Ampere

The ampere ($\mathrm{A}$) is the SI base unit for electric Current.

Operationally, current is charge flow per unit time:

$$I=\frac{Q}{t}$$

So one ampere means one Coulomb of Charge passes a point each Second:

$$1\mathrm{A}=1\frac{\mathrm{C}}{\mathrm{s}}$$

Since the 2019 SI revision, the ampere is ultimately tied to the exact value of the elementary charge:

$$e=1.602176634\times 10^{-19}\mathrm{C}$$

That means a current of $1\mathrm{A}$ corresponds to about $6.24\times 10^{18}$ elementary charges passing a point per second, if the charge carriers each have charge magnitude $e$.

Analogy that usually helps: ampere relates to coulomb as Watt relates to Joule.

$$\mathrm{A}=\mathrm{C}{\mathrm{s}}^{-1},\mathrm{W}=\mathrm{J}{\mathrm{s}}^{-1}$$

Common mistake: saying current is “used up”. Charge usually circulates; Energy is transferred from sources to loads. The current into a circuit element equals the current out, but the Voltage drop tells you how much energy per charge was transferred.

Quick check: if a problem gives charge and time, divide to get current; if it gives current and time, multiply to get total charge moved.

Quick check

If a circuit has a current of $2\mathrm{A}$, then $2\mathrm{C}$ of charge passes a cross-section each second. Current is therefore a flow rate of charge, not a stockpile of charge. This links directly to Coulomb, Current, and Voltage in circuit problems.