Simple Nature - Light and Matter

divE= 4πkρ

divB= 0

curlE=−

∂B

∂t

c^2 curlB=

∂E

∂t

+ 4πkj

Although 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 astronomy
and 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 takingk= 1
as a matter of definition, so the Coulomb force law isF =q 1 q 2 /r^2. This equation indirectly
defines a unit of charge called the elestrostatic unit, with 1 C = 2.998× 109 esu, the factor of
2.998 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 avoid
confusion, magnetic fields are normally written using the equivalent unit of gauss, 1 gauss=1
dyne/esu=10−^4 T. The force on a charged particle isF=qE+qvc×B, which differs from the
mks version by the 1/cfactor in the magnetic term. Maxwell’s equations are:

ΦE= 4πqin

ΦB= 0

ΓE=−

1

c

∂ΦB

∂t

ΓB=

1

c

∂ΦE

∂t

+

4 π

c

Ithrough

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