Current

Current ($I$) measures how quickly electric Charge passes through a surface:

$I_{avg}=\frac{\Delta Q}{\Delta t},I=\frac{dQ}{dt}$

The SI unit is the Ampere, where $1\mathrm{A}=1\mathrm{C}{\mathrm{s}}^{-1}$. In circuit diagrams, current direction is conventional current: the direction a positive charge would move. In metal wires the mobile charges are electrons, so their drift direction is opposite conventional current.

For a simple resistor, Voltage, Current, and Resistance are related by Ohm’s law:

$V=IR\Rightarrow I=\frac{V}{R}$

Interpretation: current is not “used up” by components. In a single-loop series circuit the same current passes through every element. What changes across components is energy per unit charge: the voltage drop.

Useful mental model: voltage is the push per coulomb, current is the rate of coulombs moving, and power is their product:

$P=IV$

Example: a $2\Omega$ resistor across a $6\mathrm{V}$ battery carries $I=6/2=3\mathrm{A}$ and dissipates $P=18\mathrm{W}$.

Measurement note: an ammeter is placed in series so the same current passes through the meter and the component. A voltmeter is placed in parallel; swapping these is a classic way to short a circuit or get nonsense readings.

Common mistake: confusing current with speed. A large current means lots of charge per second, not necessarily fast individual electron motion.