# Fundamental unit

1. Aug 10, 2006

### ajay mahajan

Why the current is fundamental quantity?
Why the unit of fundamental quantity current is defined in terms of force?

2. Aug 10, 2006

### Staff: Mentor

Current is not defined in terms of force, it is simply the number of electrons that passes a certain point every second.

Voltage is analagous to force.

3. Aug 10, 2006

### Meir Achuz

The SI ampere is defined in terms of the force between two parallel wires.

4. Aug 10, 2006

### Meir Achuz

Your two questions show why the SI attempt to call current a fundamental unit is illogical. Current should not be considered a fundamental quantity because it is defined in terms of force.
In the Gaussian system, current is also defined in terms of force, but is not considered a fundamental quantity.

5. Aug 10, 2006

### Staff: Mentor

SI units are defined the way they are because the definitions can be implemented precisely in the real world, not because they are ideally logical. With our technology it's probably easier to measure forces between currents precisely than it is to measure forces between charges precisely, or to count electrons directly.

Similarly, the kilogram is defined as the mass of a certain specific lump of metal stored in a basement in Paris, rather than as the mass of a certain number of hydrogen atoms (or some other atom, or some elementary particle).

6. Aug 10, 2006

### rbj

russ, i think i have to agree with Meir.

they could have, in a more orderly world, define the unit charge to be a fixed number of electrons, possibly 6 241 509 479 607 717 888 elementary charges per Coulomb, but they didn't. they defined the Coulomb to be the amount of charge that passes in 1 second in a conductor carrying 1 Ampere of current. so the Ampere had to be defined first and they defined it in such a manner that the permeability of free space is exactly $\mu_0 \equiv 4 \pi \times 10^{-7} N/A^2$:

so my answer to the OP is that it seemed more convenient to physicists about a century ago to define the unit of current first with a mechanical definition and then define the unit charge in terms of the unit current and the unit time (the second).

someday they could redefine the kilogram so that the Coulomb is precisely 6 241 509 479 607 717 888 elementary charges and keep $\mu_0 \equiv 4 \pi \times 10^{-7} N/A^2$, but i am rooting for defining the kg so that Planck's constant is a defined constant.

7. Aug 10, 2006

### Staff: Mentor

I didn't realize they did that.

8. Aug 11, 2006

### rbj

the world is full of things to learn. it wasn't until i took a Real Analysis course in the math department and learned about Lebesgue integration, that i found out that the way that Electrical Engineering texts and classes define the Dirac delta function is faulty. (but it works fine for engineering and physical science.)