Resistance

Resistance is defined as:

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

where $V$ is the potential difference (V) and $I$ is the current (A).

Factors affecting the resistance of a wire:

• Length: longer wire = more resistance ($R \propto L$)
• Cross-sectional area: thinner wire = more resistance ($R \propto 1/A$)
• Material: some materials resist more than others
• Temperature: for metals, higher temperature = more resistance

In series: $R_{\text{total}} = R_1 + R_2 + ...$

In parallel: $\frac{1}{R_{\text{total}}} = \frac{1}{R_1} + \frac{1}{R_2} + ...$