# Why is the Ampere the only SI fundamental unit for electricity?

According to wikipedia the only SI fundamental unit for Matters Electrickal is the ampere. Don't you at least need the ohm to derive anything? How would you make volts from only amps?

Perhaps I misunderstand the meaning of "fundamental unit".

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I can't speak to why other units aren't there, but Amps are defined as "The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 × 10−7 newton per metre of length" of which you only have to know about "newton" and "metre". So no need to have knowledge of Ohms or Voltage for the sake of the definition. –  Kellenjb Dec 9 '11 at 16:07
I'm no expert here, but for a pretty good discussion of the fundamental units and how other units are derived from them you might want to take a look at Frink and it's units file. A great tool to have on your PC anyway. For example, volts is defined as $m^2·kg·s^{-3}·A ^{-1}$, which is W/A ($W = m^2·kg·s^{-3}$), and Ohms as $m^2·kg·s^{-3}·A^{-2}$ which is just the last equation divided by Amperes (e.g W/A = V) –  Oli Glaser Dec 9 '11 at 16:25
No, but Watts are derived from mass length and time ($m^2·kg·s^{-3}$), which are fundamental units. –  Oli Glaser Dec 9 '11 at 16:30
@Oli, I think your comment is the answer -- the other units are not fundamental units because they can be expressed in terms of the fundamental units of meters, kilograms, seconds, and amps. –  The Photon Dec 9 '11 at 16:55
@Oli - I agree with The Photon that your comments are the answer. I noticed you were having trouble with the TeX; the syntax is x^{-3} to get negative exponents. The {} causes its contents to be treated as one element, like parentheses. I've fixed them, but I can't make them into an answer from you. –  Kevin Vermeer Dec 9 '11 at 17:02

Volt is defined as Work done for unit charge. Charge can be derived from product of current and time. So volt can be expressed in terms of mass, distance, time and current.

Now for ohms, it can be defined as the ratio of voltage and current. So it can also be expressed in terms of mass, distance, time and current.

So with just a unit for current combined with other fundamental quantities, we can define all the other electrical quantities.

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The Ampere is actually not a fundamental unit. It is Coulombs/second, with Coulombs and seconds being the fundamental units. Other common electrical units can be derived from the non-electrical fundamental units and the Coulomb. For example, a Volt is a Joule/Coulomb, or expressed in fundamental units is a Netwon-meter/Coulomb. A Ohm is a Newton-meter-second / Coulomb^2. You can continue on and derive Farads, Henries, etc, similarly.