Resistance and Ohm's Law

Resistance opposes current. For an ohmic conductor:

$V = I R$

So $I = V/R$ and $R = V/I$. Example: a 5 V supply across a 220 Ω resistor drives $I = 5/220 \approx 22.7$ mA.

Resistance of a wire depends on its material and shape:

$R = \rho \frac{L}{A}$

where $\rho$ is resistivity (copper ≈ $1.68 \times 10^{-8}$ Ω·m), $L$ the length and $A$ the cross-sectional area. Conductance $G = 1/R$ is measured in siemens (S).