Electric Current, Resistance, and Ohm's Law

Electric current is the rate at which electric charge flows past a point in a circuit. If a charge $\Delta Q$ passes a cross-section in time $\Delta t$, the current is

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

Current is measured in amperes ($\text{A}$), where one ampere is one coulomb per second. In metal conductors the moving charges are free electrons, but by a longstanding convention the direction of conventional current is taken as the direction in which positive charge would flow, that is, from the positive terminal of a battery, through the external circuit, to the negative terminal. This is opposite to the actual drift direction of the electrons, but the convention is used consistently throughout circuit analysis.

Although we speak loosely of electricity moving at the speed of light, the i