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In my last blog entry, Units - Part I - Mechanics, I presented the SI units used to describe motion and collisions. There were a few fundamental units and many derived units. In the same way, I will now present the fundamental and derived SI units for Electrodynamics.
Electrodynamics
Current - Ampere (A)
Current Density - A/m2
Charge - Coulomb (C) - A·s
Charge Density - A·s/m3
Electric Power - Watt (W) - kg·m2/s3
Electric Potential* - Volt (V) - W/A - m2·kg/s3·A
Resistance - Ohm (Ω) - V/A - kg·m2/s3·A2
Conductance - Siemen (S) - s3·A2/kg·m2
Capacitance - Farad (F) - C/V - A2·s4/kg·m3
Elastance -(1/F) - V/C
Inductance - Henry (Wb/A) - kg·m2/s2A2
Reluctance - (A/Wb)
Conductivity - S/m
Resistivity - Ω·m
Magnetic Flux - Weber (Wb) - V·s - kg·m2/s2·A
Magnetic Induction - Tesla (T) - N/A·m - kg/s2·A
Magnetic Field - A/m
Magnetic Susceptability - Unitless
Electric Field - V/m
Electric Susceptablility - Unitless
Polarization - C/m2 - A·s/m2
*Please note that Electric Potential is not Energy, which would have units Joules (J). However Volts (V) x Coulombs (A·s) = Joules (J) which you can easily verify.
Although it is my instinct to view the Coulomb as a fundamental unit and Ampere (C/s) as a derived unit, SI views it the opposite, with Ampere being listed as the fundamental unit and Coulomb (A·s) being the derived unit. I don't know why this is and welcome any explanations. Irregardless, that's the way it is and I think its just easier to accept it. The other thing you should take away from this list of units is that the best way to get a unit named after you in Electrodynamics is to be a Physicist or Engineer born between 1700 and 1900.
Electrodynamic Constants
Permittivity of Free Space (ε0) - C2/N·m2 - A2·s2/N·m2
Permeability of Free Space (μ0) - N/A2
Charge of the Electron (e) - C
Example
Electric Fields store energy. The expression for the potential energy (u) stored in an Electric Field is;
= (A2·s2/N·m2) x (V/m)2 x m3
= (A2·s2/N·m2) x (m2·kg/s3·A)2 x m
= m3·kg2/N·s4
= m3·kg2/kg·m·s2
= kg·m2/s2 = Joules
So the energy stored in an electric field has units of Joules (as it better).
Thanks for the help of the following sites. Part three coming soon will discuss Thermodynamic Units. In part three I will also include an addendum that covers units related to Light and units related to radioactivity.
http://en.wikipedia.org/wiki/Electromagnetism
http://www.physics.nist.gov/cuu/Units/
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