Ampere (A)

Also called A, Amp

The ampere is the SI base unit of electric current, equal to one coulomb of charge passing a point each second. Current is measured in amperes through a circuit element, while potential difference is measured in volts across it.

Of the seven SI base units, this is the electrical one, and the rest of the electricity chapter is built on top of it. A coulomb is an ampere-second, a volt is a watt per ampere, an ohm is a volt per ampere, a tesla is a newton per ampere-metre. Its own definition is a rate, from the sheet's

I = \frac{\Delta q}{\Delta t}

so one ampere is one coulomb per second.

Being a rate fixes the grammar. Current is through something; potential difference is across something. A wire carries $2.0$ A through it, a resistor has $6.0$ V across it. Keeping those prepositions straight removes a whole class of circuit error, because "the voltage through the resistor" is usually written just before two quantities that do not add get added.

A sense of scale, which the other electrical units do not offer: a $60$ W bulb on a $120$ V supply draws $I = P/\Delta V = 0.50$ A. Amperes are the size of everyday currents, unlike the coulomb, the farad and the tesla, each of which dwarfs the values that quantity usually takes in an AP problem.

Direction without being a vector. Essential knowledge 11.1.A.2 says current is not a vector quantity although it does have a direction, and that the direction is associated with the motion positive charge would have, not with any coordinate system in space. The AP Physics 2 conventions box states that current is conventional current; the C: Electricity and Magnetism box puts the same rule as the direction in which positive charges would drift. In a metal, electrons drift the other way.

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