1 ∂Φ Bc ∂tΓ B = 1 ∂Φ E+ 4π c ∂t c I through915