Definition:Ampere (Unit)

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The ampere is the SI base unit of electric current.

It is defined as being:

The constant current which will produce a force of attraction whose value is $2 \times 10^{–7}$ newtons per metre of length between two straight, parallel conductors of infinite length and of infinitesimal circular cross-section placed one metre apart in a vacuum.

By Force between Infinite Parallel Straight Conductors carrying Steady Current, we have:

$\mathbf F = \dfrac {\mu_0 I_1 I_2} {2 \pi r}$


$I_1$ and $I_2$ denote the current through $s_1$ and $s_2$
$r$ denotes the distance between $s_1$ and $s_2$
$\mu_0$ denotes the vacuum permeability.

Setting $r = 1$ metre, this becomes:

$\mathbf F = \dfrac {\mu_0 I_1 I_2} {2 \pi}$

If $\mathbf F$ is measured in newtons, then $I_1$ and $I_2$ are in amperes.


The symbol for the ampere is $\mathrm A$.

Also known as

In the everyday context of electricity as a utility, the word amp is usually used.

Source of Name

This entry was named for André-Marie Ampère.