# S.I Unit

1. Dec 11, 2009

### trust143_raj

Why ampere is not a S.I unit?

2. Dec 11, 2009

### Pagan Harpoon

3. Dec 11, 2009

### f95toli

The Ampere is even a base unit in the SI.

4. Dec 11, 2009

### trust143_raj

Sorry i posted the wrong question? I tried to say why Ampere is the basic/fundamental unit although it is derived from charge/time(columb/sec)

5. Dec 11, 2009

### Integral

Staff Emeritus

An amp is defined in terms of the current required to achieve a certain force between parallel conductors. Not in terms of Coulomb per sec.

6. Dec 11, 2009

### D H

Staff Emeritus
With the notable exception of the kilogram, the SI base units are defined in a way as to make the units independently observable. In SI terminology, the coulomb is a derived unit. The reason is that measuring an ampere is a considerably easier (and more precise) task than is measuring a coulomb.

7. Dec 11, 2009

### Staff: Mentor

In the SI unit system current is the fundamental quantity and charge is a derived quantity (1 C = 1 A * 1 s).

The reason is simply that current is easier to measure precisely than charge.

Edit: I see D H was faster than I was

8. Dec 12, 2009

### f95toli

But it is still a very very difficult experiment if the accuracy is to be good enough. No one has -as far as I know- actually realized the Ampere this is way in quite a while (although I am sure there are a few old setups around).
In the "real world" the Ampere actually a derived unit, calculated from the realization of the Volt (Josephson effect) and the Ohm (quantum hall effect).
One of the big topics in metrology over the past few years has been to come up with a useful current standard, something that would allow us to define current in term of charges/second OR to realize the Ampere in terms of time, the latter would be useful since we can meaure time with higher accuracy than anything else (this is how the Volt is realzied. V=Kj*f, Kj being the Josephson constant adn f the frequency)