2. Electric Current, Voltage, and Resistance

Electric current is the flow of electric charge through a conductor. In a wire, electrons move from the negative terminal toward the positive terminal. Current is measured in amperes (A).

Voltage (also called potential difference) is the "push" that drives current through a circuit. Think of it like water pressure — the higher the pressure, the more water flows. Voltage is measured in volts (V).

Resistance is the opposition a material offers to the flow of current. A thick copper wire has low resistance; a thin tungsten filament has high resistance. Resistance is measured in ohms (Ω).

Ohm's Law connects all three:
\[ V = I \times R \]
where \(V\) = voltage (V), \(I\) = current (A), \(R\) = resistance (Ω).

Rearranging:
\[ I = \frac{V}{R} \qquad R = \frac{V}{I} \]

Worked example: A 12 V battery drives current through a 4 Ω resistor. What is the current?
\[ I = \frac{V}{R} = \frac{12}{4} = 3 \text{ A.} \]

Good conductors (copper, aluminum, silver) have very low resistance and allow current to flow easily. Insulators (rubber, glass, plastic) have very high resistance and block current flow — that is why wires are coated in plastic.