# Is there a reason the Ampere is defined in terms of negative charge per unit time?

1. Jun 5, 2010

### diagopod

Learning about the nature of current, the Ampere and its role in Epsilon0. I'm assuming that the fact that the Ampere is defined in terms of electrons per second is arbitrary and that it could just as well have been protons per second? And if it had been protons per second, then Epsilon0 would still have the same value and sign because it makes use of Amperes squared, which will come out positive regardless of whether or not Amperes are defined in terms of negative or positive charge per unit time. Is this true?

2. Jun 5, 2010

### alxm

Re: Is there a reason the Ampere is defined in terms of negative charge per unit time

Ampere is not defined (yet, it's been proposed) in terms of electrons per second. An Ampere is defined as a Coulomb of charge per second.

It's a quantity of charge; the sign of the charge isn't relevant. In fact, when it was defined they still assumed charged flowed from positive to negative, and didn't know that electrons were the primary charge carriers, or the fact that charge comes in discrete quantities.

3. Jun 5, 2010

### zeebek

Re: Is there a reason the Ampere is defined in terms of negative charge per unit time

What alxm said +

By the definition positive current direction is the direction of positive charge flow. However, like alxm said, it turned out that in most case it is the electrons that are moving and making the current. So the current is kinda "negative". Normally, people speak about current absolute value and indicate the direction. Check Khirhoff's rules and their application.

4. Jun 5, 2010

### diagopod

Re: Is there a reason the Ampere is defined in terms of negative charge per unit time

that makes perfect sense, thanks for the clarification alxm

5. Jun 5, 2010

### diagopod

Re: Is there a reason the Ampere is defined in terms of negative charge per unit time

Thanks, yeah, the bit about positive and negative current has always thrown me - absolute value to the rescue :)

6. Jun 6, 2010

### Staff: Mentor

Re: Is there a reason the Ampere is defined in terms of negative charge per unit time

Actually, the ampere is defined in terms of the magnetic force between two long straight parallel wires. If the wires have equal magnitude currents, and are exactly 1 m apart, and the magnetic force between them is $4\pi \times 10^{-7}$ newtons per meter of wire length, then the current in each wire is exactly 1 ampere.

The coulomb is defined in terms of the ampere, as the amount of charge delivered by a current of one ampere in one second.

7. Jun 6, 2010

### Staff: Mentor

Re: Is there a reason the Ampere is defined in terms of negative charge per unit time

Blame Benjamin Franklin for guessing wrong about the direction of flow of "electric fluid" when rubbing two objects together to charge them.

8. Jun 6, 2010

### diagopod

Re: Is there a reason the Ampere is defined in terms of negative charge per unit time

Thanks. Would you say it's also arbitrary the direction of the electric field, as in the direction of the arrows we see in field lines diverging from positive charges and converging on negative ones, or is that different?