Electric current, denoted by the symbol \$I\$, is the rate at which electric charge flows through a conductor. Think of it like a stream of tiny cars (the charge carriers) moving along a road (the conductor). The faster and more cars that pass a point, the higher the current. 🚗
For a uniform, straight conductor the current can be calculated using:
\$I = A\,n\,v\,q\$
Imagine a pipe filled with water. The amount of water that passes a cross‑section per second is analogous to current. The pipe’s width (\$A\$) determines how many water molecules can flow side‑by‑side, the density of molecules (\$n\$) tells how many are in the water, their speed (\$v\$) is like the water’s velocity, and each molecule carries a tiny charge (\$q\$). The product of all four gives the total charge flow per second. 💧
| Symbol | Description | Units |
|---|---|---|
| \$A\$ | Cross‑sectional area of the wire | m² |
| \$n\$ | Number density of free electrons | m⁻³ |
| \$v\$ | Drift velocity of electrons | m/s |
| \$q\$ | Charge of one electron | C |
| \$I\$ | Electric current | A (ampere) |
Calculate the current in a copper wire that is 1 mm² in cross‑section, carries electrons with a drift velocity of \$1.0\times10^{-4}\,\text{m/s}\$, and has a free‑electron density of \$8.5\times10^{28}\,\text{m}^{-3}\$. The charge of an electron is \$q = 1.6\times10^{-19}\,\text{C}\$.
\$A = 1\,\text{mm}^2 = 1\times10^{-6}\,\text{m}^2\$
\$I = (1\times10^{-6})\,(8.5\times10^{28})\,(1.0\times10^{-4})\,(1.6\times10^{-19})\$